72 lines
1.5 KiB
Plaintext
72 lines
1.5 KiB
Plaintext
//===---------------------------------------------------------------------===//
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// Random ideas for the X86 backend: MMX-specific stuff.
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//===---------------------------------------------------------------------===//
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//===---------------------------------------------------------------------===//
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This:
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#include <mmintrin.h>
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__v2si qux(int A) {
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return (__v2si){ 0, A };
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}
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is compiled into:
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_qux:
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subl $28, %esp
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movl 32(%esp), %eax
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movd %eax, %mm0
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movq %mm0, (%esp)
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movl (%esp), %eax
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movl %eax, 20(%esp)
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movq %mm0, 8(%esp)
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movl 12(%esp), %eax
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movl %eax, 16(%esp)
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movq 16(%esp), %mm0
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addl $28, %esp
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ret
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Yuck!
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GCC gives us:
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_qux:
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subl $12, %esp
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movl 16(%esp), %eax
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movl 20(%esp), %edx
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movl $0, (%eax)
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movl %edx, 4(%eax)
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addl $12, %esp
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ret $4
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//===---------------------------------------------------------------------===//
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We generate crappy code for this:
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__m64 t() {
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return _mm_cvtsi32_si64(1);
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}
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_t:
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subl $12, %esp
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movl $1, %eax
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movd %eax, %mm0
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movq %mm0, (%esp)
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movl (%esp), %eax
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movl 4(%esp), %edx
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addl $12, %esp
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ret
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The extra stack traffic is covered in the previous entry. But the other reason
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is we are not smart about materializing constants in MMX registers. With -m64
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movl $1, %eax
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movd %eax, %mm0
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movd %mm0, %rax
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ret
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We should be using a constantpool load instead:
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movq LC0(%rip), %rax
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