[InstCombine] Canonicalize icmp ult (add X, C2), C pattern

`icmp ult (add X, C2), C` can be folded to `icmp ne (and X, C), 2C`,
subject to `C == -C2` and C2 being a power of 2.

Proofs: https://alive2.llvm.org/ce/z/P-VVmQ.

Author: antoniofrighetto

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -3130,6 +3130,13 @@ Instruction *InstCombinerImpl::foldICmpAddConstant(ICmpInst &Cmp,
     return new ICmpInst(ICmpInst::ICMP_EQ, Builder.CreateAnd(X, -C),
                         ConstantExpr::getNeg(cast<Constant>(Y)));
 
+  // X+C2 <u C -> (X & C) == 2C
+  //   iff C == -(C2)
+  //       C2 is a power of 2
+  if (Pred == ICmpInst::ICMP_ULT && C2->isPowerOf2() && C == -*C2)
+    return new ICmpInst(ICmpInst::ICMP_NE, Builder.CreateAnd(X, C),
+                        ConstantInt::get(Ty, C * 2));
+
   // X+C >u C2 -> (X & ~C2) != C
   //   iff C & C2 == 0
   //       C2+1 is a power of 2

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

@@ -3025,8 +3025,8 @@ define i1 @icmp_addnuw_nonzero_fail_multiuse(i32 %x, i32 %y) {
 
 define i1 @ult_add_C2_pow2_C_neg(i8 %x) {
 ; CHECK-LABEL: @ult_add_C2_pow2_C_neg(
-; CHECK-NEXT:    [[I:%.*]] = add i8 [[X:%.*]], 32
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i8 [[I]], -32
+; CHECK-NEXT:    [[TMP1:%.*]] = and i8 [[X:%.*]], -32
+; CHECK-NEXT:    [[C:%.*]] = icmp ne i8 [[TMP1]], -64
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %i = add i8 %x, 32
@@ -3036,8 +3036,8 @@ define i1 @ult_add_C2_pow2_C_neg(i8 %x) {
 
 define i1 @ult_add_nsw_C2_pow2_C_neg(i8 %x) {
 ; CHECK-LABEL: @ult_add_nsw_C2_pow2_C_neg(
-; CHECK-NEXT:    [[I:%.*]] = add nsw i8 [[X:%.*]], 32
-; CHECK-NEXT:    [[C:%.*]] = icmp ult i8 [[I]], -32
+; CHECK-NEXT:    [[TMP1:%.*]] = and i8 [[X:%.*]], -32
+; CHECK-NEXT:    [[C:%.*]] = icmp ne i8 [[TMP1]], -64
 ; CHECK-NEXT:    ret i1 [[C]]
 ;
   %i = add nsw i8 %x, 32
@@ -3068,8 +3068,8 @@ define i1 @ult_add_C2_neg_C_pow2(i8 %x) {
 
 define <2 x i1> @ult_add_C2_pow2_C_neg_vec(<2 x i8> %x) {
 ; CHECK-LABEL: @ult_add_C2_pow2_C_neg_vec(
-; CHECK-NEXT:    [[I:%.*]] = add <2 x i8> [[X:%.*]], <i8 32, i8 32>
-; CHECK-NEXT:    [[C:%.*]] = icmp ult <2 x i8> [[I]], <i8 -32, i8 -32>
+; CHECK-NEXT:    [[TMP1:%.*]] = and <2 x i8> [[X:%.*]], <i8 -32, i8 -32>
+; CHECK-NEXT:    [[C:%.*]] = icmp ne <2 x i8> [[TMP1]], <i8 -64, i8 -64>
 ; CHECK-NEXT:    ret <2 x i1> [[C]]
 ;
   %i = add <2 x i8> %x, <i8 32, i8 32>