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Implement fast-isel conversion of a branch instruction that's branching on an

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overflow/carry from the "arithmetic with overflow" intrinsics. It searches the
machine basic block from bottom to top to find the SETO/SETC instruction that is
its conditional. If an instruction modifies EFLAGS before it reaches the
SETO/SETC instruction, then it defaults to the normal instruction emission.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60807 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2008-12-09 23:19:12 +00:00
parent 54e4c36a73
commit 30a64a7649
2 changed files with 65 additions and 0 deletions
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63
lib/Target/X86/X86FastISel.cpp
View File

@ -748,6 +748,69 @@ bool X86FastISel::X86SelectBranch(Instruction *I) {
MBB->addSuccessor(TrueMBB);
return true;
}
} else if (ExtractValueInst *EI =
dyn_cast<ExtractValueInst>(BI->getCondition())) {
// Check to see if the branch instruction is from an "arithmetic with
// overflow" intrinsic. The main way these intrinsics are used is:
//
// %t = call { i32, i1 } @llvm.sadd.with.overflow.i32(i32 %v1, i32 %v2)
// %sum = extractvalue { i32, i1 } %t, 0
// %obit = extractvalue { i32, i1 } %t, 1
// br i1 %obit, label %overflow, label %normal
//
// The %sum and %obit are converted in an ADD and a SETO/SETC before
// reaching the branch. Therefore, we search backwards through the MBB
// looking for the SETO/SETC instruction. If an instruction modifies the
// EFLAGS register before we reach the SETO/SETC instruction, then we can't
// convert the branch into a JO/JC instruction.
const MachineInstr *SetMI = 0;
unsigned Reg = lookUpRegForValue(EI);
for (MachineBasicBlock::const_reverse_iterator
RI = MBB->rbegin(), RE = MBB->rend(); RI != RE; ++RI) {
const MachineInstr &MI = *RI;
if (MI.modifiesRegister(Reg)) {
unsigned Src, Dst;
if (getInstrInfo()->isMoveInstr(MI, Src, Dst)) {
Reg = Src;
continue;
}
SetMI = &MI;
break;
}
const TargetInstrDesc &TID = MI.getDesc();
const unsigned *ImpDefs = TID.getImplicitDefs();
if (TID.hasUnmodeledSideEffects()) break;
bool ModifiesEFlags = false;
if (ImpDefs) {
for (unsigned u = 0; ImpDefs[u]; ++u)
if (ImpDefs[u] == X86::EFLAGS) {
ModifiesEFlags = true;
break;
}
}
if (ModifiesEFlags) break;
}
if (SetMI) {
unsigned OpCode = SetMI->getOpcode();
if (OpCode == X86::SETOr || OpCode == X86::SETCr) {
BuildMI(MBB, TII.get((OpCode == X86::SETOr) ?
X86::JO : X86::JC)).addMBB(TrueMBB);
FastEmitBranch(FalseMBB);
MBB->addSuccessor(TrueMBB);
return true;
}
}
}
// Otherwise do a clumsy setcc and re-test it.

2
test/CodeGen/X86/add-with-overflow.ll
View File

@ -1,5 +1,7 @@
; RUN: llvm-as < %s | llc -march=x86 | grep {jo} | count 1
; RUN: llvm-as < %s | llc -march=x86 | grep {jc} | count 1
; RUN: llvm-as < %s | llc -march=x86 -fast | grep {jo} | count 1
; RUN: llvm-as < %s | llc -march=x86 -fast | grep {jc} | count 1
@ok = internal constant [4 x i8] c"%d\0A\00"
@no = internal constant [4 x i8] c"no\0A\00"