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A problem that's exposed when machine LICM is enabled. Consider this code:

Browse Source 
LBB1_3:   # bb
...
        xorl    %ebp, %ebp
        subl    (%ebx), %ebp
...
        incl    %ecx
        cmpl    %edi, %ecx
        jl      LBB1_3  # bb

Whe using machine LICM, LLVM converts it into:

        xorl %esi, %esi
LBB1_3: # bb
...
        movl    %esi, %ebp
        subl    (%ebx), %ebp
...
        incl    %ecx
        cmpl    %edi, %ecx
        jl      LBB1_3  # bb

Two address conversion inserts the copy instruction. However, it's cheaper to
rematerialize it, and remat helps reduce register pressure.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@51562 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2008-05-26 05:18:34 +00:00
parent 505242f9b6
commit 48f7f237ea
1 changed files with 40 additions and 1 deletions
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41
lib/CodeGen/TwoAddressInstructionPass.cpp
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@ -39,6 +39,7 @@
#include "llvm/Target/TargetMachine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/STLExtras.h"
using namespace llvm;
@ -200,6 +201,8 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
DOUT << "********** REWRITING TWO-ADDR INSTRS **********\n";
DOUT << "********** Function: " << MF.getFunction()->getName() << '\n';
SmallPtrSet<MachineInstr*, 8> ReMattedInstrs;
for (MachineFunction::iterator mbbi = MF.begin(), mbbe = MF.end();
mbbi != mbbe; ++mbbi) {
for (MachineBasicBlock::iterator mi = mbbi->begin(), me = mbbi->end();
@ -321,7 +324,14 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
InstructionRearranged:
const TargetRegisterClass* rc = MF.getRegInfo().getRegClass(regA);
TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc);
MachineInstr *Orig = MRI->getVRegDef(regB);
if (Orig && TII->isTriviallyReMaterializable(Orig)) {
TII->reMaterialize(*mbbi, mi, regA, Orig);
ReMattedInstrs.insert(Orig);
} else {
TII->copyRegToReg(*mbbi, mi, regA, regB, rc, rc);
}
MachineBasicBlock::iterator prevMi = prior(mi);
DOUT << "\t\tprepend:\t"; DEBUG(prevMi->print(*cerr.stream(), &TM));
@ -357,5 +367,34 @@ bool TwoAddressInstructionPass::runOnMachineFunction(MachineFunction &MF) {
}
}
SmallPtrSet<MachineInstr*, 8>::iterator I = ReMattedInstrs.begin();
SmallPtrSet<MachineInstr*, 8>::iterator E = ReMattedInstrs.end();
for (; I != E; ++I) {
MachineInstr *MI = *I;
bool InstrDead = true;
for (unsigned i = 0, e = MI->getNumOperands(); i != e; ++i) {
const MachineOperand &MO = MI->getOperand(i);
if (!MO.isRegister())
continue;
unsigned MOReg = MO.getReg();
if (!MOReg)
continue;
if (MO.isDef()) {
if (MO.isImplicit())
continue;
if (MRI->use_begin(MOReg) != MRI->use_end()) {
InstrDead = false;
break;
}
}
}
if (InstrDead && MI->getNumOperands() > 0)
MI->eraseFromParent();
}
return MadeChange;
}