[PATCH] D38195: [InstCombine] Move an optimization from foldICmpAndConstConst to foldICmpUsingKnownBits

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.

All this optimization cares about is knowing how many low bits of LHS is known to be zero and whether that means that the result is 0 or greater than the RHS constant. It doesn't matter where the zeros in the low bits came from. So we don't need to specifically look for an AND. Instead we can use known bits.


https://reviews.llvm.org/D38195

Files:
  lib/Transforms/InstCombine/InstCombineCompares.cpp


Index: lib/Transforms/InstCombine/InstCombineCompares.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -1714,22 +1714,6 @@
     }
   }
 
-  // (X & C2) > C1 --> (X & C2) != 0, if any bit set in (X & C2) will produce a
- // result greater than C1. Also handle (X & C2) < C1 --> (X & C2) == 0.
-  if (!C2->isNullValue()) {
-    unsigned NumTZ = C2->countTrailingZeros();
-    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;
 }
 
@@ -4221,6 +4205,11 @@
       if (Op1Min == Op0Min + 1)
         return new ICmpInst(ICmpInst::ICMP_EQ, Op0,
                             ConstantInt::get(Op0->getType(), Op1Min - 1));
+      // X <u C --> X == 0, if the number of zero bits in the bottom of X
+      // exceeds the log2 of C.
+      if (Op0Known.countMinTrailingZeros() >= Op1Min.ceilLogBase2())
+        return new ICmpInst(ICmpInst::ICMP_EQ, Op0,
+                            Constant::getNullValue(Op1->getType()));
     }
     break;
   }
@@ -4237,6 +4226,11 @@
       if (Op1Min == Op0Max - 1)
         return new ICmpInst(ICmpInst::ICMP_EQ, Op0,
                             ConstantInt::get(Op1->getType(), Op1Min + 1));
+      // X >u C --> X != 0, if the number of zero bits in the bottom of X
+      // exceeds the log2 of C.
+      if (Op0Known.countMinTrailingZeros() >= Op1Min.getActiveBits())
+        return new ICmpInst(ICmpInst::ICMP_NE, Op0,
+                            Constant::getNullValue(Op1->getType()));
     }
     break;
   }


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