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//===---------------------------------------------------------------------===//
// Random ideas for the X86 backend: MMX-specific stuff.
//===---------------------------------------------------------------------===//
//===---------------------------------------------------------------------===//
This:
#include <mmintrin.h>
__v2si qux(int A) {
return (__v2si){ 0, A };
}
is compiled into:
_qux:
subl $28, %esp
movl 32(%esp), %eax
movd %eax, %mm0
movq %mm0, (%esp)
movl (%esp), %eax
movl %eax, 20(%esp)
movq %mm0, 8(%esp)
movl 12(%esp), %eax
movl %eax, 16(%esp)
movq 16(%esp), %mm0
addl $28, %esp
ret
Yuck!
GCC gives us:
_qux:
subl $12, %esp
movl 16(%esp), %eax
movl 20(%esp), %edx
movl $0, (%eax)
movl %edx, 4(%eax)
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
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