[llvm] [X86] EmitCmp - use existing XOR node to check for equality (PR #125506)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: Simon Pilgrim (RKSimon)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/125506.diff


3 Files Affected:

- (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+10-3) 
- (modified) llvm/test/CodeGen/X86/cmp-xor.ll (+10-18) 
- (modified) llvm/test/CodeGen/X86/pr32284.ll (+9-13) 


``````````diff
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 8f904209d8a3ad9..662da7df113e5f9 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/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,
diff --git a/llvm/test/CodeGen/X86/cmp-xor.ll b/llvm/test/CodeGen/X86/cmp-xor.ll
index 6be5cea38303284..a8a7da4871ba96e 100644
--- a/llvm/test/CodeGen/X86/cmp-xor.ll
+++ b/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
 {
diff --git a/llvm/test/CodeGen/X86/pr32284.ll b/llvm/test/CodeGen/X86/pr32284.ll
index 90fb76a23e9a5d5..8a726a469b61eb6 100644
--- a/llvm/test/CodeGen/X86/pr32284.ll
+++ b/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

``````````

</details>


https://github.com/llvm/llvm-project/pull/125506