[PATCH] D38065: [instCombine] Handle (X & C2) < C1 --> (X & C2) == 0

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

We already did (X & C2) > C1 --> (X & C2) != 0, if any bit set in (X & C2) will produce a result greater than C1.   But there is an equivalent inverse condition with <= C1 (which will be canonicalized to < C1+1)


https://reviews.llvm.org/D38065

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
@@ -1141,7 +1141,7 @@
 define i1 @test67inverse(i32 %x) {
 ; CHECK-LABEL: @test67inverse(
 ; CHECK-NEXT:    [[AND:%.*]] = and i32 [[X:%.*]], 96
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ult i32 [[AND]], 32
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[AND]], 0
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %and = and i32 %x, 127
Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1715,14 +1715,22 @@
   }
 
   // (X & C2) > C1 --> (X & C2) != 0, if any bit set in (X & C2) will produce a
-  // result greater than C1.
+  // result greater than C1. Also handle (X & C2) < C1 --> (X & C2) == 0.
   unsigned NumTZ = C2->countTrailingZeros();
-  if (Cmp.getPredicate() == ICmpInst::ICMP_UGT && NumTZ < C2->getBitWidth() &&
-      APInt::getOneBitSet(C2->getBitWidth(), NumTZ).ugt(*C1)) {
-    Constant *Zero = Constant::getNullValue(And->getType());
-    return new ICmpInst(ICmpInst::ICMP_NE, And, Zero);
+  if (NumTZ < C2->getBitWidth()) {
+    if (Cmp.getPredicate() == ICmpInst::ICMP_UGT &&
+        NumTZ >= C1->getActiveBits()) {
+      Constant *Zero = Constant::getNullValue(And->getType());
+      return new ICmpInst(ICmpInst::ICMP_NE, And, Zero);
+    }
+    if (Cmp.getPredicate() == ICmpInst::ICMP_ULT &&
+        NumTZ >= C1->ceilLogBase2()) {
+      Constant *Zero = Constant::getNullValue(And->getType());
+      return new ICmpInst(ICmpInst::ICMP_EQ, And, Zero);
+    }
   }
 
+
   return nullptr;
 }
 


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