[InstCombine] remove constant by inverting compare + logic (PR27105)

[spatel]

Summary

This is a fix for PR27105:
https://llvm.org/bugs/show_bug.cgi?id=27105

We can check if all bits outside of a constant mask are set with a single constant.

As noted in the bug report, although this form should be considered the canonical IR, backends may want to transform this into an 'andn' / 'andc' comparison against zero because that could be a single machine instruction.

[spatel]

Ping.

[hfinkel]

I'm not sure how much reducing the number of independent constants matters for canonicalization, but this form also looks more natural for extracting known bits by later optimizations. LGTM.

llvm/trunk/lib/Transforms/InstCombine/InstCombineCompares.cpp

if (BinaryOperator *BO = dyn_cast<BinaryOperator>(LHSI)) {
         break;
       case Instruction::Or:
         // If bits are being or'd in that are not present in the constant we
         // are comparing against, then the comparison could never succeed!
         if (ConstantInt *BOC = dyn_cast<ConstantInt>(BO->getOperand(1))) {
           Constant *NotCI = ConstantExpr::getNot(RHS);
           if (!ConstantExpr::getAnd(BOC, NotCI)->isNullValue())
             return replaceInstUsesWith(ICI, Builder->getInt1(isICMP_NE));
+
+          // Comparing if all bits outside of a constant mask are set?
+          // Replace (X | C) == -1 with (X & ~C) == ~C.
+          // This removes the -1 constant.
+          if (BO->hasOneUse() && RHS->isAllOnesValue()) {
+            Constant *NotBOC = ConstantExpr::getNot(BOC);
+            Value *And = Builder->CreateAnd(BO->getOperand(0), NotBOC);
+            return new ICmpInst(ICI.getPredicate(), And, NotBOC);
+          }
         }
         break;
 
       case Instruction::And:
         if (ConstantInt *BOC = dyn_cast<ConstantInt>(BO->getOperand(1))) {
           // If bits are being compared against that are and'd out, then the
           // comparison can never succeed!
           if ((RHSV & ~BOC->getValue()) != 0)

llvm/trunk/test/Transforms/InstCombine/icmp.ll

 ; CHECK-NEXT:    [[CMP:%.*]] = icmp uge i32 [[ADDX]], [[ADDY]]
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %addx = add i32 %x, 2147483647
   %addy = add i32 %y, 2147483647
   %cmp = icmp uge i32 %addx, %addy
   ret i1 %cmp
 }
+
+define i1 @cmp_inverse_mask_bits_set_eq(i32 %x) {
+; CHECK-LABEL: @cmp_inverse_mask_bits_set_eq(
+; CHECK-NEXT:    [[TMP1:%.*]] = and i32 %x, -43
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[TMP1]], -43
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %or = or i32 %x, 42
+  %cmp = icmp eq i32 %or, -1
+  ret i1 %cmp
+}
+
+define i1 @cmp_inverse_mask_bits_set_ne(i32 %x) {
+; CHECK-LABEL: @cmp_inverse_mask_bits_set_ne(
+; CHECK-NEXT:    [[TMP1:%.*]] = and i32 %x, -43
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ne i32 [[TMP1]], -43
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %or = or i32 %x, 42
+  %cmp = icmp ne i32 %or, -1
+  ret i1 %cmp
+}
+