optimize bitcast(trunc(bitcast(x))) where the result is a float and 'x'

is a vector to be a vector element extraction.  This allows clang to
compile:

struct S { float A, B, C, D; };
float foo(struct S A) { return A.A + A.B+A.C+A.D; }

into:

_foo:                                   ## @foo
## BB#0:                                ## %entry
	movd	%xmm0, %rax
	shrq	$32, %rax
	movd	%eax, %xmm2
	addss	%xmm0, %xmm2
	movapd	%xmm1, %xmm3
	addss	%xmm2, %xmm3
	movd	%xmm1, %rax
	shrq	$32, %rax
	movd	%eax, %xmm0
	addss	%xmm3, %xmm0
	ret

instead of:

_foo:                                   ## @foo
## BB#0:                                ## %entry
	movd	%xmm0, %rax
	movd	%eax, %xmm0
	shrq	$32, %rax
	movd	%eax, %xmm2
	addss	%xmm0, %xmm2
	movd	%xmm1, %rax
	movd	%eax, %xmm1
	addss	%xmm2, %xmm1
	shrq	$32, %rax
	movd	%eax, %xmm0
	addss	%xmm1, %xmm0
	ret

... eliminating half of the horribleness.

llvm-svn: 112227

1 files changed, 22 insertions, 0 deletions

diff --git a/llvm/test/Transforms/InstCombine/bitcast.ll b/llvm/test/Transforms/InstCombine/bitcast.ll
index 88fa9a43e54..c248b5e4bc8 100644
--- a/llvm/test/Transforms/InstCombine/bitcast.ll
+++ b/llvm/test/Transforms/InstCombine/bitcast.ll
@@ -13,3 +13,25 @@ define i32 @test1(i64 %a) {
 ; CHECK: ret i32 0
 }
 
+; Optimize bitcasts that are extracting low element of vector.  This happens
+; because of SRoA.
+; rdar://7892780
+define float @test2(<2 x float> %A, <2 x i32> %B) {
+  %tmp28 = bitcast <2 x float> %A to i64  ; <i64> [#uses=2]
+  %tmp23 = trunc i64 %tmp28 to i32                ; <i32> [#uses=1]
+  %tmp24 = bitcast i32 %tmp23 to float            ; <float> [#uses=1]
+
+  %tmp = bitcast <2 x i32> %B to i64
+  %tmp2 = trunc i64 %tmp to i32                ; <i32> [#uses=1]
+  %tmp4 = bitcast i32 %tmp2 to float            ; <float> [#uses=1]
+
+  %add = fadd float %tmp24, %tmp4
+  ret float %add
+  
+; CHECK: @test2
+; CHECK-NEXT:  %tmp24 = extractelement <2 x float> %A, i32 0
+; CHECK-NEXT:  bitcast <2 x i32> %B to <2 x float>
+; CHECK-NEXT:  %tmp4 = extractelement <2 x float> {{.*}}, i32 0
+; CHECK-NEXT:  %add = fadd float %tmp24, %tmp4
+; CHECK-NEXT:  ret float %add
+}