[llvm-commits] [llvm] r65275 - /llvm/trunk/lib/Target/X86/README-MMX.txt

[Evan Cheng]

Author: evancheng
Date: Sun Feb 22 02:13:45 2009
New Revision: 65275

URL: http://llvm.org/viewvc/llvm-project?rev=65275&view=rev
Log:
Add a note.

Modified:
    llvm/trunk/lib/Target/X86/README-MMX.txt

Modified: llvm/trunk/lib/Target/X86/README-MMX.txt
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/README-MMX.txt?rev=65275&r1=65274&r2=65275&view=diff

==============================================================================
--- llvm/trunk/lib/Target/X86/README-MMX.txt (original)
+++ llvm/trunk/lib/Target/X86/README-MMX.txt Sun Feb 22 02:13:45 2009
@@ -41,3 +41,31 @@
         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