From 6639e547bcd8412a4c2999da240fcceb5be18c1d Mon Sep 17 00:00:00 2001
From: Rohan <rohanam2000@gmail.com>
Date: Tue, 19 Aug 2025 10:00:45 +0530
Subject: [PATCH] InstCombine: fold(select C, (X | A), X) | B into X | select
 C, (A | B), B

---
 .../InstCombine/InstCombineAndOrXor.cpp       | 70 +++++++++++++++++++
 .../InstCombine/or-select-factor.ll           | 33 +++++++++
 2 files changed, 103 insertions(+)
 create mode 100644 llvm/test/Transforms/InstCombine/or-select-factor.ll

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
index 6e46898634070..ac78b0276d594 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
@@ -2695,6 +2695,40 @@ Instruction *InstCombinerImpl::visitAnd(BinaryOperator &I) {
   if (Instruction *FoldedLogic = foldBinOpIntoSelectOrPhi(I))
     return FoldedLogic;
 
+  // Factor a common operand across select feeding an 'or':
+  // (select Cond, (X | A), X) | B  ->  X | select Cond, (A | B), B
+  // (select Cond, X, (X | A)) | B  ->  X | select Cond, B, (A | B)
+  // and commuted variants swapping the operands of the outer 'or'.
+  auto TryFactorSelectOr = [&](Value *SelOp, Value *OtherOp) -> Instruction * {
+    auto *SI = dyn_cast<SelectInst>(SelOp);
+    if (!SI)
+      return nullptr;
+
+    Value *Cond = SI->getCondition();
+    Value *T = SI->getTrueValue();
+    Value *F = SI->getFalseValue();
+    Value *X, *A;
+
+    // Match: select Cond, (X|A), X
+    if (match(T, m_c_Or(m_Value(X), m_Value(A))) && F == X) {
+      Value *AorB = Builder.CreateOr(A, OtherOp);
+      Value *InnerSel = Builder.CreateSelect(Cond, AorB, OtherOp);
+      return BinaryOperator::CreateOr(X, InnerSel);
+    }
+    // Match: select Cond, X, (X|A)
+    if (match(F, m_c_Or(m_Value(X), m_Value(A))) && T == X) {
+      Value *AorB = Builder.CreateOr(A, OtherOp);
+      Value *InnerSel = Builder.CreateSelect(Cond, OtherOp, AorB);
+      return BinaryOperator::CreateOr(X, InnerSel);
+    }
+    return nullptr;
+  };
+
+  if (Instruction *R = TryFactorSelectOr(Op0, Op1))
+    return R;
+  if (Instruction *R = TryFactorSelectOr(Op1, Op0))
+    return R;
+
   if (Instruction *DeMorgan = matchDeMorgansLaws(I, *this))
     return DeMorgan;
 
@@ -4022,6 +4056,42 @@ Instruction *InstCombinerImpl::visitOr(BinaryOperator &I) {
     }
   }
 
+  // Factor a common operand across select feeding an 'or' before pushing the
+  // constant into the select arms:
+  // (select Cond, (X | A), X) | B  ->  X | select Cond, (A | B), B
+  // (select Cond, X, (X | A)) | B  ->  X | select Cond, B, (A | B)
+  // and commuted variants swapping the operands of the outer 'or'.
+  auto TryFactorSelectOrEarly = [&](Value *SelOp,
+                                    Value *OtherOp) -> Instruction * {
+    auto *SI = dyn_cast<SelectInst>(SelOp);
+    if (!SI)
+      return nullptr;
+
+    Value *Cond = SI->getCondition();
+    Value *T = SI->getTrueValue();
+    Value *F = SI->getFalseValue();
+    Value *X, *A;
+
+    // Match: select Cond, (X|A), X
+    if (match(T, m_c_Or(m_Value(X), m_Value(A))) && F == X) {
+      Value *AorB = Builder.CreateOr(A, OtherOp);
+      Value *InnerSel = Builder.CreateSelect(Cond, AorB, OtherOp);
+      return BinaryOperator::CreateOr(X, InnerSel);
+    }
+    // Match: select Cond, X, (X|A)
+    if (match(F, m_c_Or(m_Value(X), m_Value(A))) && T == X) {
+      Value *AorB = Builder.CreateOr(A, OtherOp);
+      Value *InnerSel = Builder.CreateSelect(Cond, OtherOp, AorB);
+      return BinaryOperator::CreateOr(X, InnerSel);
+    }
+    return nullptr;
+  };
+
+  if (Instruction *R = TryFactorSelectOrEarly(Op0, Op1))
+    return R;
+  if (Instruction *R = TryFactorSelectOrEarly(Op1, Op0))
+    return R;
+
   if (Instruction *FoldedLogic = foldBinOpIntoSelectOrPhi(I))
     return FoldedLogic;
 
diff --git a/llvm/test/Transforms/InstCombine/or-select-factor.ll b/llvm/test/Transforms/InstCombine/or-select-factor.ll
new file mode 100644
index 0000000000000..75f62b2679ca0
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/or-select-factor.ll
@@ -0,0 +1,33 @@
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+; Fold: (select C, (x | a), x) | b  ->  x | select C, (a | b), b
+
+define i8 @src(i8 %x, i8 %y) {
+; CHECK-LABEL: @src(
+; CHECK-NEXT:    [[V0:%.*]] = icmp eq i8 [[Y:%.*]], -1
+; CHECK-NEXT:    [[V1:%.*]] = or i8 [[X:%.*]], 4
+; CHECK-NEXT:    [[V2:%.*]] = select i1 [[V0]], i8 [[V1]], i8 [[X]]
+; CHECK-NEXT:    [[V3:%.*]] = or i8 [[V2]], 1
+; CHECK-NEXT:    ret i8 [[V3]]
+;
+  %v0 = icmp eq i8 %y, -1
+  %v1 = or i8 %x, 4
+  %v2 = select i1 %v0, i8 %v1, i8 %x
+  %v3 = or i8 %v2, 1
+  ret i8 %v3
+}
+
+define i8 @tgt(i8 %x, i8 %y) {
+; CHECK-LABEL: @tgt(
+; CHECK-NEXT:    [[V0:%.*]] = icmp eq i8 [[Y:%.*]], -1
+; CHECK-NEXT:    [[V1:%.*]] = select i1 [[V0]], i8 5, i8 1
+; CHECK-NEXT:    [[V2:%.*]] = or i8 [[X:%.*]], [[V1]]
+; CHECK-NEXT:    ret i8 [[V2]]
+;
+  %v0 = icmp eq i8 %y, -1
+  %v1 = select i1 %v0, i8 5, i8 1
+  %v2 = or i8 %x, %v1
+  ret i8 %v2
+}
+
+