reference, declarationdefinition
definition → references, declarations, derived classes, virtual overrides
reference to multiple definitions → definitions
unreferenced
    1
    2
    3
    4
    5
    6
    7
    8
    9
   10
   11
   12
   13
   14
   15
   16
   17
   18
   19
   20
   21
   22
   23
   24
   25
   26
   27
   28
   29
   30
   31
   32
   33
   34
   35
   36
   37
   38
   39
   40
   41
   42
   43
   44
   45
   46
   47
   48
   49
   50
   51
   52
   53
   54
   55
   56
   57
   58
   59
   60
   61
   62
   63
   64
   65
   66
   67
   68
   69
   70
   71
   72
   73
   74
   75
   76
   77
   78
   79
   80
   81
   82
   83
   84
   85
   86
   87
   88
   89
   90
   91
   92
   93
   94
   95
   96
   97
   98
   99
  100
  101
  102
  103
  104
  105
  106
  107
  108
  109
  110
  111
  112
  113
  114
  115
  116
  117
  118
  119
  120
  121
  122
  123
  124
  125
  126
  127
  128
  129
  130
  131
  132
  133
  134
  135
  136
  137
  138
  139
  140
  141
  142
  143
  144
  145
  146
  147
  148
  149
  150
  151
  152
  153
  154
  155
  156
  157
  158
  159
  160
  161
  162
  163
  164
  165
  166
  167
  168
  169
  170
  171
  172
  173
  174
  175
  176
  177
  178
  179
  180
  181
  182
  183
  184
  185
  186
  187
  188
  189
  190
  191
  192
  193
  194
  195
  196
  197
  198
  199
  200
  201
  202
  203
  204
  205
  206
  207
  208
  209
  210
  211
  212
  213
  214
  215
  216
  217
  218
  219
  220
  221
  222
  223
  224
  225
  226
  227
  228
  229
  230
  231
  232
  233
  234
  235
  236
  237
  238
  239
  240
  241
  242
  243
  244
  245
  246
  247
  248
  249
  250
  251
  252
  253
  254
  255
  256
  257
  258
  259
  260
  261
  262
  263
  264
  265
  266
  267
  268
  269
  270
  271
  272
  273
  274
  275
  276
  277
  278
  279
  280
  281
  282
  283
  284
  285
  286
  287
  288
  289
  290
  291
  292
  293
  294
  295
  296
  297
  298
  299
  300
  301
  302
  303
  304
  305
  306
  307
  308
  309
  310
  311
  312
  313
  314
  315
  316
  317
  318
  319
  320
  321
  322
  323
  324
  325
  326
  327
  328
  329
  330
  331
  332
  333
  334
  335
  336
  337
  338
  339
  340
  341
  342
  343
  344
  345
  346
  347
  348
  349
  350
  351
  352
  353
  354
  355
  356
  357
  358
  359
  360
  361
  362
  363
  364
  365
  366
  367
  368
  369
  370
  371
  372
  373
  374
  375
  376
  377
  378
  379
  380
  381
  382
  383
  384
  385
  386
  387
  388
  389
  390
  391
  392
  393
  394
  395
  396
  397
  398
  399
  400
  401
  402
  403
  404
  405
  406
  407
  408
  409
  410
  411
  412
  413
  414
  415
  416
  417
  418
  419
  420
  421
  422
  423
  424
  425
  426
  427
  428
  429
  430
  431
  432
  433
  434
  435
  436
  437
  438
  439
  440
  441
  442
  443
  444
  445
  446
  447
  448
  449
  450
  451
  452
  453
  454
  455
  456
  457
  458
  459
  460
  461
  462
  463
  464
  465
  466
  467
  468
  469
  470
  471
  472
  473
  474
  475
  476
  477
  478
  479
  480
  481
  482
  483
  484
  485
  486
  487
  488
  489
  490
  491
  492
  493
  494
  495
  496
  497
  498
  499
  500
  501
  502
  503
  504
  505
  506
  507
  508
  509
  510
  511
  512
  513
  514
  515
  516
  517
  518
  519
  520
  521
  522
  523
  524
  525
  526
  527
  528
  529
  530
  531
  532
  533
  534
  535
  536
  537
  538
  539
  540
  541
  542
  543
  544
  545
  546
  547
  548
  549
  550
  551
  552
  553
  554
  555
  556
  557
  558
  559
  560
  561
  562
  563
  564
  565
  566
  567
  568
  569
  570
  571
  572
  573
  574
  575
  576
  577
  578
  579
  580
  581
  582
  583
  584
  585
  586
  587
  588
  589
  590
  591
  592
  593
  594
  595
  596
  597
  598
  599
  600
  601
  602
  603
  604
  605
  606
  607
  608
  609
  610
  611
  612
  613
  614
  615
  616
  617
  618
  619
  620
  621
  622
  623
  624
  625
  626
  627
  628
  629
  630
  631
  632
  633
  634
  635
  636
  637
  638
  639
  640
  641
  642
  643
  644
  645
  646
  647
  648
  649
  650
  651
  652
  653
  654
  655
  656
  657
  658
  659
  660
  661
  662
  663
  664
  665
  666
  667
  668
  669
  670
  671
  672
  673
  674
  675
  676
  677
  678
  679
  680
  681
  682
  683
  684
  685
  686
  687
  688
  689
  690
  691
  692
  693
  694
  695
  696
  697
  698
  699
  700
  701
  702
  703
  704
  705
  706
  707
  708
  709
  710
  711
  712
  713
  714
  715
  716
  717
  718
  719
  720
  721
  722
  723
  724
  725
  726
  727
  728
  729
  730
  731
  732
  733
  734
  735
  736
  737
  738
  739
  740
  741
  742
  743
  744
  745
  746
  747
  748
  749
  750
  751
  752
  753
  754
  755
  756
  757
  758
  759
  760
  761
  762
  763
  764
  765
  766
  767
  768
  769
  770
  771
  772
  773
  774
  775
  776
  777
  778
  779
  780
  781
  782
  783
  784
  785
  786
  787
  788
  789
  790
  791
  792
  793
  794
  795
  796
  797
  798
  799
  800
  801
  802
  803
  804
  805
  806
  807
  808
  809
  810
  811
  812
  813
  814
  815
  816
  817
  818
  819
  820
  821
  822
  823
  824
  825
  826
  827
  828
  829
  830
  831
  832
  833
  834
  835
  836
  837
  838
  839
  840
  841
  842
  843
  844
  845
  846
  847
  848
  849
  850
  851
  852
  853
  854
  855
  856
  857
  858
  859
  860
  861
  862
  863
  864
  865
  866
  867
  868
  869
  870
  871
  872
  873
  874
  875
  876
=====================================
Garbage Collection Safepoints in LLVM
=====================================

.. contents::
   :local:
   :depth: 2

Status
=======

This document describes a set of extensions to LLVM to support garbage
collection.  By now, these mechanisms are well proven with commercial java 
implementation with a fully relocating collector having shipped using them.  
There are a couple places where bugs might still linger; these are called out
below.

They are still listed as "experimental" to indicate that no forward or backward
compatibility guarantees are offered across versions.  If your use case is such 
that you need some form of forward compatibility guarantee, please raise the 
issue on the llvm-dev mailing list.  

LLVM still supports an alternate mechanism for conservative garbage collection 
support using the ``gcroot`` intrinsic.  The ``gcroot`` mechanism is mostly of
historical interest at this point with one exception - its implementation of
shadow stacks has been used successfully by a number of language frontends and
is still supported.  

Overview
========

To collect dead objects, garbage collectors must be able to identify
any references to objects contained within executing code, and,
depending on the collector, potentially update them.  The collector
does not need this information at all points in code - that would make
the problem much harder - but only at well-defined points in the
execution known as 'safepoints' For most collectors, it is sufficient
to track at least one copy of each unique pointer value.  However, for
a collector which wishes to relocate objects directly reachable from
running code, a higher standard is required.

One additional challenge is that the compiler may compute intermediate
results ("derived pointers") which point outside of the allocation or
even into the middle of another allocation.  The eventual use of this
intermediate value must yield an address within the bounds of the
allocation, but such "exterior derived pointers" may be visible to the
collector.  Given this, a garbage collector can not safely rely on the
runtime value of an address to indicate the object it is associated
with.  If the garbage collector wishes to move any object, the
compiler must provide a mapping, for each pointer, to an indication of
its allocation.

To simplify the interaction between a collector and the compiled code,
most garbage collectors are organized in terms of three abstractions:
load barriers, store barriers, and safepoints.

#. A load barrier is a bit of code executed immediately after the
   machine load instruction, but before any use of the value loaded.
   Depending on the collector, such a barrier may be needed for all
   loads, merely loads of a particular type (in the original source
   language), or none at all.

#. Analogously, a store barrier is a code fragment that runs
   immediately before the machine store instruction, but after the
   computation of the value stored.  The most common use of a store
   barrier is to update a 'card table' in a generational garbage
   collector.

#. A safepoint is a location at which pointers visible to the compiled
   code (i.e. currently in registers or on the stack) are allowed to
   change.  After the safepoint completes, the actual pointer value
   may differ, but the 'object' (as seen by the source language)
   pointed to will not.

  Note that the term 'safepoint' is somewhat overloaded.  It refers to
  both the location at which the machine state is parsable and the
  coordination protocol involved in bring application threads to a
  point at which the collector can safely use that information.  The
  term "statepoint" as used in this document refers exclusively to the
  former.

This document focuses on the last item - compiler support for
safepoints in generated code.  We will assume that an outside
mechanism has decided where to place safepoints.  From our
perspective, all safepoints will be function calls.  To support
relocation of objects directly reachable from values in compiled code,
the collector must be able to:

#. identify every copy of a pointer (including copies introduced by
   the compiler itself) at the safepoint,
#. identify which object each pointer relates to, and
#. potentially update each of those copies.

This document describes the mechanism by which an LLVM based compiler
can provide this information to a language runtime/collector, and
ensure that all pointers can be read and updated if desired.  

At a high level, LLVM has been extended to support compiling to an abstract 
machine which extends the actual target with a non-integral pointer type 
suitable for representing a garbage collected reference to an object.  In 
particular, such non-integral pointer type have no defined mapping to an 
integer representation.  This semantic quirk allows the runtime to pick a 
integer mapping for each point in the program allowing relocations of objects 
without visible effects.

Warning: Non-Integral Pointer Types are a newly added concept in LLVM IR.  
It's possible that we've missed disabling some of the optimizations which 
assume an integral value for pointers.  If you find such a case, please 
file a bug or share a patch.

Warning: There is one currently known semantic hole in the definition of 
non-integral pointers which has not been addressed upstream.  To work around
this, you need to disable speculation of loads unless the memory type 
(non-integral pointer vs anything else) is known to unchanged.  That is, it is 
not safe to speculate a load if doing causes a non-integral pointer value to 
be loaded as any other type or vice versa.  In practice, this restriction is 
well isolated to isSafeToSpeculate in ValueTracking.cpp.

This high level abstract machine model is used for most of the LLVM optimizer.
Before starting code generation, we switch representations to an explicit form.
In theory, a frontend could directly generate this low level explicit form, but 
doing so is likely to inhibit optimization.  

The heart of the explicit approach is to construct (or rewrite) the IR in a 
manner where the possible updates performed by the garbage collector are
explicitly visible in the IR.  Doing so requires that we:

#. create a new SSA value for each potentially relocated pointer, and
   ensure that no uses of the original (non relocated) value is
   reachable after the safepoint,
#. specify the relocation in a way which is opaque to the compiler to
   ensure that the optimizer can not introduce new uses of an
   unrelocated value after a statepoint. This prevents the optimizer
   from performing unsound optimizations.
#. recording a mapping of live pointers (and the allocation they're
   associated with) for each statepoint.

At the most abstract level, inserting a safepoint can be thought of as
replacing a call instruction with a call to a multiple return value
function which both calls the original target of the call, returns
its result, and returns updated values for any live pointers to
garbage collected objects.

  Note that the task of identifying all live pointers to garbage
  collected values, transforming the IR to expose a pointer giving the
  base object for every such live pointer, and inserting all the
  intrinsics correctly is explicitly out of scope for this document.
  The recommended approach is to use the :ref:`utility passes 
  <statepoint-utilities>` described below. 

This abstract function call is concretely represented by a sequence of
intrinsic calls known collectively as a "statepoint relocation sequence".

Let's consider a simple call in LLVM IR:

.. code-block:: llvm

  define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj) 
         gc "statepoint-example" {
    call void ()* @foo()
    ret i8 addrspace(1)* %obj
  }

Depending on our language we may need to allow a safepoint during the execution 
of ``foo``. If so, we need to let the collector update local values in the 
current frame.  If we don't, we'll be accessing a potential invalid reference 
once we eventually return from the call.

In this example, we need to relocate the SSA value ``%obj``.  Since we can't 
actually change the value in the SSA value ``%obj``, we need to introduce a new 
SSA value ``%obj.relocated`` which represents the potentially changed value of
``%obj`` after the safepoint and update any following uses appropriately.  The 
resulting relocation sequence is:

.. code-block:: llvm

  define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj) 
         gc "statepoint-example" {
    %0 = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj)
    %obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 7, i32 7)
    ret i8 addrspace(1)* %obj.relocated
  }

Ideally, this sequence would have been represented as a M argument, N
return value function (where M is the number of values being
relocated + the original call arguments and N is the original return
value + each relocated value), but LLVM does not easily support such a
representation.

Instead, the statepoint intrinsic marks the actual site of the
safepoint or statepoint.  The statepoint returns a token value (which
exists only at compile time).  To get back the original return value
of the call, we use the ``gc.result`` intrinsic.  To get the relocation
of each pointer in turn, we use the ``gc.relocate`` intrinsic with the
appropriate index.  Note that both the ``gc.relocate`` and ``gc.result`` are
tied to the statepoint.  The combination forms a "statepoint relocation 
sequence" and represents the entirety of a parseable call or 'statepoint'.

When lowered, this example would generate the following x86 assembly:

.. code-block:: gas
  
	  .globl	test1
	  .align	16, 0x90
	  pushq	%rax
	  callq	foo
  .Ltmp1:
	  movq	(%rsp), %rax  # This load is redundant (oops!)
	  popq	%rdx
	  retq

Each of the potentially relocated values has been spilled to the
stack, and a record of that location has been recorded to the
:ref:`Stack Map section <stackmap-section>`.  If the garbage collector
needs to update any of these pointers during the call, it knows
exactly what to change.

The relevant parts of the StackMap section for our example are:

.. code-block:: gas
  
  # This describes the call site
  # Stack Maps: callsite 2882400000
	  .quad	2882400000
	  .long	.Ltmp1-test1
	  .short	0
  # .. 8 entries skipped ..
  # This entry describes the spill slot which is directly addressable
  # off RSP with offset 0.  Given the value was spilled with a pushq, 
  # that makes sense.
  # Stack Maps:   Loc 8: Direct RSP     [encoding: .byte 2, .byte 8, .short 7, .int 0]
	  .byte	2
	  .byte	8
	  .short	7
	  .long	0

This example was taken from the tests for the :ref:`RewriteStatepointsForGC`
utility pass.  As such, its full StackMap can be easily examined with the
following command.

.. code-block:: bash

  opt -rewrite-statepoints-for-gc test/Transforms/RewriteStatepointsForGC/basics.ll -S | llc -debug-only=stackmaps

Base & Derived Pointers
^^^^^^^^^^^^^^^^^^^^^^^

A "base pointer" is one which points to the starting address of an allocation
(object).  A "derived pointer" is one which is offset from a base pointer by
some amount.  When relocating objects, a garbage collector needs to be able 
to relocate each derived pointer associated with an allocation to the same 
offset from the new address.

"Interior derived pointers" remain within the bounds of the allocation 
they're associated with.  As a result, the base object can be found at 
runtime provided the bounds of allocations are known to the runtime system.

"Exterior derived pointers" are outside the bounds of the associated object;
they may even fall within *another* allocations address range.  As a result,
there is no way for a garbage collector to determine which allocation they 
are associated with at runtime and compiler support is needed.

The ``gc.relocate`` intrinsic supports an explicit operand for describing the
allocation associated with a derived pointer.  This operand is frequently 
referred to as the base operand, but does not strictly speaking have to be
a base pointer, but it does need to lie within the bounds of the associated
allocation.  Some collectors may require that the operand be an actual base
pointer rather than merely an internal derived pointer. Note that during 
lowering both the base and derived pointer operands are required to be live 
over the associated call safepoint even if the base is otherwise unused 
afterwards.

If we extend our previous example to include a pointless derived pointer, 
we get:

.. code-block:: llvm

  define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj) 
         gc "statepoint-example" {
    %gep = getelementptr i8, i8 addrspace(1)* %obj, i64 20000
    %token = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj, i8 addrspace(1)* %gep)
    %obj.relocated = call i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %token, i32 7, i32 7)
    %gep.relocated = call i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %token, i32 7, i32 8)
    %p = getelementptr i8, i8 addrspace(1)* %gep, i64 -20000
    ret i8 addrspace(1)* %p
  }

Note that in this example %p and %obj.relocate are the same address and we
could replace one with the other, potentially removing the derived pointer
from the live set at the safepoint entirely.

.. _gc_transition_args:

GC Transitions
^^^^^^^^^^^^^^^^^^

As a practical consideration, many garbage-collected systems allow code that is
collector-aware ("managed code") to call code that is not collector-aware
("unmanaged code"). It is common that such calls must also be safepoints, since
it is desirable to allow the collector to run during the execution of
unmanaged code. Furthermore, it is common that coordinating the transition from
managed to unmanaged code requires extra code generation at the call site to
inform the collector of the transition. In order to support these needs, a
statepoint may be marked as a GC transition, and data that is necessary to
perform the transition (if any) may be provided as additional arguments to the
statepoint.

  Note that although in many cases statepoints may be inferred to be GC
  transitions based on the function symbols involved (e.g. a call from a
  function with GC strategy "foo" to a function with GC strategy "bar"),
  indirect calls that are also GC transitions must also be supported. This
  requirement is the driving force behind the decision to require that GC
  transitions are explicitly marked.

Let's revisit the sample given above, this time treating the call to ``@foo``
as a GC transition. Depending on our target, the transition code may need to
access some extra state in order to inform the collector of the transition.
Let's assume a hypothetical GC--somewhat unimaginatively named "hypothetical-gc"
--that requires that a TLS variable must be written to before and after a call
to unmanaged code. The resulting relocation sequence is:

.. code-block:: llvm

  @flag = thread_local global i32 0, align 4

  define i8 addrspace(1)* @test1(i8 addrspace(1) *%obj)
         gc "hypothetical-gc" {

    %0 = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @foo, i32 0, i32 1, i32* @Flag, i32 0, i8 addrspace(1)* %obj)
    %obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 7, i32 7)
    ret i8 addrspace(1)* %obj.relocated
  }

During lowering, this will result in a instruction selection DAG that looks
something like:

::

  CALLSEQ_START
  ...
  GC_TRANSITION_START (lowered i32 *@Flag), SRCVALUE i32* Flag
  STATEPOINT
  GC_TRANSITION_END (lowered i32 *@Flag), SRCVALUE i32 *Flag
  ...
  CALLSEQ_END

In order to generate the necessary transition code, the backend for each target
supported by "hypothetical-gc" must be modified to lower ``GC_TRANSITION_START``
and ``GC_TRANSITION_END`` nodes appropriately when the "hypothetical-gc"
strategy is in use for a particular function. Assuming that such lowering has
been added for X86, the generated assembly would be:

.. code-block:: gas

	  .globl	test1
	  .align	16, 0x90
	  pushq	%rax
	  movl $1, %fs:Flag@TPOFF
	  callq	foo
	  movl $0, %fs:Flag@TPOFF
  .Ltmp1:
	  movq	(%rsp), %rax  # This load is redundant (oops!)
	  popq	%rdx
	  retq

Note that the design as presented above is not fully implemented: in particular,
strategy-specific lowering is not present, and all GC transitions are emitted as
as single no-op before and after the call instruction. These no-ops are often
removed by the backend during dead machine instruction elimination.


Intrinsics
===========

'llvm.experimental.gc.statepoint' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Syntax:
"""""""

::

      declare token
        @llvm.experimental.gc.statepoint(i64 <id>, i32 <num patch bytes>,
                       func_type <target>, 
                       i64 <#call args>, i64 <flags>,
                       ... (call parameters),
                       i64 <# transition args>, ... (transition parameters),
                       i64 <# deopt args>, ... (deopt parameters),
                       ... (gc parameters))

Overview:
"""""""""

The statepoint intrinsic represents a call which is parse-able by the
runtime.

Operands:
"""""""""

The 'id' operand is a constant integer that is reported as the ID
field in the generated stackmap.  LLVM does not interpret this
parameter in any way and its meaning is up to the statepoint user to
decide.  Note that LLVM is free to duplicate code containing
statepoint calls, and this may transform IR that had a unique 'id' per
lexical call to statepoint to IR that does not.

If 'num patch bytes' is non-zero then the call instruction
corresponding to the statepoint is not emitted and LLVM emits 'num
patch bytes' bytes of nops in its place.  LLVM will emit code to
prepare the function arguments and retrieve the function return value
in accordance to the calling convention; the former before the nop
sequence and the latter after the nop sequence.  It is expected that
the user will patch over the 'num patch bytes' bytes of nops with a
calling sequence specific to their runtime before executing the
generated machine code.  There are no guarantees with respect to the
alignment of the nop sequence.  Unlike :doc:`StackMaps` statepoints do
not have a concept of shadow bytes.  Note that semantically the
statepoint still represents a call or invoke to 'target', and the nop
sequence after patching is expected to represent an operation
equivalent to a call or invoke to 'target'.

The 'target' operand is the function actually being called.  The
target can be specified as either a symbolic LLVM function, or as an
arbitrary Value of appropriate function type.  Note that the function
type must match the signature of the callee and the types of the 'call
parameters' arguments.

The '#call args' operand is the number of arguments to the actual
call.  It must exactly match the number of arguments passed in the
'call parameters' variable length section.

The 'flags' operand is used to specify extra information about the
statepoint. This is currently only used to mark certain statepoints
as GC transitions. This operand is a 64-bit integer with the following
layout, where bit 0 is the least significant bit:

  +-------+---------------------------------------------------+
  | Bit # | Usage                                             |
  +=======+===================================================+
  |     0 | Set if the statepoint is a GC transition, cleared |
  |       | otherwise.                                        |
  +-------+---------------------------------------------------+
  |  1-63 | Reserved for future use; must be cleared.         |
  +-------+---------------------------------------------------+

The 'call parameters' arguments are simply the arguments which need to
be passed to the call target.  They will be lowered according to the
specified calling convention and otherwise handled like a normal call
instruction.  The number of arguments must exactly match what is
specified in '# call args'.  The types must match the signature of
'target'.

The 'transition parameters' arguments contain an arbitrary list of
Values which need to be passed to GC transition code. They will be
lowered and passed as operands to the appropriate GC_TRANSITION nodes
in the selection DAG. It is assumed that these arguments must be
available before and after (but not necessarily during) the execution
of the callee. The '# transition args' field indicates how many operands
are to be interpreted as 'transition parameters'.

The 'deopt parameters' arguments contain an arbitrary list of Values
which is meaningful to the runtime.  The runtime may read any of these
values, but is assumed not to modify them.  If the garbage collector
might need to modify one of these values, it must also be listed in
the 'gc pointer' argument list.  The '# deopt args' field indicates
how many operands are to be interpreted as 'deopt parameters'.

The 'gc parameters' arguments contain every pointer to a garbage
collector object which potentially needs to be updated by the garbage
collector.  Note that the argument list must explicitly contain a base
pointer for every derived pointer listed.  The order of arguments is
unimportant.  Unlike the other variable length parameter sets, this
list is not length prefixed.

Semantics:
""""""""""

A statepoint is assumed to read and write all memory.  As a result,
memory operations can not be reordered past a statepoint.  It is
illegal to mark a statepoint as being either 'readonly' or 'readnone'.

Note that legal IR can not perform any memory operation on a 'gc
pointer' argument of the statepoint in a location statically reachable
from the statepoint.  Instead, the explicitly relocated value (from a
``gc.relocate``) must be used.

'llvm.experimental.gc.result' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Syntax:
"""""""

::

      declare type*
        @llvm.experimental.gc.result(token %statepoint_token)

Overview:
"""""""""

``gc.result`` extracts the result of the original call instruction
which was replaced by the ``gc.statepoint``.  The ``gc.result``
intrinsic is actually a family of three intrinsics due to an
implementation limitation.  Other than the type of the return value,
the semantics are the same.

Operands:
"""""""""

The first and only argument is the ``gc.statepoint`` which starts
the safepoint sequence of which this ``gc.result`` is a part.
Despite the typing of this as a generic token, *only* the value defined 
by a ``gc.statepoint`` is legal here.

Semantics:
""""""""""

The ``gc.result`` represents the return value of the call target of
the ``statepoint``.  The type of the ``gc.result`` must exactly match
the type of the target.  If the call target returns void, there will
be no ``gc.result``.

A ``gc.result`` is modeled as a 'readnone' pure function.  It has no
side effects since it is just a projection of the return value of the
previous call represented by the ``gc.statepoint``.

'llvm.experimental.gc.relocate' Intrinsic
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Syntax:
"""""""

::

      declare <pointer type>
        @llvm.experimental.gc.relocate(token %statepoint_token, 
                                       i32 %base_offset, 
                                       i32 %pointer_offset)

Overview:
"""""""""

A ``gc.relocate`` returns the potentially relocated value of a pointer
at the safepoint.

Operands:
"""""""""

The first argument is the ``gc.statepoint`` which starts the
safepoint sequence of which this ``gc.relocation`` is a part.
Despite the typing of this as a generic token, *only* the value defined 
by a ``gc.statepoint`` is legal here.

The second argument is an index into the statepoints list of arguments
which specifies the allocation for the pointer being relocated.
This index must land within the 'gc parameter' section of the
statepoint's argument list.  The associated value must be within the
object with which the pointer being relocated is associated. The optimizer
is free to change *which* interior derived pointer is reported, provided that
it does not replace an actual base pointer with another interior derived 
pointer.  Collectors are allowed to rely on the base pointer operand 
remaining an actual base pointer if so constructed.

The third argument is an index into the statepoint's list of arguments
which specify the (potentially) derived pointer being relocated.  It
is legal for this index to be the same as the second argument
if-and-only-if a base pointer is being relocated. This index must land
within the 'gc parameter' section of the statepoint's argument list.

Semantics:
""""""""""

The return value of ``gc.relocate`` is the potentially relocated value
of the pointer specified by its arguments.  It is unspecified how the
value of the returned pointer relates to the argument to the
``gc.statepoint`` other than that a) it points to the same source
language object with the same offset, and b) the 'based-on'
relationship of the newly relocated pointers is a projection of the
unrelocated pointers.  In particular, the integer value of the pointer
returned is unspecified.

A ``gc.relocate`` is modeled as a ``readnone`` pure function.  It has no
side effects since it is just a way to extract information about work
done during the actual call modeled by the ``gc.statepoint``.

.. _statepoint-stackmap-format:

Stack Map Format
================

Locations for each pointer value which may need read and/or updated by
the runtime or collector are provided via the :ref:`Stack Map format
<stackmap-format>` specified in the PatchPoint documentation.

Each statepoint generates the following Locations:

* Constant which describes the calling convention of the call target. This
  constant is a valid :ref:`calling convention identifier <callingconv>` for
  the version of LLVM used to generate the stackmap. No additional compatibility
  guarantees are made for this constant over what LLVM provides elsewhere w.r.t.
  these identifiers.
* Constant which describes the flags passed to the statepoint intrinsic
* Constant which describes number of following deopt *Locations* (not
  operands)
* Variable number of Locations, one for each deopt parameter listed in
  the IR statepoint (same number as described by previous Constant).  At 
  the moment, only deopt parameters with a bitwidth of 64 bits or less 
  are supported.  Values of a type larger than 64 bits can be specified 
  and reported only if a) the value is constant at the call site, and b) 
  the constant can be represented with less than 64 bits (assuming zero 
  extension to the original bitwidth).
* Variable number of relocation records, each of which consists of 
  exactly two Locations.  Relocation records are described in detail
  below.

Each relocation record provides sufficient information for a collector to 
relocate one or more derived pointers.  Each record consists of a pair of 
Locations.  The second element in the record represents the pointer (or 
pointers) which need updated.  The first element in the record provides a 
pointer to the base of the object with which the pointer(s) being relocated is
associated.  This information is required for handling generalized derived 
pointers since a pointer may be outside the bounds of the original allocation,
but still needs to be relocated with the allocation.  Additionally:

* It is guaranteed that the base pointer must also appear explicitly as a 
  relocation pair if used after the statepoint. 
* There may be fewer relocation records then gc parameters in the IR
  statepoint. Each *unique* pair will occur at least once; duplicates
  are possible.  
* The Locations within each record may either be of pointer size or a 
  multiple of pointer size.  In the later case, the record must be 
  interpreted as describing a sequence of pointers and their corresponding 
  base pointers. If the Location is of size N x sizeof(pointer), then
  there will be N records of one pointer each contained within the Location.
  Both Locations in a pair can be assumed to be of the same size.

Note that the Locations used in each section may describe the same
physical location.  e.g. A stack slot may appear as a deopt location,
a gc base pointer, and a gc derived pointer.

The LiveOut section of the StkMapRecord will be empty for a statepoint
record.

Safepoint Semantics & Verification
==================================

The fundamental correctness property for the compiled code's
correctness w.r.t. the garbage collector is a dynamic one.  It must be
the case that there is no dynamic trace such that a operation
involving a potentially relocated pointer is observably-after a
safepoint which could relocate it.  'observably-after' is this usage
means that an outside observer could observe this sequence of events
in a way which precludes the operation being performed before the
safepoint.

To understand why this 'observable-after' property is required,
consider a null comparison performed on the original copy of a
relocated pointer.  Assuming that control flow follows the safepoint,
there is no way to observe externally whether the null comparison is
performed before or after the safepoint.  (Remember, the original
Value is unmodified by the safepoint.)  The compiler is free to make
either scheduling choice.

The actual correctness property implemented is slightly stronger than
this.  We require that there be no *static path* on which a
potentially relocated pointer is 'observably-after' it may have been
relocated.  This is slightly stronger than is strictly necessary (and
thus may disallow some otherwise valid programs), but greatly
simplifies reasoning about correctness of the compiled code.

By construction, this property will be upheld by the optimizer if
correctly established in the source IR.  This is a key invariant of
the design.

The existing IR Verifier pass has been extended to check most of the
local restrictions on the intrinsics mentioned in their respective
documentation.  The current implementation in LLVM does not check the
key relocation invariant, but this is ongoing work on developing such
a verifier.  Please ask on llvm-dev if you're interested in
experimenting with the current version.

.. _statepoint-utilities:

Utility Passes for Safepoint Insertion
======================================

.. _RewriteStatepointsForGC:

RewriteStatepointsForGC
^^^^^^^^^^^^^^^^^^^^^^^^

The pass RewriteStatepointsForGC transforms a function's IR to lower from the
abstract machine model described above to the explicit statepoint model of 
relocations.  To do this, it replaces all calls or invokes of functions which
might contain a safepoint poll with a ``gc.statepoint`` and associated full
relocation sequence, including all required ``gc.relocates``.  

Note that by default, this pass only runs for the "statepoint-example" or 
"core-clr" gc strategies.  You will need to add your custom strategy to this 
whitelist or use one of the predefined ones. 

As an example, given this code:

.. code-block:: llvm

  define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj) 
         gc "statepoint-example" {
    call void @foo()
    ret i8 addrspace(1)* %obj
  }

The pass would produce this IR:

.. code-block:: llvm

  define i8 addrspace(1)* @test1(i8 addrspace(1)* %obj) 
         gc "statepoint-example" {
    %0 = call token (i64, i32, void ()*, i32, i32, ...)* @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @foo, i32 0, i32 0, i32 0, i32 5, i32 0, i32 -1, i32 0, i32 0, i32 0, i8 addrspace(1)* %obj)
    %obj.relocated = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %0, i32 12, i32 12)
    ret i8 addrspace(1)* %obj.relocated
  }

In the above examples, the addrspace(1) marker on the pointers is the mechanism
that the ``statepoint-example`` GC strategy uses to distinguish references from
non references.  The pass assumes that all addrspace(1) pointers are non-integral
pointer types.  Address space 1 is not globally reserved for this purpose.

This pass can be used an utility function by a language frontend that doesn't 
want to manually reason about liveness, base pointers, or relocation when 
constructing IR.  As currently implemented, RewriteStatepointsForGC must be 
run after SSA construction (i.e. mem2ref).

RewriteStatepointsForGC will ensure that appropriate base pointers are listed
for every relocation created.  It will do so by duplicating code as needed to
propagate the base pointer associated with each pointer being relocated to
the appropriate safepoints.  The implementation assumes that the following 
IR constructs produce base pointers: loads from the heap, addresses of global 
variables, function arguments, function return values. Constant pointers (such
as null) are also assumed to be base pointers.  In practice, this constraint
can be relaxed to producing interior derived pointers provided the target 
collector can find the associated allocation from an arbitrary interior 
derived pointer.

By default RewriteStatepointsForGC passes in ``0xABCDEF00`` as the statepoint
ID and ``0`` as the number of patchable bytes to the newly constructed
``gc.statepoint``.  These values can be configured on a per-callsite
basis using the attributes ``"statepoint-id"`` and
``"statepoint-num-patch-bytes"``.  If a call site is marked with a
``"statepoint-id"`` function attribute and its value is a positive
integer (represented as a string), then that value is used as the ID
of the newly constructed ``gc.statepoint``.  If a call site is marked
with a ``"statepoint-num-patch-bytes"`` function attribute and its
value is a positive integer, then that value is used as the 'num patch
bytes' parameter of the newly constructed ``gc.statepoint``.  The
``"statepoint-id"`` and ``"statepoint-num-patch-bytes"`` attributes
are not propagated to the ``gc.statepoint`` call or invoke if they
could be successfully parsed.

In practice, RewriteStatepointsForGC should be run much later in the pass 
pipeline, after most optimization is already done.  This helps to improve 
the quality of the generated code when compiled with garbage collection support.

.. _PlaceSafepoints:

PlaceSafepoints
^^^^^^^^^^^^^^^^

The pass PlaceSafepoints inserts safepoint polls sufficient to ensure running 
code checks for a safepoint request on a timely manner. This pass is expected 
to be run before RewriteStatepointsForGC and thus does not produce full 
relocation sequences.  

As an example, given input IR of the following:

.. code-block:: llvm

  define void @test() gc "statepoint-example" {
    call void @foo()
    ret void
  }

  declare void @do_safepoint()
  define void @gc.safepoint_poll() {
    call void @do_safepoint()
    ret void
  }


This pass would produce the following IR:

.. code-block:: llvm

  define void @test() gc "statepoint-example" {
    call void @do_safepoint()
    call void @foo()
    ret void
  }

In this case, we've added an (unconditional) entry safepoint poll.  Note that 
despite appearances, the entry poll is not necessarily redundant.  We'd have to 
know that ``foo`` and ``test`` were not mutually recursive for the poll to be 
redundant.  In practice, you'd probably want to your poll definition to contain 
a conditional branch of some form.

At the moment, PlaceSafepoints can insert safepoint polls at method entry and 
loop backedges locations.  Extending this to work with return polls would be 
straight forward if desired.

PlaceSafepoints includes a number of optimizations to avoid placing safepoint 
polls at particular sites unless needed to ensure timely execution of a poll 
under normal conditions.  PlaceSafepoints does not attempt to ensure timely 
execution of a poll under worst case conditions such as heavy system paging.

The implementation of a safepoint poll action is specified by looking up a 
function of the name ``gc.safepoint_poll`` in the containing Module.  The body
of this function is inserted at each poll site desired.  While calls or invokes
inside this method are transformed to a ``gc.statepoints``, recursive poll 
insertion is not performed.

This pass is useful for any language frontend which only has to support
garbage collection semantics at safepoints.  If you need other abstract
frame information at safepoints (e.g. for deoptimization or introspection),
you can insert safepoint polls in the frontend.  If you have the later case,
please ask on llvm-dev for suggestions.  There's been a good amount of work
done on making such a scheme work well in practice which is not yet documented
here.  


Supported Architectures
=======================

Support for statepoint generation requires some code for each backend.
Today, only X86_64 is supported.  

Problem Areas and Active Work
=============================

#. Support for languages which allow unmanaged pointers to garbage collected
   objects (i.e. pass a pointer to an object to a C routine) via pinning.

#. Support for garbage collected objects allocated on the stack.  Specifically,
   allocas are always assumed to be in address space 0 and we need a
   cast/promotion operator to let rewriting identify them.

#. The current statepoint lowering is known to be somewhat poor.  In the very
   long term, we'd like to integrate statepoints with the register allocator;
   in the near term this is unlikely to happen.  We've found the quality of
   lowering to be relatively unimportant as hot-statepoints are almost always
   inliner bugs.

#. Concerns have been raised that the statepoint representation results in a
   large amount of IR being produced for some examples and that this
   contributes to higher than expected memory usage and compile times.  There's
   no immediate plans to make changes due to this, but alternate models may be
   explored in the future.

#. Relocations along exceptional paths are currently broken in ToT.  In
   particular, there is current no way to represent a rethrow on a path which
   also has relocations.  See `this llvm-dev discussion
   <https://groups.google.com/forum/#!topic/llvm-dev/AE417XjgxvI>`_ for more
   detail.

Bugs and Enhancements
=====================

Currently known bugs and enhancements under consideration can be
tracked by performing a `bugzilla search
<https://bugs.llvm.org/buglist.cgi?cmdtype=runnamed&namedcmd=Statepoint%20Bugs&list_id=64342>`_
for [Statepoint] in the summary field. When filing new bugs, please
use this tag so that interested parties see the newly filed bug.  As
with most LLVM features, design discussions take place on `llvm-dev
<http://lists.llvm.org/mailman/listinfo/llvm-dev>`_, and patches
should be sent to `llvm-commits
<http://lists.llvm.org/mailman/listinfo/llvm-commits>`_ for review.