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| //===-- PPCTOCRegDeps.cpp - Add Extra TOC Register Dependencies -----------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// When resolving an address using the ELF ABI TOC pointer, two relocations are
// generally required: one for the high part and one for the low part. Only
// the high part generally explicitly depends on r2 (the TOC pointer). And, so,
// we might produce code like this:
//
// .Ltmp526:
// addis 3, 2, .LC12@toc@ha
// .Ltmp1628:
// std 2, 40(1)
// ld 5, 0(27)
// ld 2, 8(27)
// ld 11, 16(27)
// ld 3, .LC12@toc@l(3)
// rldicl 4, 4, 0, 32
// mtctr 5
// bctrl
// ld 2, 40(1)
//
// And there is nothing wrong with this code, as such, but there is a linker bug
// in binutils (https://sourceware.org/bugzilla/show_bug.cgi?id=18414) that will
// misoptimize this code sequence to this:
// nop
// std r2,40(r1)
// ld r5,0(r27)
// ld r2,8(r27)
// ld r11,16(r27)
// ld r3,-32472(r2)
// clrldi r4,r4,32
// mtctr r5
// bctrl
// ld r2,40(r1)
// because the linker does not know (and does not check) that the value in r2
// changed in between the instruction using the .LC12@toc@ha (TOC-relative)
// relocation and the instruction using the .LC12@toc@l(3) relocation.
// Because it finds these instructions using the relocations (and not by
// scanning the instructions), it has been asserted that there is no good way
// to detect the change of r2 in between. As a result, this bug may never be
// fixed (i.e. it may become part of the definition of the ABI). GCC was
// updated to add extra dependencies on r2 to instructions using the @toc@l
// relocations to avoid this problem, and we'll do the same here.
//
// This is done as a separate pass because:
// 1. These extra r2 dependencies are not really properties of the
// instructions, but rather due to a linker bug, and maybe one day we'll be
// able to get rid of them when targeting linkers without this bug (and,
// thus, keeping the logic centralized here will make that
// straightforward).
// 2. There are ISel-level peephole optimizations that propagate the @toc@l
// relocations to some user instructions, and so the exta dependencies do
// not apply only to a fixed set of instructions (without undesirable
// definition replication).
//
//===----------------------------------------------------------------------===//
#include "MCTargetDesc/PPCPredicates.h"
#include "PPC.h"
#include "PPCInstrBuilder.h"
#include "PPCInstrInfo.h"
#include "PPCMachineFunctionInfo.h"
#include "PPCTargetMachine.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/CodeGen/MachineFrameInfo.h"
#include "llvm/CodeGen/MachineFunctionPass.h"
#include "llvm/CodeGen/MachineInstr.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/MC/MCAsmInfo.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/TargetRegistry.h"
#include "llvm/Support/raw_ostream.h"
using namespace llvm;
#define DEBUG_TYPE "ppc-toc-reg-deps"
namespace {
// PPCTOCRegDeps pass - For simple functions without epilogue code, move
// returns up, and create conditional returns, to avoid unnecessary
// branch-to-blr sequences.
struct PPCTOCRegDeps : public MachineFunctionPass {
static char ID;
PPCTOCRegDeps() : MachineFunctionPass(ID) {
initializePPCTOCRegDepsPass(*PassRegistry::getPassRegistry());
}
protected:
bool hasTOCLoReloc(const MachineInstr &MI) {
if (MI.getOpcode() == PPC::LDtocL ||
MI.getOpcode() == PPC::ADDItocL ||
MI.getOpcode() == PPC::LWZtocL)
return true;
for (const MachineOperand &MO : MI.operands()) {
if ((MO.getTargetFlags() & PPCII::MO_ACCESS_MASK) == PPCII::MO_TOC_LO)
return true;
}
return false;
}
bool processBlock(MachineBasicBlock &MBB) {
bool Changed = false;
const bool isPPC64 =
MBB.getParent()->getSubtarget<PPCSubtarget>().isPPC64();
const unsigned TOCReg = isPPC64 ? PPC::X2 : PPC::R2;
for (auto &MI : MBB) {
if (!hasTOCLoReloc(MI))
continue;
MI.addOperand(MachineOperand::CreateReg(TOCReg,
false /*IsDef*/,
true /*IsImp*/));
Changed = true;
}
return Changed;
}
public:
bool runOnMachineFunction(MachineFunction &MF) override {
bool Changed = false;
for (MachineFunction::iterator I = MF.begin(); I != MF.end();) {
MachineBasicBlock &B = *I++;
if (processBlock(B))
Changed = true;
}
return Changed;
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
MachineFunctionPass::getAnalysisUsage(AU);
}
};
}
INITIALIZE_PASS(PPCTOCRegDeps, DEBUG_TYPE,
"PowerPC TOC Register Dependencies", false, false)
char PPCTOCRegDeps::ID = 0;
FunctionPass*
llvm::createPPCTOCRegDepsPass() { return new PPCTOCRegDeps(); }
|