[llvm] [X86] Create special case for (a-b) - (a<b) -> sbb a, b (PR #161388)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: AZero13 (AZero13)

<details>
<summary>Changes</summary>



---
Full diff: https://github.com/llvm/llvm-project/pull/161388.diff


2 Files Affected:

- (modified) llvm/lib/Target/X86/X86ISelLowering.cpp (+10) 
- (modified) llvm/test/CodeGen/X86/sbb.ll (+14) 


``````````diff
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index cd04ff5bc7ef4..23861913d36a1 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -52380,6 +52380,16 @@ static SDValue combineAddOrSubToADCOrSBB(bool IsSub, const SDLoc &DL, EVT VT,
                          DAG.getVTList(VT, MVT::i32), X,
                          DAG.getConstant(0, DL, VT), NewEFLAGS);
     }
+
+    if (IsSub && X.getOpcode() == X86ISD::SUB && X.getValueType().isInteger() &&
+        EFLAGS == X.getValue(1) && !isa<ConstantSDNode>(X.getOperand(1))) {
+      SDValue NewSub =
+          DAG.getNode(X86ISD::SUB, SDLoc(X), X.getNode()->getVTList(),
+                      X.getOperand(1), X.getOperand(0));
+      SDValue NewEFLAGS = NewSub.getValue(1);
+      return DAG.getNode(X86ISD::SBB, DL, DAG.getVTList(VT, MVT::i32),
+                         X.getOperand(0), X.getOperand(1), NewEFLAGS);
+    }
   }
 
   if (CC == X86::COND_AE) {
diff --git a/llvm/test/CodeGen/X86/sbb.ll b/llvm/test/CodeGen/X86/sbb.ll
index 78d609d3a17e6..579608b628582 100644
--- a/llvm/test/CodeGen/X86/sbb.ll
+++ b/llvm/test/CodeGen/X86/sbb.ll
@@ -365,3 +365,17 @@ define i32 @uge_sext_add(i32 %0, i32 %1, i32 %2) {
   %6 = add nsw i32 %5, %0
   ret i32 %6
 }
+
+define i32 @sub_sub_ugt(i32 %a, i32 %b) {
+; CHECK-LABEL: sub_sub_ugt:
+; CHECK:       # %bb.0:
+; CHECK-NEXT:    movl %edi, %eax
+; CHECK-NEXT:    cmpl %edi, %esi
+; CHECK-NEXT:    sbbl %esi, %eax
+; CHECK-NEXT:    retq
+  %cmp = icmp ugt i32 %a, %b
+  %conv = zext i1 %cmp to i32
+  %sub = sub i32 %a, %b
+  %res = sub i32 %sub, %conv
+  ret i32 %res
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/161388