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X86: Fix optimizeCompare to correctly check safe condition.

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It is safe if EFLAGS is killed or re-defined.
When we are done with the basic block, check whether EFLAGS is live-out.
Do not optimize away cmp if EFLAGS is live-out.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159888 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Manman Ren 2012-07-07 03:34:46 +00:00
parent 845b4c2f0b
commit 2d4215f759
2 changed files with 43 additions and 5 deletions
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30
lib/Target/X86/X86InstrInfo.cpp
View File

@ -3107,22 +3107,28 @@ optimizeCompareInstr(MachineInstr *CmpInstr, unsigned SrcReg, unsigned SrcReg2,
if (!Sub)
return false;
bool IsSwapped = (SrcReg2 != 0 && Sub->getOperand(1).getReg() == SrcReg2 &&
Sub->getOperand(2).getReg() == SrcReg);
// Scan forward from the instruction after CmpInstr for uses of EFLAGS.
// It is safe to remove CmpInstr if EFLAGS is redefined or killed.
// If we are done with the basic block, we need to check whether EFLAGS is
// live-out.
bool IsSafe = false;
SmallVector<std::pair<MachineInstr*, unsigned /*NewOpc*/>, 4> OpsToUpdate;
MachineBasicBlock::iterator E = CmpInstr->getParent()->end();
for (++I; I != E; ++I) {
const MachineInstr &Instr = *I;
if (Instr.modifiesRegister(X86::EFLAGS, TRI))
if (Instr.modifiesRegister(X86::EFLAGS, TRI)) {
// It is safe to remove CmpInstr if EFLAGS is updated again.
IsSafe = true;
break;
}
if (!Instr.readsRegister(X86::EFLAGS, TRI))
continue;
// EFLAGS is used by this instruction.
if (SrcReg2 != 0 && Sub->getOperand(1).getReg() == SrcReg2 &&
Sub->getOperand(2).getReg() == SrcReg) {
if (IsSwapped) {
// If we have SUB(r1, r2) and CMP(r2, r1), the condition code needs
// to be changed from r2 > r1 to r1 < r2, from r2 < r1 to r1 > r2, etc.
unsigned NewOpc = getSwappedConditionForSET(Instr.getOpcode());
@ -3135,6 +3141,20 @@ optimizeCompareInstr(MachineInstr *CmpInstr, unsigned SrcReg, unsigned SrcReg2,
// instructions will be modified.
OpsToUpdate.push_back(std::make_pair(&*I, NewOpc));
}
if (Instr.killsRegister(X86::EFLAGS, TRI)) {
IsSafe = true;
break;
}
}
// If EFLAGS is not killed nor re-defined, we should check whether it is
// live-out. If it is live-out, do not optimize.
if (IsSwapped && !IsSafe) {
MachineBasicBlock *MBB = CmpInstr->getParent();
for (MachineBasicBlock::succ_iterator SI = MBB->succ_begin(),
SE = MBB->succ_end(); SI != SE; ++SI)
if ((*SI)->isLiveIn(X86::EFLAGS))
return false;
}
// Make sure Sub instruction defines EFLAGS.

18
test/CodeGen/X86/jump_sign.ll
View File

@ -102,6 +102,24 @@ entry:
%cond = select i1 %cmp, i32 %b, i32 %sub
ret i32 %cond
}
; If EFLAGS is live-out, we can't remove cmp if there exists
; a swapped sub.
define i32 @l2(i32 %a, i32 %b) nounwind {
entry:
; CHECK: l2:
; CHECK: cmp
%cmp = icmp eq i32 %b, %a
%sub = sub nsw i32 %a, %b
br i1 %cmp, label %if.then, label %if.else
if.then:
%cmp2 = icmp sgt i32 %b, %a
%sel = select i1 %cmp2, i32 %sub, i32 %a
ret i32 %sel
if.else:
ret i32 %sub
}
; rdar://11540023
define i32 @n(i32 %x, i32 %y) nounwind {
entry: