Fold fcmp in cases where value is provably non-negative. By Arch Robison.

This patch folds fcmp in some cases of interest in Julia. The patch adds a function CannotBeOrderedLessThanZero that returns true if a value is provably not less than zero. I.e. the function returns true if the value is provably -0, +0, positive, or a NaN. The patch extends InstructionSimplify.cpp to fold instances of fcmp where:
 - the predicate is olt or uge
 - the first operand is provably not less than zero
 - the second operand is zero
The motivation for handling these cases optimizing away domain checks for sqrt in Julia for common idioms such as sqrt(x*x+y*y)..

http://reviews.llvm.org/D6972

llvm-svn: 227298

1 files changed, 14 insertions, 0 deletions

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 3fbbd7cbd22..588d625910f 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -3087,6 +3087,20 @@ static Value *SimplifyFCmpInst(unsigned Predicate, Value *LHS, Value *RHS,
           }
         }
       }
+      if (CFP->getValueAPF().isZero()) {
+        switch (Pred) {
+        case FCmpInst::FCMP_UGE:
+          if (CannotBeOrderedLessThanZero(LHS)) 
+            return ConstantInt::getTrue(CFP->getContext());
+          break;
+        case FCmpInst::FCMP_OLT:
+          if (CannotBeOrderedLessThanZero(LHS)) 
+            return ConstantInt::getFalse(CFP->getContext());
+          break;
+        default:
+          break;
+        }
+     }
     }
   }