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[X86] Avoid emitting unnecessary test instructions.

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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.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210468 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Andrea Di Biagio 2014-06-09 12:34:50 +00:00
parent bc72c8f0d8
commit 592d439efe
2 changed files with 43 additions and 2 deletions
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21
lib/Target/X86/X86ISelLowering.cpp
View File

@ -10074,10 +10074,27 @@ SDValue X86TargetLowering::EmitTest(SDValue Op, unsigned X86CC, SDLoc dl,
break;
case X86::COND_G: case X86::COND_GE:
case X86::COND_L: case X86::COND_LE:
case X86::COND_O: case X86::COND_NO:
NeedOF = true;
case X86::COND_O: case X86::COND_NO: {
// 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;
}
}
// 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
// we prove that the arithmetic won't overflow, we can't use OF or CF.

24
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
}