Messages in this thread | | | Date | Thu, 26 Mar 2015 11:25:51 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns |
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On Thu, Mar 26, 2015 at 11:07:42AM +0100, Denys Vlasenko wrote: > In my experiment, GAS uses 10-byte insn only for constants which > won't work with 7-byte encoding; or if I explicitly ask for "movabs": > > _start: .globl _start > mov $0x12345678,%edi # 5 bytes > mov $0x12345678,%rdi # 7 bytes > movq $0x12345678,%rdi # 7 bytes > mov $0x80000000,%rdi # 10 bytes
Right, and since they're signed immediates, the AUDIT_ARCH_X86_64 thing is 0xc000003e and does not fit in an s32, thus the 64-bit immediate with bf opcode:
mov $0x80000000-1,%rdi mov $0x80000000,%rdi mov $0xc000003e,%rdi mov $0xc000003e,%edi
...
21: 48 c7 c7 ff ff ff 7f mov $0x7fffffff,%rdi 28: 48 bf 00 00 00 80 00 movabs $0x80000000,%rdi 2f: 00 00 00 32: 48 bf 3e 00 00 c0 00 movabs $0xc000003e,%rdi 39: 00 00 00 3c: bf 3e 00 00 c0 mov $0xc000003e,%edi
Makes sense to me.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply. --
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