[FastISel][x86] Teach how to select SSE2/AVX bitcasts between 128/256-bit vector types.

This patch teaches FastIsel the following two things:
1) On SSE2, no instructions are needed for bitcasts between 128-bit vector types;
2) On AVX, no instructions are needed for bitcasts between 256-bit vector types.

Example:

  %1 = bitcast <4 x i31> %V to <2 x i64>

Before (-fast-isel -fast-isel-abort=1):

  FastIsel miss: %1 = bitcast <4 x i31> %V to <2 x i64>

Now we don't fall back to SelectionDAG and we correctly fold that computation
propagating the register associated to %V.

Differential Revision: http://reviews.llvm.org/D13347

llvm-svn: 249121

1 files changed, 24 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86FastISel.cpp b/llvm/lib/Target/X86/X86FastISel.cpp
index 5eabd7a756f..17704da91c2 100644
--- a/llvm/lib/Target/X86/X86FastISel.cpp
+++ b/llvm/lib/Target/X86/X86FastISel.cpp
@@ -3234,6 +3234,30 @@ X86FastISel::fastSelectInstruction(const Instruction *I)  {
     updateValueMap(I, Reg);
     return true;
   }
+  case Instruction::BitCast: {
+    // Select SSE2/AVX bitcasts between 128/256 bit vector types.
+    if (!Subtarget->hasSSE2())
+      return false;
+
+    EVT SrcVT = TLI.getValueType(DL, I->getOperand(0)->getType());
+    EVT DstVT = TLI.getValueType(DL, I->getType());
+
+    if (!SrcVT.isSimple() || !DstVT.isSimple())
+      return false;
+
+    if (!SrcVT.is128BitVector() &&
+        !(Subtarget->hasAVX() && SrcVT.is256BitVector()))
+      return false;
+
+    unsigned Reg = getRegForValue(I->getOperand(0));
+    if (Reg == 0)
+      return false;
+      
+    // No instruction is needed for conversion. Reuse the register used by
+    // the fist operand.
+    updateValueMap(I, Reg);
+    return true;
+  }
   }
 
   return false;