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

[Denys Vlasenko]

On Thu, Mar 26, 2015 at 11:25 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
> * Denys Vlasenko <vda.linux@xxxxxxxxxxxxxx> 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
>>                 mov $0x123456789,%rdi  # 10 bytes
>>                 movabs $0x12345678,%rdi # 10 bytes
>>
>>
>> $ gcc -nostartfiles -nostdlib -c z.S && objdump -dr z.o
>> z.o:     file format elf64-x86-64
>> Disassembly of section .text:
>> 0000000000000000 <_start>:
>>    0:    bf 78 56 34 12           mov    $0x12345678,%edi
>>    5:    48 c7 c7 78 56 34 12     mov    $0x12345678,%rdi
>>    c:    48 c7 c7 78 56 34 12     mov    $0x12345678,%rdi
>>   13:    48 bf 00 00 00 80 00     movabs $0x80000000,%rdi
>>   1a:    00 00 00
>>   1d:    48 bf 89 67 45 23 01     movabs $0x123456789,%rdi
>>   24:    00 00 00
>>   27:    48 bf 78 56 34 12 00     movabs $0x12345678,%rdi
>>   2e:    00 00 00
>
> I see, so:
>
>        movq $AUDIT_ARCH_X86_64, %rsi
>
> generated a 10-byte MOVABS opcode, while moving into %esi generates
> the 5-byte 32-bit MOV opcode?

Exactly.
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