[llvm] [InstCombine] Fold `select Cond, not X, X` into `Cond ^ X` (PR #90089)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

See the following example:
```
define i1 @<!-- -->src(i64 %x, i1 %y) {
  %1526 = icmp ne i64 %x, 0
  %1527 = icmp eq i64 %x, 0
  %sel = select i1 %y, i1 %1526, i1 %1527
  ret i1 %sel
}

define i1 @<!-- -->tgt(i64 %x, i1 %y) {
  %1527 = icmp eq i64 %x, 0
  %sel = xor i1 %y, %1527
  ret i1 %sel
}
```
I find that this pattern is common in C/C++/Rust code base.
This patch folds `select Cond, Y, X` into `Cond ^ X` iff:
1. X has the same type as Cond
2. X is poison -> Y is poison
3. X == !Y

Alive2: https://alive2.llvm.org/ce/z/hSmkHS

---
Full diff: https://github.com/llvm/llvm-project/pull/90089.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+5) 
- (modified) llvm/test/Transforms/InstCombine/select-cmp.ll (+74) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index 117eb7a1dcc933..69341c8d3f41cf 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -3985,5 +3985,10 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
     }
   }
 
+  if (CondVal->getType() == SI.getType() && impliesPoison(FalseVal, TrueVal) &&
+      isImpliedCondition(FalseVal, TrueVal, DL, /*LHSIsTrue=*/true) == false &&
+      isImpliedCondition(FalseVal, TrueVal, DL, /*LHSIsTrue=*/false) == true)
+    return BinaryOperator::CreateXor(CondVal, FalseVal);
+
   return nullptr;
 }
diff --git a/llvm/test/Transforms/InstCombine/select-cmp.ll b/llvm/test/Transforms/InstCombine/select-cmp.ll
index 711fac542179f8..3ec89af8d868c7 100644
--- a/llvm/test/Transforms/InstCombine/select-cmp.ll
+++ b/llvm/test/Transforms/InstCombine/select-cmp.ll
@@ -345,4 +345,78 @@ define i1 @icmp_no_common(i1 %c, i8 %x, i8 %y, i8 %z) {
   ret i1 %r
 }
 
+define i1 @test_select_inverse1(i64 %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse1(
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq i64 [[X:%.*]], 0
+; CHECK-NEXT:    [[SEL:%.*]] = xor i1 [[CMP2]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[SEL]]
+;
+  %cmp1 = icmp ne i64 %x, 0
+  %cmp2 = icmp eq i64 %x, 0
+  %sel = select i1 %y, i1 %cmp1, i1 %cmp2
+  ret i1 %sel
+}
+
+define i1 @test_select_inverse2(i64 %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse2(
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp slt i64 [[X:%.*]], 0
+; CHECK-NEXT:    [[SEL:%.*]] = xor i1 [[CMP2]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[SEL]]
+;
+  %cmp1 = icmp sgt i64 %x, -1
+  %cmp2 = icmp slt i64 %x, 0
+  %sel = select i1 %y, i1 %cmp1, i1 %cmp2
+  ret i1 %sel
+}
+
+define i1 @test_select_inverse3(ptr %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse3(
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp ne ptr [[X:%.*]], null
+; CHECK-NEXT:    [[SEL:%.*]] = xor i1 [[CMP2]], [[Y:%.*]]
+; CHECK-NEXT:    ret i1 [[SEL]]
+;
+  %cmp1 = icmp eq ptr %x, null
+  %cmp2 = icmp ne ptr %x, null
+  %sel = select i1 %y, i1 %cmp1, i1 %cmp2
+  ret i1 %sel
+}
+
+define i1 @test_select_inverse_fail(i64 %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse_fail(
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp sgt i64 [[X:%.*]], 0
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp slt i64 [[X]], 0
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[Y:%.*]], i1 [[CMP1]], i1 [[CMP2]]
+; CHECK-NEXT:    ret i1 [[SEL]]
+;
+  %cmp1 = icmp sgt i64 %x, 0
+  %cmp2 = icmp slt i64 %x, 0
+  %sel = select i1 %y, i1 %cmp1, i1 %cmp2
+  ret i1 %sel
+}
+
+define <2 x i1> @test_select_inverse_vec(<2 x i64> %x, <2 x i1> %y) {
+; CHECK-LABEL: @test_select_inverse_vec(
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq <2 x i64> [[X:%.*]], zeroinitializer
+; CHECK-NEXT:    [[SEL:%.*]] = xor <2 x i1> [[CMP2]], [[Y:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[SEL]]
+;
+  %cmp1 = icmp ne <2 x i64> %x, zeroinitializer
+  %cmp2 = icmp eq <2 x i64> %x, zeroinitializer
+  %sel = select <2 x i1> %y, <2 x i1> %cmp1, <2 x i1> %cmp2
+  ret <2 x i1> %sel
+}
+
+define <2 x i1> @test_select_inverse_vec_fail(<2 x i64> %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse_vec_fail(
+; CHECK-NEXT:    [[CMP1:%.*]] = icmp ne <2 x i64> [[X:%.*]], zeroinitializer
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp eq <2 x i64> [[X]], zeroinitializer
+; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[Y:%.*]], <2 x i1> [[CMP1]], <2 x i1> [[CMP2]]
+; CHECK-NEXT:    ret <2 x i1> [[SEL]]
+;
+  %cmp1 = icmp ne <2 x i64> %x, zeroinitializer
+  %cmp2 = icmp eq <2 x i64> %x, zeroinitializer
+  %sel = select i1 %y, <2 x i1> %cmp1, <2 x i1> %cmp2
+  ret <2 x i1> %sel
+}
+
 declare void @use(i1)

``````````

</details>


https://github.com/llvm/llvm-project/pull/90089