X86: replace SUB with CMP if possible
This patch will optimize the following
movq %rdi, %rax
subq %rsi, %rax
cmovsq %rsi, %rdi
movq %rdi, %rax
to
cmpq %rsi, %rdi
cmovsq %rsi, %rdi
movq %rdi, %rax
Perform this optimization if the actual result of SUB is not used.
rdar: 11540023
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158126 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Target/X86/X86ISelLowering.cpp b/lib/Target/X86/X86ISelLowering.cpp
index 4baa1a6..57f61ab 100644
--- a/lib/Target/X86/X86ISelLowering.cpp
+++ b/lib/Target/X86/X86ISelLowering.cpp
@@ -8271,7 +8271,13 @@
// Otherwise use a regular EFLAGS-setting instruction.
switch (Op.getNode()->getOpcode()) {
default: llvm_unreachable("unexpected operator!");
- case ISD::SUB: Opcode = X86ISD::SUB; break;
+ case ISD::SUB:
+ // If the only use of SUB is EFLAGS, use CMP instead.
+ if (Op.hasOneUse())
+ Opcode = X86ISD::CMP;
+ else
+ Opcode = X86ISD::SUB;
+ break;
case ISD::OR: Opcode = X86ISD::OR; break;
case ISD::XOR: Opcode = X86ISD::XOR; break;
case ISD::AND: Opcode = X86ISD::AND; break;
@@ -8297,6 +8303,13 @@
return DAG.getNode(X86ISD::CMP, dl, MVT::i32, Op,
DAG.getConstant(0, Op.getValueType()));
+ if (Opcode == X86ISD::CMP) {
+ SDValue New = DAG.getNode(Opcode, dl, MVT::i32, Op.getOperand(0),
+ Op.getOperand(1));
+ DAG.ReplaceAllUsesWith(Op, New);
+ return SDValue(New.getNode(), 0);
+ }
+
SDVTList VTs = DAG.getVTList(Op.getValueType(), MVT::i32);
SmallVector<SDValue, 4> Ops;
for (unsigned i = 0; i != NumOperands; ++i)