[X86] EmitCmp - use existing XOR node to check for equality

Normally, we use the result of the SUB flag to scalar comparison as its more compatible with CMP, but if we're testing for equality and already have a XOR we can reuse that instead.

Fixes #6146

Author: RKSimon

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -23157,10 +23157,17 @@ static SDValue EmitCmp(SDValue Op0, SDValue Op1, unsigned X86CC,
     return Add.getValue(1);
   }
 
-  // Use SUB instead of CMP to enable CSE between SUB and CMP.
+  // If we already have an XOR of the ops, use that to check for equality.
+  // Else use SUB instead of CMP to enable CSE between SUB and CMP.
+  unsigned X86Opc = X86ISD::SUB;
+  if ((X86CC == X86::COND_E || X86CC == X86::COND_NE) &&
+      (DAG.doesNodeExist(ISD::XOR, DAG.getVTList({CmpVT}), {Op0, Op1}) ||
+       DAG.doesNodeExist(ISD::XOR, DAG.getVTList({CmpVT}), {Op1, Op0})))
+    X86Opc = X86ISD::XOR; 
+
   SDVTList VTs = DAG.getVTList(CmpVT, MVT::i32);
-  SDValue Sub = DAG.getNode(X86ISD::SUB, dl, VTs, Op0, Op1);
-  return Sub.getValue(1);
+  SDValue CmpOp = DAG.getNode(X86Opc, dl, VTs, Op0, Op1);
+  return CmpOp.getValue(1);
 }
 
 bool X86TargetLowering::isXAndYEqZeroPreferableToXAndYEqY(ISD::CondCode Cond,

llvm/test/CodeGen/X86/cmp-xor.ll

@@ -9,22 +9,18 @@
 define i32 @cmp_xor_i32(i32 %a, i32 %b, i32 %c)
 ; X86-LABEL: cmp_xor_i32:
 ; X86:       # %bb.0:
-; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X86-NEXT:    cmpl %ecx, %eax
-; X86-NEXT:    je .LBB0_1
-; X86-NEXT:  # %bb.2:
-; X86-NEXT:    xorl %ecx, %eax
-; X86-NEXT:    retl
-; X86-NEXT:  .LBB0_1:
+; X86-NEXT:    xorl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    jne .LBB0_2
+; X86-NEXT:  # %bb.1:
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:  .LBB0_2:
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: cmp_xor_i32:
 ; X64:       # %bb.0:
 ; X64-NEXT:    movl %edi, %eax
 ; X64-NEXT:    xorl %esi, %eax
-; X64-NEXT:    cmpl %esi, %edi
 ; X64-NEXT:    cmovel %edx, %eax
 ; X64-NEXT:    retq
 {
@@ -37,22 +33,18 @@ define i32 @cmp_xor_i32(i32 %a, i32 %b, i32 %c)
 define i32 @cmp_xor_i32_commute(i32 %a, i32 %b, i32 %c)
 ; X86-LABEL: cmp_xor_i32_commute:
 ; X86:       # %bb.0:
-; X86-NEXT:    movl {{[0-9]+}}(%esp), %ecx
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
-; X86-NEXT:    cmpl %eax, %ecx
-; X86-NEXT:    je .LBB1_1
-; X86-NEXT:  # %bb.2:
-; X86-NEXT:    xorl %ecx, %eax
-; X86-NEXT:    retl
-; X86-NEXT:  .LBB1_1:
+; X86-NEXT:    xorl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:    jne .LBB1_2
+; X86-NEXT:  # %bb.1:
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %eax
+; X86-NEXT:  .LBB1_2:
 ; X86-NEXT:    retl
 ;
 ; X64-LABEL: cmp_xor_i32_commute:
 ; X64:       # %bb.0:
-; X64-NEXT:    movl %esi, %eax
-; X64-NEXT:    xorl %edi, %eax
-; X64-NEXT:    cmpl %esi, %edi
+; X64-NEXT:    movl %edi, %eax
+; X64-NEXT:    xorl %esi, %eax
 ; X64-NEXT:    cmovel %edx, %eax
 ; X64-NEXT:    retq
 {

llvm/test/CodeGen/X86/pr32284.ll

@@ -321,11 +321,9 @@ define void @f2() {
 ; X64-NEXT:    xorl %ecx, %ecx
 ; X64-NEXT:    testl %eax, %eax
 ; X64-NEXT:    sete %cl
-; X64-NEXT:    movl %eax, %edx
-; X64-NEXT:    xorl %ecx, %edx
-; X64-NEXT:    movw %dx, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    xorl %edx, %edx
-; X64-NEXT:    cmpl %eax, %ecx
+; X64-NEXT:    xorl %eax, %ecx
+; X64-NEXT:    movw %cx, -{{[0-9]+}}(%rsp)
 ; X64-NEXT:    sete %dl
 ; X64-NEXT:    movw %dx, (%rax)
 ; X64-NEXT:    retq
@@ -366,17 +364,15 @@ define void @f2() {
 ; X86:       # %bb.0: # %entry
 ; X86-NEXT:    subl $2, %esp
 ; X86-NEXT:    .cfi_def_cfa_offset 6
-; X86-NEXT:    movzbl var_7, %ecx
+; X86-NEXT:    movzbl var_7, %edx
 ; X86-NEXT:    xorl %eax, %eax
-; X86-NEXT:    testl %ecx, %ecx
+; X86-NEXT:    testl %edx, %edx
 ; X86-NEXT:    sete %al
-; X86-NEXT:    movl %ecx, %edx
-; X86-NEXT:    xorl %eax, %edx
-; X86-NEXT:    movw %dx, (%esp)
-; X86-NEXT:    xorl %edx, %edx
-; X86-NEXT:    cmpl %ecx, %eax
-; X86-NEXT:    sete %dl
-; X86-NEXT:    movw %dx, (%eax)
+; X86-NEXT:    xorl %ecx, %ecx
+; X86-NEXT:    xorl %edx, %eax
+; X86-NEXT:    movw %ax, (%esp)
+; X86-NEXT:    sete %cl
+; X86-NEXT:    movw %cx, (%eax)
 ; X86-NEXT:    addl $2, %esp
 ; X86-NEXT:    .cfi_def_cfa_offset 4
 ; X86-NEXT:    retl