[PATCH] D18842: [InstCombine] remove constant by inverting compare + logic (PR27105)

[Sanjay Patel via llvm-commits]

spatel created this revision.
spatel added reviewers: hfinkel, majnemer, nlopes, amehsan.
spatel added a subscriber: llvm-commits.
Herald added a subscriber: mcrosier.

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.

http://reviews.llvm.org/D18842

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp
  test/Transforms/InstCombine/icmp.ll

Index: test/Transforms/InstCombine/icmp.ll
===================================================================
--- test/Transforms/InstCombine/icmp.ll
+++ test/Transforms/InstCombine/icmp.ll
@@ -1956,3 +1956,26 @@
   %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
+}
+
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -2248,6 +2248,15 @@
           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;
 


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