[PATCH] D57169: [X86] Fold X86ISD::SBB(ISD::SUB(X, Y), 0) -> X86ISD::SBB(X, Y) (PR25858)

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon updated this revision to Diff 183499.
RKSimon added a comment.

Limit the combine to cases where we don't use the output overflow flag


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D57169/new/

https://reviews.llvm.org/D57169

Files:
  lib/Target/X86/X86ISelLowering.cpp
  test/CodeGen/X86/combine-sbb.ll


Index: test/CodeGen/X86/combine-sbb.ll
===================================================================
--- test/CodeGen/X86/combine-sbb.ll
+++ test/CodeGen/X86/combine-sbb.ll
@@ -13,11 +13,10 @@
 ; X86-NEXT:    movl {{[0-9]+}}(%esp), %edx
 ; X86-NEXT:    movl (%ecx), %esi
 ; X86-NEXT:    movl 4(%ecx), %ecx
-; X86-NEXT:    subl 4(%edx), %ecx
 ; X86-NEXT:    subl (%edx), %esi
-; X86-NEXT:    sbbl $0, %ecx
-; X86-NEXT:    movl %esi, (%eax)
+; X86-NEXT:    sbbl 4(%edx), %ecx
 ; X86-NEXT:    movl %ecx, 4(%eax)
+; X86-NEXT:    movl %esi, (%eax)
 ; X86-NEXT:    popl %esi
 ; X86-NEXT:    retl $4
 ;
@@ -26,11 +25,10 @@
 ; X64-NEXT:    movq %rdi, %rax
 ; X64-NEXT:    movl (%rsi), %ecx
 ; X64-NEXT:    movl 4(%rsi), %esi
-; X64-NEXT:    subl 4(%rdx), %esi
 ; X64-NEXT:    subl (%rdx), %ecx
-; X64-NEXT:    sbbl $0, %esi
-; X64-NEXT:    movl %ecx, (%rdi)
+; X64-NEXT:    sbbl 4(%rdx), %esi
 ; X64-NEXT:    movl %esi, 4(%rdi)
+; X64-NEXT:    movl %ecx, (%rdi)
 ; X64-NEXT:    retq
 top:
   %3 = bitcast %WideUInt32* %1 to i32*
@@ -94,11 +92,10 @@
 ; X64-NEXT:    movq %rdi, %rax
 ; X64-NEXT:    movq (%rsi), %rcx
 ; X64-NEXT:    movq 8(%rsi), %rsi
-; X64-NEXT:    subq 8(%rdx), %rsi
 ; X64-NEXT:    subq (%rdx), %rcx
-; X64-NEXT:    sbbq $0, %rsi
-; X64-NEXT:    movq %rcx, (%rdi)
+; X64-NEXT:    sbbq 8(%rdx), %rsi
 ; X64-NEXT:    movq %rsi, 8(%rdi)
+; X64-NEXT:    movq %rcx, (%rdi)
 ; X64-NEXT:    retq
 top:
   %3 = bitcast %WideUInt64* %1 to i64*
Index: lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- lib/Target/X86/X86ISelLowering.cpp
+++ lib/Target/X86/X86ISelLowering.cpp
@@ -40455,6 +40455,15 @@
                        Flags);
   }
 
+  // Fold SBB(SUB(X,Y),0,Carry) -> SBB(X,Y,Carry)
+  // iff the flag result is dead.
+  SDValue Op0 = N->getOperand(0);
+  SDValue Op1 = N->getOperand(1);
+  if (Op0.getOpcode() == ISD::SUB && isNullConstant(Op1) &&
+      !N->hasAnyUseOfValue(1))
+    return DAG.getNode(X86ISD::SBB, SDLoc(N), N->getVTList(), Op0.getOperand(0),
+                       Op0.getOperand(1), N->getOperand(2));
+
   return SDValue();
 }
 


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