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

Linus Torvalds
2015-03-25T23:51:54Z

On Wed, Mar 25, 2015 at 10:18 AM, Denys Vlasenko <dvlasenk@redhat.com> wrote:
> The $AUDIT_ARCH_X86_64 parameter to syscall_trace_enter_phase1/2
> is a 32-bit constant, loading it with 32-bit MOV produces 5-byte insn
> instead of 10-byte one.

Side note: has anybody talked to the assembler people? This would seem
to be very much something that the assembler could have noticed and
done on its own. It's a bit sad that we need to overspecify these
things..

If it had actually been a 64-bit constant, the assembler would have
ended up silently using a different instruction encoding *anyway*
("movabs"), so it's not like the "movq" in any way specifies one
particular instruction representation, and the assembler already picks
different instruction versions for different constant values. Why not
this one?

Linus