[llvm] [InstCombine] Fold `select Cond, not X, X` into `Cond ^ X` (PR #93591)
via llvm-commits
llvm-commits at lists.llvm.org
Tue May 28 10:58:17 PDT 2024
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
It is an alternative to https://github.com/llvm/llvm-project/pull/90089.
---
Full diff: https://github.com/llvm/llvm-project/pull/93591.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+34)
- (modified) llvm/test/Transforms/InstCombine/select-cmp.ll (+86)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index a3ddb402bf662..011b1c2e09c03 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -3502,6 +3502,34 @@ static bool matchFMulByZeroIfResultEqZero(InstCombinerImpl &IC, Value *Cmp0,
return false;
}
+/// Return true iff:
+/// 1. X is poison implies Y is poison.
+/// 2. X is true implies Y is false.
+/// 3. X is false implies Y is true.
+/// Otherwise, return false.
+static bool isKnownInversion(Value *X, Value *Y) {
+ // Handle X = icmp pred V, C1, Y = icmp pred V, C2.
+ Value *V;
+ Constant *C1, *C2;
+ ICmpInst::Predicate Pred1, Pred2;
+ if (!match(X, m_ICmp(Pred1, m_Value(V), m_Constant(C1))) ||
+ !match(Y, m_ICmp(Pred2, m_Specific(V), m_Constant(C2))))
+ return false;
+
+ if (C1 == C2)
+ return Pred1 == ICmpInst::getInversePredicate(Pred2);
+
+ // Try to infer the relationship from constant ranges.
+ const APInt *RHSC1, *RHSC2;
+ if (!match(C1, m_APInt(RHSC1)) || !match(C2, m_APInt(RHSC2)))
+ return false;
+
+ const auto CR1 = ConstantRange::makeExactICmpRegion(Pred1, *RHSC1);
+ const auto CR2 = ConstantRange::makeExactICmpRegion(Pred2, *RHSC2);
+
+ return CR1.inverse() == CR2;
+}
+
Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
Value *CondVal = SI.getCondition();
Value *TrueVal = SI.getTrueValue();
@@ -3996,5 +4024,11 @@ Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
}
}
+ // select Cond, !X, X -> xor Cond, X
+ // Note: We don't fold select Cond, Y, X -> X (iff X->Y & !X->!Y) here as
+ // it indicates that these two patterns should be canonicalized.
+ if (CondVal->getType() == SI.getType() && isKnownInversion(FalseVal, TrueVal))
+ 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 711fac542179f..8609bc5041ad8 100644
--- a/llvm/test/Transforms/InstCombine/select-cmp.ll
+++ b/llvm/test/Transforms/InstCombine/select-cmp.ll
@@ -345,4 +345,90 @@ define i1 @icmp_no_common(i1 %c, i8 %x, i8 %y, i8 %z) {
ret i1 %r
}
+define i1 @test_select_inverse_eq(i64 %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse_eq(
+; 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_inverse_signed(i64 %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse_signed(
+; 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_inverse_unsigned(i64 %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse_unsigned(
+; CHECK-NEXT: [[CMP2:%.*]] = icmp ugt i64 [[X:%.*]], 10
+; CHECK-NEXT: [[SEL:%.*]] = xor i1 [[CMP2]], [[Y:%.*]]
+; CHECK-NEXT: ret i1 [[SEL]]
+;
+ %cmp1 = icmp ult i64 %x, 11
+ %cmp2 = icmp ugt i64 %x, 10
+ %sel = select i1 %y, i1 %cmp1, i1 %cmp2
+ ret i1 %sel
+}
+
+define i1 @test_select_inverse_eq_ptr(ptr %x, i1 %y) {
+; CHECK-LABEL: @test_select_inverse_eq_ptr(
+; 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/93591
More information about the llvm-commits
mailing list