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[X86] Avoid emitting unnecessary test instructions. 

This patch teaches the backend how to check for the 'NoSignedWrap' flag on
binary operations to improve the emission of 'test' instructions.

If the result of a binary operation is known not to overflow we know that
resetting the Overflow flag is unnecessary and so we can avoid emitting
the test instruction.

Patch by Marcello Maggioni.

llvm-svn: 210468
This commit is contained in:
Andrea Di Biagio 2014-06-09 12:34:50 +00:00
parent 4db1abea15
commit dfbdc71ea1
2 changed files with 43 additions and 2 deletions
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21
llvm/lib/Target/X86/X86ISelLowering.cpp
View File

@ -10074,10 +10074,27 @@ SDValue X86TargetLowering::EmitTest(SDValue Op, unsigned X86CC, SDLoc dl,
break; break;
case X86::COND_G: case X86::COND_GE: case X86::COND_G: case X86::COND_GE:
case X86::COND_L: case X86::COND_LE: case X86::COND_L: case X86::COND_LE:
case X86::COND_O: case X86::COND_NO: case X86::COND_O: case X86::COND_NO: {
NeedOF = true; // Check if we really need to set the
// Overflow flag. If NoSignedWrap is present
// that is not actually needed.
switch (Op->getOpcode()) {
case ISD::ADD:
case ISD::SUB:
case ISD::MUL:
case ISD::SHL: {
const BinaryWithFlagsSDNode *BinNode =
cast<BinaryWithFlagsSDNode>(Op.getNode());
if (BinNode->hasNoSignedWrap())
break;
}
default:
NeedOF = true;
break;
}
break; break;
} }
}
// See if we can use the EFLAGS value from the operand instead of // See if we can use the EFLAGS value from the operand instead of
// doing a separate TEST. TEST always sets OF and CF to 0, so unless // doing a separate TEST. TEST always sets OF and CF to 0, so unless
// we prove that the arithmetic won't overflow, we can't use OF or CF. // we prove that the arithmetic won't overflow, we can't use OF or CF.

24
llvm/test/CodeGen/X86/2014-05-29-factorial.ll Normal file
View File

@ -0,0 +1,24 @@
; RUN: llc < %s -march=x86-64 | FileCheck %s
; CHECK: decq %rdi
; CHECK-NOT: testq %rdi, %rdi
define i64 @fact2(i64 %x) {
entry:
br label %while.body
while.body:
%result.06 = phi i64 [ %mul, %while.body ], [ 1, %entry ]
%x.addr.05 = phi i64 [ %dec, %while.body ], [ %x, %entry ]
%mul = mul nsw i64 %result.06, %x.addr.05
%dec = add nsw i64 %x.addr.05, -1
%cmp = icmp sgt i64 %dec, 0
br i1 %cmp, label %while.body, label %while.end.loopexit
while.end.loopexit:
%mul.lcssa = phi i64 [ %mul, %while.body ]
br label %while.end
while.end:
%result.0.lcssa = phi i64 [ %mul.lcssa, %while.end.loopexit ]
ret i64 %result.0.lcssa
}