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

Denys Vlasenko
2015-03-26T10:08:09Z

On Thu, Mar 26, 2015 at 10:37 AM, Ingo Molnar <mingo@kernel.org> wrote:
>> On Wed, Mar 25, 2015 at 05:05:50PM -0700, Linus Torvalds wrote:
>> > 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?
>>
>> I think the destination register mandates which insn to use:
>>
>> mov $0x12345678, %rdi
>> mov $0x12345678, %edi
>>
>> ...
>>
>> 15: 48 c7 c7 78 56 34 12 mov $0x12345678,%rdi
>> 1c: bf 78 56 34 12 mov $0x12345678,%edi
>>
>> and 'as' is perhaps not insolent enough to go and change it when
>> seeing the 32-bit constant.
>
> Well, GAS generally has the freedom to use more optimal opcodes, as
> long as behavior on the CPU matches what the specified opcode does.
>
> Saying that 'movq $0x1, %rdi' should map to the longer variant is
> defensible: there should probably always be a way to specify a very
> specific opcode even if it's not optimal - say I want to fill in an
> alignment space and don't want to use an extra NOP.
>
> But here GAS generates the 10-byte opcode even if 'mov $0x12345678,
> %rdi' is used, which is an unforced error.

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