Messages in this thread | | | Date | Wed, 25 Mar 2015 17:05:50 -0700 | Subject | Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns | From | Linus Torvalds <> |
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On Wed, Mar 25, 2015 at 4:56 PM, H. Peter Anvin <hpa@zytor.com> wrote: > No, movabs is yet another instruction (with a 64-bit absolute address.) But movq can mean 10 or 7 bytes...
I mentioned movabs because that is literally what as generates at least for me (or then objdump is confused):
[torvalds@i7 ~]$ as -v GNU assembler version 2.24 (x86_64-redhat-linux) using BFD version version 2.24
[torvalds@i7 ~]$ cat t.s main: movq $0x12, %rdi movq $0x1234, %rdi movq $0x123456, %rdi movq $0x12345678, %rdi movq $0x123456789ab, %rdi
[torvalds@i7 ~]$ as t.s [torvalds@i7 ~]$ objdump -d a.out ... 0: 48 c7 c7 12 00 00 00 mov $0x12,%rdi 7: 48 c7 c7 34 12 00 00 mov $0x1234,%rdi e: 48 c7 c7 56 34 12 00 mov $0x123456,%rdi 15: 48 c7 c7 78 56 34 12 mov $0x12345678,%rdi 1c: 48 bf ab 89 67 45 23 movabs $0x123456789ab,%rdi 23: 01 00 00
so 'as' is clearly just stupid. It already takes the size of the constant into account and generates different instructions. Why not for the common 32-bit case too?
Oh well.
Linus
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