[InstCombine] remove constant by inverting compare + logic (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.

Differential Revision: http://reviews.llvm.org/D18842


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@266362 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -2248,6 +2248,15 @@ Instruction *InstCombiner::visitICmpInstWithInstAndIntCst(ICmpInst &ICI,
           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;
 

test/Transforms/InstCombine/icmp.ll

@@ -1956,3 +1956,26 @@ define i1 @PR26407(i32 %x, i32 %y) {
   %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
+}
+