[InstCombine] Fold `(icmp eq/ne (zext i1 X) (sext i1 Y))`-> `(icmp eq/ne (or X, Y), 0)`

[goldstein.w.n]

Summary

This comes up when adding two bool types in C/C++

    bool foo(bool a, bool b) {
        return a + b;
    }
    ...
    ->
    define i1 @foo(i1 %a, i1 %b) {
        %conv = zext i1 %a to i32
        %conv3.neg = sext i1 %b to i32
        %tobool4 = icmp ne i32 %conv, %conv3.neg
        ret i1 %tobool4
}

Proof: https://alive2.llvm.org/ce/z/HffWAN

[nikic]

LGTM

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

5264

and -> are

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

Instruction *InstCombinerImpl::foldICmpWithZextOrSext(ICmpInst &ICmp) {
   bool IsSignedCmp = ICmp.isSigned();
 
   // icmp Pred (ext X), (ext Y)
   Value *Y;
   if (match(ICmp.getOperand(1), m_ZExtOrSExt(m_Value(Y)))) {
     bool IsZext0 = isa<ZExtOperator>(ICmp.getOperand(0));
     bool IsZext1 = isa<ZExtOperator>(ICmp.getOperand(1));
 
+    if (IsZext0 != IsZext1) {
+        // If X and Y and both i1
+        // (icmp eq/ne (zext X) (sext Y))
+        //      eq -> (icmp eq (or X, Y), 0)
+        //      ne -> (icmp ne (or X, Y), 0)
+      if (ICmp.isEquality() && X->getType()->isIntOrIntVectorTy(1) &&
+          Y->getType()->isIntOrIntVectorTy(1))
+        return new ICmpInst(ICmp.getPredicate(), Builder.CreateOr(X, Y),
+                            Constant::getNullValue(X->getType()));
+
       // If we have mismatched casts, treat the zext of a non-negative source as
       // a sext to simulate matching casts. Otherwise, we are done.
       // TODO: Can we handle some predicates (equality) without non-negative?
-    if (IsZext0 != IsZext1) {
       if ((IsZext0 && isKnownNonNegative(X, DL, 0, &AC, &ICmp, &DT)) ||
           (IsZext1 && isKnownNonNegative(Y, DL, 0, &AC, &ICmp, &DT)))
         IsSignedExt = true;
       else
         return nullptr;
     }
 
     // Not an extension from the same type?

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

;
   %b = zext i8 %n to i32
   %c = icmp uge i32 %a, %b
   ret i1 %c
 }
 
 
 define i1 @zext_eq_sext(i1 %a, i1 %b) {
 ; CHECK-LABEL: @zext_eq_sext(
-; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[A:%.*]] to i32
-; CHECK-NEXT:    [[CONV3_NEG:%.*]] = sext i1 [[B:%.*]] to i32
-; CHECK-NEXT:    [[TOBOOL4:%.*]] = icmp eq i32 [[CONV]], [[CONV3_NEG]]
+; CHECK-NEXT:    [[TMP1:%.*]] = or i1 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[TOBOOL4:%.*]] = xor i1 [[TMP1]], true
 ; CHECK-NEXT:    ret i1 [[TOBOOL4]]
 ;
   %conv = zext i1 %a to i32
   %conv3.neg = sext i1 %b to i32
   %tobool4 = icmp eq i32 %conv, %conv3.neg
   ret i1 %tobool4
 }
 
 define i1 @zext_eq_sext_fail_not_i1(i1 %a, i8 %b) {
 ; CHECK-LABEL: @zext_eq_sext_fail_not_i1(
 ; CHECK-NEXT:    [[CONV:%.*]] = zext i1 [[A:%.*]] to i32
 ; CHECK-NEXT:    [[CONV3_NEG:%.*]] = sext i8 [[B:%.*]] to i32
 ; CHECK-NEXT:    [[TOBOOL4:%.*]] = icmp eq i32 [[CONV]], [[CONV3_NEG]]
 ; CHECK-NEXT:    ret i1 [[TOBOOL4]]
 ;
   %conv = zext i1 %a to i32
   %conv3.neg = sext i8 %b to i32
   %tobool4 = icmp eq i32 %conv, %conv3.neg
   ret i1 %tobool4
 }
 
 define <2 x i1> @zext_ne_sext(<2 x i1> %a, <2 x i1> %b) {
 ; CHECK-LABEL: @zext_ne_sext(
-; CHECK-NEXT:    [[CONV:%.*]] = zext <2 x i1> [[A:%.*]] to <2 x i8>
-; CHECK-NEXT:    [[CONV3_NEG:%.*]] = sext <2 x i1> [[B:%.*]] to <2 x i8>
-; CHECK-NEXT:    [[TOBOOL4:%.*]] = icmp ne <2 x i8> [[CONV3_NEG]], [[CONV]]
-; CHECK-NEXT:    ret <2 x i1> [[TOBOOL4]]
+; CHECK-NEXT:    [[TMP1:%.*]] = or <2 x i1> [[B:%.*]], [[A:%.*]]
+; CHECK-NEXT:    ret <2 x i1> [[TMP1]]
 ;
   %conv = zext <2 x i1> %a to <2 x i8>
   %conv3.neg = sext <2 x i1> %b to <2 x i8>
   %tobool4 = icmp ne <2 x i8> %conv3.neg, %conv
   ret <2 x i1> %tobool4
 }