[InstCombine] Introduce `foldICmpBinOpWithConstantViaTruthTable` folding #139109

antoniofrighetto · 2025-05-08T17:08:10Z

Match icmps of binops where both operands are select with constant arms, i.e., icmp pred (select A ? C1 : C2) binop (select B ? C3 : C4), C5. Fold such patterns by creating a truth table of the possible four constant variants, and materialize back the optimal logic from it via createLogicFromTable helper. This also generalizes an existing fold, which has therefore been dropped.

Proofs: https://alive2.llvm.org/ce/z/NS7Vzu.

Fixes: #138212.

llvmbot · 2025-05-08T17:08:42Z

@llvm/pr-subscribers-llvm-transforms

Author: Antonio Frighetto (antoniofrighetto)

Changes

Match icmps of binops where both operands are select with constant arms, i.e., icmp pred (select A ? C1 : C2) binop (select B ? C3 : C4), C5. Fold such patterns by creating a truth table of the possible four constant variants, and materialize back the optimal logic from it via createLogicFromTable helper.

Proofs: https://alive2.llvm.org/ce/z/_kkUfJ.

Fixes: #138212.

Full diff: https://github.com/llvm/llvm-project/pull/139109.diff

4 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+45-1)
(modified) llvm/lib/Transforms/InstCombine/InstCombineInternal.h (+3)
(modified) llvm/test/Transforms/InstCombine/icmp-binop.ll (+96)
(modified) llvm/test/Transforms/InstCombine/icmp-select.ll (+12-40)

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index 252781e54ab06..9cb3a80c2ea20 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -3110,6 +3110,44 @@ static Value *createLogicFromTable(const std::bitset<4> &Table, Value *Op0,
   return nullptr;
 }
 
+Instruction *InstCombinerImpl::foldICmpBinOpWithConstantViaTruthTable(
+    ICmpInst &Cmp, BinaryOperator *BO, const APInt &C) {
+  Value *A, *B;
+  Constant *C1, *C2, *C3, *C4;
+  if (match(BO->getOperand(0),
+            m_Select(m_Value(A), m_Constant(C1), m_Constant(C2))) &&
+      match(BO->getOperand(1),
+            m_Select(m_Value(B), m_Constant(C3), m_Constant(C4)))) {
+    std::bitset<4> Table;
+    auto ComputeTable = [&](bool First, bool Second) -> std::optional<bool> {
+      Constant *L = First ? C1 : C2;
+      Constant *R = Second ? C3 : C4;
+      if (auto *Res = ConstantFoldBinaryOpOperands(BO->getOpcode(), L, R, DL)) {
+        auto *Val = Res->getType()->isVectorTy() ? Res->getSplatValue() : Res;
+        if (auto *CI = dyn_cast_or_null<ConstantInt>(Val))
+          return ICmpInst::compare(CI->getValue(), C, Cmp.getPredicate());
+      }
+      return std::nullopt;
+    };
+
+    for (unsigned I = 0; I < 4; ++I) {
+      bool First = (I >> 1) & 1;
+      bool Second = I & 1;
+      if (auto Res = ComputeTable(First, Second))
+        Table[I] = *Res;
+      else
+        return nullptr;
+    }
+
+    // Synthesize optimal logic.
+    if (auto *Cond =
+            createLogicFromTable(Table, A, B, Builder, BO->hasOneUse()))
+      return replaceInstUsesWith(Cmp, Cond);
+  }
+
+  return nullptr;
+}
+
 /// Fold icmp (add X, Y), C.
 Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
                                                    BinaryOperator *Add,
@@ -4014,7 +4052,13 @@ Instruction *InstCombinerImpl::foldICmpBinOpWithConstant(ICmpInst &Cmp,
   }
 
   // TODO: These folds could be refactored to be part of the above calls.
-  return foldICmpBinOpEqualityWithConstant(Cmp, BO, C);
+  if (Instruction *I = foldICmpBinOpEqualityWithConstant(Cmp, BO, C))
+    return I;
+
+  // Fall back to handling `icmp pred (select A ? C1 : C2) binop (select B ? C3
+  // : C4), C5` pattern, by computing a truth table of the four constant
+  // variants.
+  return foldICmpBinOpWithConstantViaTruthTable(Cmp, BO, C);
 }
 
 static Instruction *
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index 324738ef8c88e..8b657b3f8555c 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -736,6 +736,9 @@ class LLVM_LIBRARY_VISIBILITY InstCombinerImpl final
   Instruction *foldICmpShlConstConst(ICmpInst &I, Value *ShAmt, const APInt &C1,
                                      const APInt &C2);
 
+  Instruction *foldICmpBinOpWithConstantViaTruthTable(ICmpInst &Cmp,
+                                                      BinaryOperator *BO,
+                                                      const APInt &C);
   Instruction *foldICmpBinOpEqualityWithConstant(ICmpInst &Cmp,
                                                  BinaryOperator *BO,
                                                  const APInt &C);
diff --git a/llvm/test/Transforms/InstCombine/icmp-binop.ll b/llvm/test/Transforms/InstCombine/icmp-binop.ll
index 356489716fff9..e240bf1cd95f9 100644
--- a/llvm/test/Transforms/InstCombine/icmp-binop.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-binop.ll
@@ -359,3 +359,99 @@ define i1 @test_icmp_sgt_and_negpow2_invalid_c(i32 %add) {
   %cmp = icmp sgt i32 %and, 48
   ret i1 %cmp
 }
+
+define i1 @icmp_eq_or_of_selects_with_constant(i1 %a, i1 %b) {
+; CHECK-LABEL: @icmp_eq_or_of_selects_with_constant(
+; CHECK-NEXT:    [[CMP:%.*]] = and i1 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %s1 = select i1 %a, i64 65536, i64 0
+  %s2 = select i1 %b, i64 256, i64 0
+  %or = or i64 %s1, %s2
+  %cmp = icmp eq i64 %or, 65792
+  ret i1 %cmp
+}
+
+define i1 @icmp_slt_and_of_selects_with_constant(i1 %a, i1 %b) {
+; CHECK-LABEL: @icmp_slt_and_of_selects_with_constant(
+; CHECK-NEXT:    [[TMP1:%.*]] = or i1 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[TMP1]], true
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %s1 = select i1 %a, i8 1, i8 254
+  %s2 = select i1 %b, i8 1, i8 253
+  %and = and i8 %s1, %s2
+  %cmp = icmp slt i8 %and, 254
+  ret i1 %cmp
+}
+
+define i1 @icmp_sge_add_of_selects_with_constant(i1 %a, i1 %b) {
+; CHECK-LABEL: @icmp_sge_add_of_selects_with_constant(
+; CHECK-NEXT:    ret i1 true
+;
+  %s1 = select i1 %a, i8 248, i8 7
+  %s2 = select i1 %b, i8 16, i8 0
+  %add = add i8 %s1, %s2
+  %cmp = icmp sge i8 %add, 247
+  ret i1 %cmp
+}
+
+define <2 x i1> @icmp_eq_or_of_selects_with_constant_vectorized(<2 x i1> %a, <2 x i1> %b) {
+; CHECK-LABEL: @icmp_eq_or_of_selects_with_constant_vectorized(
+; CHECK-NEXT:    [[CMP:%.*]] = and <2 x i1> [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+;
+  %s1 = select <2 x i1> %a, <2 x i64> <i64 65536, i64 65536>, <2 x i64> zeroinitializer
+  %s2 = select <2 x i1> %b, <2 x i64> <i64 256, i64 256>, <2 x i64> zeroinitializer
+  %or = or <2 x i64> %s1, %s2
+  %cmp = icmp eq <2 x i64> %or, <i64 65792, i64 65792>
+  ret <2 x i1> %cmp
+}
+
+; Negative tests.
+define i1 @icmp_eq_or_of_selects_with_constant_and_arg(i1 %a, i1 %b, i64 %arg) {
+; CHECK-LABEL: @icmp_eq_or_of_selects_with_constant_and_arg(
+; CHECK-NEXT:    [[S1:%.*]] = select i1 [[A:%.*]], i64 65536, i64 [[ARG:%.*]]
+; CHECK-NEXT:    [[S2:%.*]] = select i1 [[B:%.*]], i64 256, i64 0
+; CHECK-NEXT:    [[OR:%.*]] = or i64 [[S1]], [[S2]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i64 [[OR]], 65792
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %s1 = select i1 %a, i64 65536, i64 %arg
+  %s2 = select i1 %b, i64 256, i64 0
+  %or = or i64 %s1, %s2
+  %cmp = icmp eq i64 %or, 65792
+  ret i1 %cmp
+}
+
+define i1 @icmp_eq_or_of_selects_with_constant_multiuse(i1 %a, i1 %b) {
+; CHECK-LABEL: @icmp_eq_or_of_selects_with_constant_multiuse(
+; CHECK-NEXT:    [[S1:%.*]] = select i1 [[A:%.*]], i64 0, i64 65536
+; CHECK-NEXT:    [[S2:%.*]] = select i1 [[B:%.*]], i64 0, i64 256
+; CHECK-NEXT:    [[OR:%.*]] = or disjoint i64 [[S1]], [[S2]]
+; CHECK-NEXT:    call void @use64(i64 [[OR]])
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i64 [[OR]], 65792
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %s1 = select i1 %a, i64 0, i64 65536
+  %s2 = select i1 %b, i64 0, i64 256
+  %or = or i64 %s1, %s2
+  call void @use64(i64 %or)
+  %cmp = icmp eq i64 %or, 65792
+  ret i1 %cmp
+}
+
+define <2 x i1> @icmp_eq_or_of_selects_with_constant_vectorized_nonsplat(<2 x i1> %a, <2 x i1> %b) {
+; CHECK-LABEL: @icmp_eq_or_of_selects_with_constant_vectorized_nonsplat(
+; CHECK-NEXT:    [[S1:%.*]] = select <2 x i1> [[A:%.*]], <2 x i64> splat (i64 65536), <2 x i64> zeroinitializer
+; CHECK-NEXT:    [[S2:%.*]] = select <2 x i1> [[B:%.*]], <2 x i64> <i64 256, i64 128>, <2 x i64> zeroinitializer
+; CHECK-NEXT:    [[OR:%.*]] = or disjoint <2 x i64> [[S1]], [[S2]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq <2 x i64> [[OR]], <i64 65792, i64 65664>
+; CHECK-NEXT:    ret <2 x i1> [[CMP]]
+;
+  %s1 = select <2 x i1> %a, <2 x i64> <i64 65536, i64 65536>, <2 x i64> zeroinitializer
+  %s2 = select <2 x i1> %b, <2 x i64> <i64 256, i64 128>, <2 x i64> zeroinitializer
+  %or = or <2 x i64> %s1, %s2
+  %cmp = icmp eq <2 x i64> %or, <i64 65792, i64 65664>
+  ret <2 x i1> %cmp
+}
diff --git a/llvm/test/Transforms/InstCombine/icmp-select.ll b/llvm/test/Transforms/InstCombine/icmp-select.ll
index 1aae91302dab1..a038731abbc48 100644
--- a/llvm/test/Transforms/InstCombine/icmp-select.ll
+++ b/llvm/test/Transforms/InstCombine/icmp-select.ll
@@ -328,10 +328,7 @@ define i1 @select_constants_and_icmp_eq0_common_bit(i1 %x, i1 %y) {
 
 define i1 @select_constants_and_icmp_eq0_no_common_op1(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_eq0_no_common_op1(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 16, i8 3
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 24, i8 3
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[AND]], 0
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 16, i8 3
@@ -345,10 +342,7 @@ define i1 @select_constants_and_icmp_eq0_no_common_op1(i1 %x, i1 %y) {
 
 define i1 @select_constants_and_icmp_eq0_no_common_op2(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_eq0_no_common_op2(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 16, i8 3
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 16, i8 7
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[AND]], 0
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 16, i8 3
@@ -387,14 +381,9 @@ define i1 @select_constants_and_icmp_eq0_zero_fval(i1 %x, i1 %y) {
   ret i1 %cmp
 }
 
-; TODO: x & y
-
 define i1 @select_constants_and_icmp_eq_tval(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_eq_tval(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 6, i8 1
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 6, i8 1
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[AND]], 6
+; CHECK-NEXT:    [[CMP:%.*]] = and i1 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 6, i8 1
@@ -404,14 +393,10 @@ define i1 @select_constants_and_icmp_eq_tval(i1 %x, i1 %y) {
   ret i1 %cmp
 }
 
-; TODO: ~(x | y)
-
 define i1 @select_constants_and_icmp_eq_fval(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_eq_fval(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 12, i8 3
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 12, i8 3
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i8 [[AND]], 3
+; CHECK-NEXT:    [[TMP1:%.*]] = or i1 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[TMP1]], true
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 12, i8 3
@@ -512,10 +497,8 @@ define i1 @select_constants_and_icmp_ne0_common_bit(i1 %x, i1 %y) {
 
 define i1 @select_constants_and_icmp_ne0_no_common_op1(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_ne0_no_common_op1(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 16, i8 3
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 24, i8 3
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[AND]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[TMP1]], true
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 16, i8 3
@@ -529,10 +512,8 @@ define i1 @select_constants_and_icmp_ne0_no_common_op1(i1 %x, i1 %y) {
 
 define i1 @select_constants_and_icmp_ne0_no_common_op2(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_ne0_no_common_op2(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 16, i8 3
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 16, i8 7
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[AND]], 0
+; CHECK-NEXT:    [[TMP1:%.*]] = xor i1 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[TMP1]], true
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 16, i8 3
@@ -571,14 +552,10 @@ define i1 @select_constants_and_icmp_ne0_zero_fval(i1 %x, i1 %y) {
   ret i1 %cmp
 }
 
-; TODO: ~(x & y)
-
 define i1 @select_constants_and_icmp_ne_tval(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_ne_tval(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 6, i8 1
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 6, i8 1
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[AND]], 6
+; CHECK-NEXT:    [[TMP1:%.*]] = and i1 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[CMP:%.*]] = xor i1 [[TMP1]], true
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 6, i8 1
@@ -588,14 +565,9 @@ define i1 @select_constants_and_icmp_ne_tval(i1 %x, i1 %y) {
   ret i1 %cmp
 }
 
-; TODO: (x | y)
-
 define i1 @select_constants_and_icmp_ne_fval(i1 %x, i1 %y) {
 ; CHECK-LABEL: @select_constants_and_icmp_ne_fval(
-; CHECK-NEXT:    [[S1:%.*]] = select i1 [[X:%.*]], i8 12, i8 3
-; CHECK-NEXT:    [[S2:%.*]] = select i1 [[Y:%.*]], i8 12, i8 3
-; CHECK-NEXT:    [[AND:%.*]] = and i8 [[S1]], [[S2]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i8 [[AND]], 3
+; CHECK-NEXT:    [[CMP:%.*]] = or i1 [[X:%.*]], [[Y:%.*]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %s1 = select i1 %x, i8 12, i8 3

nikic

Does this subsume the fold at

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

Lines 1969 to 1991 in 948bffa

    
           // If we are testing the intersection of 2 select-of-nonzero-constants with no 
        
           // common bits set, it's the same as checking if exactly one select condition 
        
           // is set: 
        
           // ((A ? TC : FC) & (B ? TC : FC)) == 0 --> xor A, B 
        
           // ((A ? TC : FC) & (B ? TC : FC)) != 0 --> not(xor A, B) 
        
           // TODO: Generalize for non-constant values. 
        
           // TODO: Handle signed/unsigned predicates. 
        
           // TODO: Handle other bitwise logic connectors. 
        
           // TODO: Extend to handle a non-zero compare constant. 
        
           if (C.isZero() && (Pred == CmpInst::ICMP_EQ || And->hasOneUse())) { 
        
             assert(Cmp.isEquality() && "Not expecting non-equality predicates"); 
        
             Value *A, *B; 
        
             const APInt *TC, *FC; 
        
             if (match(X, m_Select(m_Value(A), m_APInt(TC), m_APInt(FC))) && 
        
                 match(Y, 
        
                       m_Select(m_Value(B), m_SpecificInt(*TC), m_SpecificInt(*FC))) && 
        
                 !TC->isZero() && !FC->isZero() && !TC->intersects(*FC)) { 
        
               Value *R = Builder.CreateXor(A, B); 
        
               if (Pred == CmpInst::ICMP_NE) 
        
                 R = Builder.CreateNot(R); 
        
               return replaceInstUsesWith(Cmp, R); 
        
             } 
        
           }

?

llvm/test/Transforms/InstCombine/icmp-binop.ll

antoniofrighetto · 2025-05-10T18:14:19Z

Does this subsume the fold at

llvm-project/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

Lines 1969 to 1991 in 948bffa

// If we are testing the intersection of 2 select-of-nonzero-constants with no

// common bits set, it's the same as checking if exactly one select condition

// is set:

// ((A ? TC : FC) & (B ? TC : FC)) == 0 --> xor A, B

// ((A ? TC : FC) & (B ? TC : FC)) != 0 --> not(xor A, B)

// TODO: Generalize for non-constant values.

// TODO: Handle signed/unsigned predicates.

// TODO: Handle other bitwise logic connectors.

// TODO: Extend to handle a non-zero compare constant.

if (C.isZero() && (Pred == CmpInst::ICMP_EQ || And->hasOneUse())) {

assert(Cmp.isEquality() && "Not expecting non-equality predicates");

Value *A, *B;

const APInt *TC, *FC;

if (match(X, m_Select(m_Value(A), m_APInt(TC), m_APInt(FC))) &&

match(Y,

m_Select(m_Value(B), m_SpecificInt(*TC), m_SpecificInt(*FC))) &&

!TC->isZero() && !FC->isZero() && !TC->intersects(*FC)) {

Value *R = Builder.CreateXor(A, B);

if (Pred == CmpInst::ICMP_NE)

R = Builder.CreateNot(R);

return replaceInstUsesWith(Cmp, R);

}

}

?

Right, thanks for catching it, PR description updated.

llvm/test/Transforms/InstCombine/icmp-binop.ll

dtcxzyw · 2025-05-11T06:57:03Z

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

+    // Synthesize optimal logic.
+    if (auto *Cond =
+            createLogicFromTable(Table, A, B, Builder, BO->hasOneUse());
+        Cond && Cmp.getType() == Cond->getType())


If Cmp.getType() != Cond->getType(), the newly created instruction will not be used. It is the root cause of https://github.com/llvm/llvm-project/pull/139109/files#r2083295338.

Oh, sure thing, check anticipated, thanks.

dtcxzyw

LGTM.

nikic

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

Match icmps of binops where both operands are select with constant arms, i.e., `icmp pred (select A ? C1 : C2) binop (select B ? C3 : C4), C5`. Fold such patterns by creating a truth table of the possible four constant variants, and materialize back the optimal logic from it via `createLogicFromTable` helper. This also generalizes an existing fold, which has therefore been dropped. Proofs: https://alive2.llvm.org/ce/z/NS7Vzu. Fixes: llvm#138212.

antoniofrighetto requested a review from dtcxzyw May 8, 2025 17:08

antoniofrighetto requested a review from nikic as a code owner May 8, 2025 17:08

llvmbot added llvm:instcombine llvm:transforms labels May 8, 2025

nikic mentioned this pull request May 8, 2025

Task submission dtcxzyw/llvm-opt-benchmark#1312

Open

dtcxzyw mentioned this pull request May 8, 2025

pre-commit: PR139109 dtcxzyw/llvm-opt-benchmark#2338

Open

nikic reviewed May 8, 2025

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antoniofrighetto force-pushed the feature/ic-binop-with-constants-table branch from f7fba64 to e3c539c Compare May 10, 2025 18:13

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Fuzz PR139109 dtcxzyw/llvm-mutation-based-fuzz-service#45

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pre-commit: PR139109 dtcxzyw/llvm-opt-benchmark#2342

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antoniofrighetto added 2 commits May 13, 2025 09:03

[InstCombine] Precommit tests for PR139109 (NFC)

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antoniofrighetto force-pushed the feature/ic-binop-with-constants-table branch from 5c728fb to adfd59f Compare May 13, 2025 07:05

antoniofrighetto merged commit adfd59f into llvm:main May 13, 2025
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[InstCombine] Introduce `foldICmpBinOpWithConstantViaTruthTable` folding #139109

[InstCombine] Introduce `foldICmpBinOpWithConstantViaTruthTable` folding #139109

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antoniofrighetto commented May 8, 2025 •

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	// If we are testing the intersection of 2 select-of-nonzero-constants with no
	// common bits set, it's the same as checking if exactly one select condition
	// is set:
	// ((A ? TC : FC) & (B ? TC : FC)) == 0 --> xor A, B
	// ((A ? TC : FC) & (B ? TC : FC)) != 0 --> not(xor A, B)
	// TODO: Generalize for non-constant values.
	// TODO: Handle signed/unsigned predicates.
	// TODO: Handle other bitwise logic connectors.
	// TODO: Extend to handle a non-zero compare constant.
	if (C.isZero() && (Pred == CmpInst::ICMP_EQ \|\| And->hasOneUse())) {
	assert(Cmp.isEquality() && "Not expecting non-equality predicates");
	Value A, B;
	const APInt TC, FC;
	if (match(X, m_Select(m_Value(A), m_APInt(TC), m_APInt(FC))) &&
	match(Y,
	m_Select(m_Value(B), m_SpecificInt(TC), m_SpecificInt(FC))) &&
	!TC->isZero() && !FC->isZero() && !TC->intersects(*FC)) {
	Value *R = Builder.CreateXor(A, B);
	if (Pred == CmpInst::ICMP_NE)
	R = Builder.CreateNot(R);
	return replaceInstUsesWith(Cmp, R);
	}
	}

[InstCombine] Introduce foldICmpBinOpWithConstantViaTruthTable folding #139109

[InstCombine] Introduce foldICmpBinOpWithConstantViaTruthTable folding #139109

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[InstCombine] Introduce `foldICmpBinOpWithConstantViaTruthTable` folding #139109

[InstCombine] Introduce `foldICmpBinOpWithConstantViaTruthTable` folding #139109

antoniofrighetto commented May 8, 2025 •

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