[InstCombine] Missed optimization for select a%2==0, (a/2*2)*(a/2*2), 0

[jf-botto]

This is my first PR contributing to LLVM.
I'm more than happy to take any advice on board so that I can improve my future contributions.

Fixes #71533
Proof: https://alive2.llvm.org/ce/z/PCkT2L

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be
notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write
permissions for the repository. In which case you can instead tag reviewers by
name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review
by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate
is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Jorge Botto (jf-botto)

Changes

This is my first PR contributing to LLVM.
I'm more than happy to take any advice on board so that I can improve my future contributions.

Fixes #71533
Proof: https://alive2.llvm.org/ce/z/PCkT2L

---

Full diff: https://github.com/llvm/llvm-project/pull/92658.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp (+39)
• (modified) llvm/test/Transforms/InstCombine/select.ll (+72)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
index a3ddb402bf662..e0cb7d951fb5f 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -1073,6 +1073,41 @@ static Value *foldAbsDiff(ICmpInst *Cmp, Value *TVal, Value *FVal,
   return nullptr;
 }
 
+// (A % 2 == 0) ? (A/2*2) : B --> (A % 2 == 0) ? A : B
+// (A % 2 == 0) ? BinOp (A/2*2), (A/2*2) : B --> (A % 2 == 0) ? BinOp A, A : B
+static Value *foldSelectWithIcmpEqAndPattern(ICmpInst *Cmp, Value *TVal,
+                                             Value *FVal,
+                                             InstCombiner::BuilderTy &Builder) {
+  Value *A;
+  ConstantInt *MaskedConstant;
+  ICmpInst::Predicate Pred = Cmp->getPredicate();
+
+  // Checks if the comparison is (A % 2 == 0) and A is not undef.
+  if (!(Pred == ICmpInst::ICMP_EQ &&
+        match(Cmp->getOperand(0), m_And(m_Value(A), m_SpecificInt(1))) &&
+        match(Cmp->getOperand(1), m_SpecificInt(0)) &&
+        isGuaranteedNotToBeUndef(A)))
+    return nullptr;
+
+  // Checks if true branch matches (A % 2).
+  if (match(TVal,
+            m_OneUse(m_And(m_Specific(A), m_ConstantInt(MaskedConstant)))) &&
+      MaskedConstant->getValue().getSExtValue() == -2)
+    return Builder.CreateSelect(Cmp, A, FVal);
+
+  // Checks if true branch matches nested (A % 2) within a binary operation.
+  Value *MulVal;
+  if (match(TVal, m_OneUse(m_BinOp(m_Value(MulVal), m_Deferred(MulVal)))))
+    if (match(MulVal, m_And(m_Specific(A), m_ConstantInt(MaskedConstant))) &&
+        MaskedConstant->getValue().getSExtValue() == -2) {
+      Instruction::BinaryOps OpCode = cast<BinaryOperator>(TVal)->getOpcode();
+      Value *NewBinop = Builder.CreateBinOp(OpCode, A, A);
+      return Builder.CreateSelect(Cmp, NewBinop, FVal);
+    }
+
+  return nullptr;
+}
+
 /// Fold the following code sequence:
 /// \code
 ///   int a = ctlz(x & -x);
@@ -1933,6 +1968,10 @@ Instruction *InstCombinerImpl::foldSelectInstWithICmp(SelectInst &SI,
   if (Value *V = foldAbsDiff(ICI, TrueVal, FalseVal, Builder))
     return replaceInstUsesWith(SI, V);
 
+  if (Value *V =
+          foldSelectWithIcmpEqAndPattern(ICI, TrueVal, FalseVal, Builder))
+    return replaceInstUsesWith(SI, V);
+
   return Changed ? &SI : nullptr;
 }
 
diff --git a/llvm/test/Transforms/InstCombine/select.ll b/llvm/test/Transforms/InstCombine/select.ll
index 2ade6faa99be3..a913552dc6cff 100644
--- a/llvm/test/Transforms/InstCombine/select.ll
+++ b/llvm/test/Transforms/InstCombine/select.ll
@@ -1456,6 +1456,78 @@ define <2 x i32> @select_icmp_slt0_xor_vec(<2 x i32> %x) {
   ret <2 x i32> %x.xor
 }
 
+define i8 @select_icmp_eq_mul_and(i8 noundef %a, i8 %b)  {
+; CHECK-LABEL: @select_icmp_eq_mul_and(
+; CHECK-NEXT:    [[TMP1:%.*]] = and i8 [[A:%.*]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[TMP1]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = mul i8 [[A]], [[A]]
+; CHECK-NEXT:    [[RETVAL_0:%.*]] = select i1 [[CMP]], i8 [[TMP2]], i8 [[B:%.*]]
+; CHECK-NEXT:    ret i8 [[RETVAL_0]]
+;
+  %1 = and i8 %a, 1
+  %cmp = icmp eq i8 %1, 0
+  %div7 = and i8 %a, -2
+  %mul = mul i8 %div7, %div7
+  %retval.0 = select i1 %cmp, i8 %mul, i8 %b
+  ret i8 %retval.0
+}
+
+define i8 @select_icmp_eq_shl_and(i8 noundef %a, i8 %b)  {
+; CHECK-LABEL: @select_icmp_eq_shl_and(
+; CHECK-NEXT:    [[TMP1:%.*]] = and i8 [[A:%.*]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[TMP1]], 0
+; CHECK-NEXT:    [[TMP2:%.*]] = shl i8 [[A]], [[A]]
+; CHECK-NEXT:    [[RETVAL_0:%.*]] = select i1 [[CMP]], i8 [[TMP2]], i8 [[B:%.*]]
+; CHECK-NEXT:    ret i8 [[RETVAL_0]]
+;
+  %1 = and i8 %a, 1
+  %cmp = icmp eq i8 %1, 0
+  %div7 = and i8 %a, -2
+  %shl = shl i8 %div7, %div7
+  %retval.0 = select i1 %cmp, i8 %shl, i8 %b
+  ret i8 %retval.0
+}
+
+define i8 @select_icmp_eq_and(i8 noundef %a, i8 %b)  {
+; CHECK-LABEL: @select_icmp_eq_and(
+; CHECK-NEXT:    [[TMP1:%.*]] = and i8 [[A:%.*]], 1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[TMP1]], 0
+; CHECK-NEXT:    [[RETVAL_0:%.*]] = select i1 [[CMP]], i8 [[A]], i8 [[B:%.*]]
+; CHECK-NEXT:    ret i8 [[RETVAL_0]]
+;
+  %1 = and i8 %a, 1
+  %cmp = icmp eq i8 %1, 0
+  %div7 = and i8 %a, -2
+  %retval.0 = select i1 %cmp, i8 %div7, i8 %b
+  ret i8 %retval.0
+}
+
+;negative test
+define i8 @select_and(i8 noundef %a, i8 %b, i1 %cmp)  {
+; CHECK-LABEL: @select_and(
+; CHECK-NEXT:    [[DIV7:%.*]] = and i8 [[A:%.*]], -2
+; CHECK-NEXT:    [[RETVAL_0:%.*]] = select i1 [[CMP:%.*]], i8 [[DIV7]], i8 [[B:%.*]]
+; CHECK-NEXT:    ret i8 [[RETVAL_0]]
+;
+  %div7 = and i8 %a, -2
+  %retval.0 = select i1 %cmp, i8 %div7, i8 %b
+  ret i8 %retval.0
+}
+
+;negative test
+define i8 @select_mul_and(i8 noundef %a, i8 %b, i1 %cmp)  {
+; CHECK-LABEL: @select_mul_and(
+; CHECK-NEXT:    [[DIV7:%.*]] = and i8 [[A:%.*]], -2
+; CHECK-NEXT:    [[MUL:%.*]] = mul i8 [[DIV7]], [[DIV7]]
+; CHECK-NEXT:    [[RETVAL_0:%.*]] = select i1 [[CMP:%.*]], i8 [[MUL]], i8 [[B:%.*]]
+; CHECK-NEXT:    ret i8 [[RETVAL_0]]
+;
+  %div7 = and i8 %a, -2
+  %mul = mul i8 %div7, %div7
+  %retval.0 = select i1 %cmp, i8 %mul, i8 %b
+  ret i8 %retval.0
+}
+
 define <4 x i32> @canonicalize_to_shuffle(<4 x i32> %a, <4 x i32> %b) {
 ; CHECK-LABEL: @canonicalize_to_shuffle(
 ; CHECK-NEXT:    [[SEL:%.*]] = shufflevector <4 x i32> [[A:%.*]], <4 x i32> [[B:%.*]], <4 x i32> <i32 0, i32 5, i32 6, i32 3>