[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

2 files changed, 18 insertions, 22 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];
diff --git a/llvm/test/Transforms/InstCombine/bitcast-bitcast.ll b/llvm/test/Transforms/InstCombine/bitcast-bitcast.ll
index a67666c60a5..0f46ff53bc1 100644
--- a/llvm/test/Transforms/InstCombine/bitcast-bitcast.ll
+++ b/llvm/test/Transforms/InstCombine/bitcast-bitcast.ll
@@ -18,8 +18,7 @@ define <2 x i32> @bitcast_bitcast_s_s_v(i64 %a) {
   ret <2 x i32> %bc2
 
 ; CHECK-LABEL: @bitcast_bitcast_s_s_v(
-; CHECK-NEXT:  %bc1 = bitcast i64 %a to double
-; CHECK-NEXT:  %bc2 = bitcast double %bc1 to <2 x i32>
+; CHECK-NEXT:  %bc2 = bitcast i64 %a to <2 x i32>
 ; CHECK-NEXT:  ret <2 x i32> %bc2
 }
 
@@ -29,8 +28,7 @@ define double @bitcast_bitcast_s_v_s(i64 %a) {
   ret double %bc2
 
 ; CHECK-LABEL: @bitcast_bitcast_s_v_s(
-; CHECK-NEXT:  %bc1 = bitcast i64 %a to <2 x i32>
-; CHECK-NEXT:  %bc2 = bitcast <2 x i32> %bc1 to double
+; CHECK-NEXT:  %bc2 = bitcast i64 %a to double
 ; CHECK-NEXT:  ret double %bc2
 }
 
@@ -40,8 +38,7 @@ define <2 x i32> @bitcast_bitcast_s_v_v(i64 %a) {
   ret <2 x i32> %bc2
 
 ; CHECK-LABEL: @bitcast_bitcast_s_v_v(
-; CHECK-NEXT:  %bc1 = bitcast i64 %a to <4 x i16>
-; CHECK-NEXT:  %bc2 = bitcast <4 x i16> %bc1 to <2 x i32>
+; CHECK-NEXT:  %bc2 = bitcast i64 %a to <2 x i32>
 ; CHECK-NEXT:  ret <2 x i32> %bc2
 }
 
@@ -51,8 +48,7 @@ define i64 @bitcast_bitcast_v_s_s(<2 x i32> %a) {
   ret i64 %bc2
 
 ; CHECK-LABEL: @bitcast_bitcast_v_s_s(
-; CHECK-NEXT:  %bc1 = bitcast <2 x i32> %a to double
-; CHECK-NEXT:  %bc2 = bitcast double %bc1 to i64
+; CHECK-NEXT:  %bc2 = bitcast <2 x i32> %a to i64
 ; CHECK-NEXT:  ret i64 %bc2
 }
 
@@ -62,8 +58,7 @@ define <4 x i16> @bitcast_bitcast_v_s_v(<2 x i32> %a) {
   ret <4 x i16> %bc2
 
 ; CHECK-LABEL: @bitcast_bitcast_v_s_v(
-; CHECK-NEXT:  %bc1 = bitcast <2 x i32> %a to double
-; CHECK-NEXT:  %bc2 = bitcast double %bc1 to <4 x i16>
+; CHECK-NEXT:  %bc2 = bitcast <2 x i32> %a to <4 x i16>
 ; CHECK-NEXT:  ret <4 x i16> %bc2
 }
 
@@ -73,8 +68,7 @@ define double @bitcast_bitcast_v_v_s(<2 x float> %a) {
   ret double %bc2
 
 ; CHECK-LABEL: @bitcast_bitcast_v_v_s(
-; CHECK-NEXT:  %bc1 = bitcast <2 x float> %a to <4 x i16>
-; CHECK-NEXT:  %bc2 = bitcast <4 x i16> %bc1 to double
+; CHECK-NEXT:  %bc2 = bitcast <2 x float> %a to double
 ; CHECK-NEXT:  ret double %bc2
 }