[InstCombine] allow any pair of bitcasts to be combined

This change is discussed in D15392 and should allow us to effectively
revert:
http://llvm.org/viewvc/llvm-project?view=revision&revision=255261
if we canonicalize bitcasts ahead of extracts.

It should be safe to convert any pair of bitcasts into a single bitcast, 
however, it was mentioned here:
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20110829/127089.html
that we're not allowed to bitcast from an x86_mmx to some other types, but I'm 
not seeing any failures from that, and we have regression tests in CodeGen/X86
that appear to cover all of those cases. 

Some day we'll get to remove that MMX wart from LLVM IR completely?

Differential Revision: http://reviews.llvm.org/D15468

llvm-svn: 255399

1 files changed, 12 insertions, 10 deletions

diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index 839908268c9..11c55865135 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -2516,17 +2516,19 @@ unsigned CastInst::isEliminableCastPair(
     {  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,13,12}, // AddrSpaceCast -+
   };
 
+  // TODO: This logic could be encoded into the table above and handled in the
+  // switch below.
   // If either of the casts are a bitcast from scalar to vector, disallow the
-  // merging. However, bitcast of A->B->A are allowed.
-  bool isFirstBitcast  = (firstOp == Instruction::BitCast);
-  bool isSecondBitcast = (secondOp == Instruction::BitCast);
-  bool chainedBitcast  = (SrcTy == DstTy && isFirstBitcast && isSecondBitcast);
-
-  // Check if any of the bitcasts convert scalars<->vectors.
-  if ((isFirstBitcast  && isa<VectorType>(SrcTy) != isa<VectorType>(MidTy)) ||
-      (isSecondBitcast && isa<VectorType>(MidTy) != isa<VectorType>(DstTy)))
-    // Unless we are bitcasting to the original type, disallow optimizations.
-    if (!chainedBitcast) return 0;
+  // merging. However, any pair of bitcasts are allowed.
+  bool IsFirstBitcast  = (firstOp == Instruction::BitCast);
+  bool IsSecondBitcast = (secondOp == Instruction::BitCast);
+  bool AreBothBitcasts = IsFirstBitcast && IsSecondBitcast;
+
+  // Check if any of the casts convert scalars <-> vectors.
+  if ((IsFirstBitcast  && isa<VectorType>(SrcTy) != isa<VectorType>(MidTy)) ||
+      (IsSecondBitcast && isa<VectorType>(MidTy) != isa<VectorType>(DstTy)))
+    if (!AreBothBitcasts)
+      return 0;
 
   int ElimCase = CastResults[firstOp-Instruction::CastOpsBegin]
                             [secondOp-Instruction::CastOpsBegin];