Rename ValueRequiresCast to ShouldOptimizeCast, to better reflect

what it does.  Enhance it to return false to optimizing vector
sign extensions from vector comparisions, which is the idiom used
to get a splatted vector for a vector comparison.

Doing this breaks vector-casts.ll, add some compensating 
transformations to handle the important case they cover without
depending on this canonicalization.

This fixes rdar://7434900 a serious pessimization of vector compares.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@95855 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 16 insertions, 7 deletions

diff --git a/lib/Transforms/InstCombine/InstCombineCasts.cpp b/lib/Transforms/InstCombine/InstCombineCasts.cpp
index 09cd21f144..68e17e59f6 100644
--- a/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -255,17 +255,26 @@ isEliminableCastPair(
   return Instruction::CastOps(Res);
 }
 
-/// ValueRequiresCast - Return true if the cast from "V to Ty" actually results
-/// in any code being generated.  It does not require codegen if V is simple
-/// enough or if the cast can be folded into other casts.
-bool InstCombiner::ValueRequiresCast(Instruction::CastOps opcode,const Value *V,
-                                     const Type *Ty) {
+/// ShouldOptimizeCast - Return true if the cast from "V to Ty" actually
+/// results in any code being generated and is interesting to optimize out. If
+/// the cast can be eliminated by some other simple transformation, we prefer
+/// to do the simplification first.
+bool InstCombiner::ShouldOptimizeCast(Instruction::CastOps opc, const Value *V,
+                                      const Type *Ty) {
+  // Noop casts and casts of constants should be eliminated trivially.
   if (V->getType() == Ty || isa<Constant>(V)) return false;
   
-  // If this is another cast that can be eliminated, it isn't codegen either.
+  // If this is another cast that can be eliminated, we prefer to have it
+  // eliminated.
   if (const CastInst *CI = dyn_cast<CastInst>(V))
-    if (isEliminableCastPair(CI, opcode, Ty, TD))
+    if (isEliminableCastPair(CI, opc, Ty, TD))
       return false;
+  
+  // If this is a vector sext from a compare, then we don't want to break the
+  // idiom where each element of the extended vector is either zero or all ones.
+  if (opc == Instruction::SExt && isa<CmpInst>(V) && isa<VectorType>(Ty))
+    return false;
+  
   return true;
 }