diff options
| -rw-r--r-- | lib/Target/X86/README-MMX.txt | 28 |
1 files changed, 28 insertions, 0 deletions
diff --git a/lib/Target/X86/README-MMX.txt b/lib/Target/X86/README-MMX.txt index 7ad623e40d..a6c8616b6d 100644 --- a/lib/Target/X86/README-MMX.txt +++ b/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 |