Subzero: Build a better bitcast.

The x86 lowering of bitcast between integers and floats forced the transfer through a stack slot (the original implementer *cough* *cough* wasn't aware of the movd instruction).  This requires excess instructions, but also a store to memory followed immediately by a load from that location is very slow.

This fixes the problem by using the movd instruction instead.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2077503002 .

src/IceTargetLoweringX86BaseImpl.h

diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index b28c009..3ed27cb 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -3059,24 +3059,10 @@
     } break;
     case IceType_i32:
     case IceType_f32: {
-      Operand *Src0RM = legalize(Src0, Legal_Reg | Legal_Mem);
-      Type SrcType = Src0RM->getType();
-      assert((DestTy == IceType_i32 && SrcType == IceType_f32) ||
-             (DestTy == IceType_f32 && SrcType == IceType_i32));
-      // a.i32 = bitcast b.f32 ==>
-      //   t.f32 = b.f32
-      //   s.f32 = spill t.f32
-      //   a.i32 = s.f32
-      Variable *T = nullptr;
-      // TODO: Should be able to force a spill setup by calling legalize() with
-      // Legal_Mem and not Legal_Reg or Legal_Imm.
-      SpillVariable *SpillVar = Func->makeVariable<SpillVariable>(SrcType);
-      SpillVar->setLinkedTo(Dest);
-      Variable *Spill = SpillVar;
-      Spill->setMustNotHaveReg();
-      _mov(T, Src0RM);
-      _mov(Spill, T);
-      _mov(Dest, Spill);
+      Variable *Src0R = legalizeToReg(Src0);
+      Variable *T = makeReg(DestTy);
+      _movd(T, Src0R);
+      _mov(Dest, T);
     } break;
     case IceType_i64: {
       assert(Src0->getType() == IceType_f64);

tests_lit/llvm2ice_tests/bitcast.ll

diff --git a/tests_lit/llvm2ice_tests/bitcast.ll b/tests_lit/llvm2ice_tests/bitcast.ll
index b5e9066..399d3fe 100644
--- a/tests_lit/llvm2ice_tests/bitcast.ll
+++ b/tests_lit/llvm2ice_tests/bitcast.ll
@@ -19,7 +19,7 @@
   ret i32 %v0
 }
 ; CHECK-LABEL: cast_f2i
-; CHECK: mov eax
+; CHECK: movd eax
 ; ARM32-LABEL: cast_f2i
 ; ARM32: vmov r{{[0-9]+}}, s{{[0-9]+}}