Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns

Linus Torvalds
2015-03-26T00:05:56Z

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