Messages in this thread | | | Date | Thu, 26 Mar 2015 10:37:32 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 3/4] x86/asm/entry/64: use smaller insns |
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* Borislav Petkov <bp@alien8.de> 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.
Thanks,
Ingo
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