move the x86-32 part of PR2108 here.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@49465 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Target/X86/README-SSE.txt b/lib/Target/X86/README-SSE.txt
index 4d7224514cb93456c4dd2cda277a0cf92545768a..bbd93bf3ed404f3b0a86ba6a091eec5d69f98909 100644 (file)
--- a/lib/Target/X86/README-SSE.txt
+++ b/lib/Target/X86/README-SSE.txt
@@ -428,6 +428,54 @@ entry:
 
 //===---------------------------------------------------------------------===//
 
+Consider (PR2108):
+
+#include <xmmintrin.h>
+__m128i doload64(unsigned long long x) { return _mm_loadl_epi64(&x);}
+__m128i doload64_2(unsigned long long *x) { return _mm_loadl_epi64(x);}
+
+These are very similar routines, but we generate significantly worse code for
+the first one on x86-32:
+
+_doload64:
+       subl    $12, %esp
+       movl    20(%esp), %eax
+       movl    %eax, 4(%esp)
+       movl    16(%esp), %eax
+       movl    %eax, (%esp)
+       movsd   (%esp), %xmm0
+       addl    $12, %esp
+       ret
+_doload64_2:
+       movl    4(%esp), %eax
+       movsd   (%eax), %xmm0
+       ret
+
+The problem is that the argument lowering logic splits the i64 argument into
+2x i32 loads early, the f64 insert doesn't match.  Here's a reduced testcase:
+
+define fastcc double @doload64(i64 %x) nounwind  {
+entry:
+       %tmp717 = bitcast i64 %x to double              ; <double> [#uses=1]
+       ret double %tmp717
+}
+
+compiles to:
+
+_doload64:
+       subl    $12, %esp
+       movl    20(%esp), %eax
+       movl    %eax, 4(%esp)
+       movl    16(%esp), %eax
+       movl    %eax, (%esp)
+       movsd   (%esp), %xmm0
+       addl    $12, %esp
+       ret
+
+instead of movsd from the stack.
+
+//===---------------------------------------------------------------------===//
+
 __m128d test1( __m128d A, __m128d B) {
   return _mm_shuffle_pd(A, B, 0x3);
 }