Add a note.

llvm-svn: 65275

1 files changed, 28 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/README-MMX.txt b/llvm/lib/Target/X86/README-MMX.txt
index 7ad623e40d5..a6c8616b6d2 100644
--- a/llvm/lib/Target/X86/README-MMX.txt
+++ b/llvm/lib/Target/X86/README-MMX.txt
@@ -41,3 +41,31 @@ _qux:
         addl    $12, %esp
         ret     $4
 
+//===---------------------------------------------------------------------===//
+
+We generate crappy code for this:
+
+__m64 t() {
+  return _mm_cvtsi32_si64(1);
+}
+
+_t:
+	subl	$12, %esp
+	movl	$1, %eax
+	movd	%eax, %mm0
+	movq	%mm0, (%esp)
+	movl	(%esp), %eax
+	movl	4(%esp), %edx
+	addl	$12, %esp
+	ret
+
+The extra stack traffic is covered in the previous entry. But the other reason
+is we are not smart about materializing constants in MMX registers. With -m64
+
+	movl	$1, %eax
+	movd	%eax, %mm0
+	movd	%mm0, %rax
+	ret
+
+We should be using a constantpool load instead:
+	movq	LC0(%rip), %rax