stack alignment in the kernel was Re: nasm over gas?

[Andi Kleen <ak@muc.de>]

Jamie Lokier <jamie@shareable.org> writes:

> Obvious the _intent_ of -O2 is to compile for speed, but it's clear
> that GCC often emits trivially redundant instructions (like stack
> adjustments) that don't serve to speed up the program at all.

The stack adjustments are for getting good performance with floating
point code. Most x86 CPUs require 16 byte alignment for floating point
stores/loads on the stack. It can make a dramatic difference in some 
FP intensive programs.

But obviously that's completely useless for the kernel which never
uses floating point.

A compiler option to turn it off would make sense to save .text space
and eliminate these useless instructions. Especially since the kernel
entry points make no attempt to align the stack to 16 byte anyways,
so most likely the stack adjustments do not even work.

(this option could also warn for floating point usage which is usually illegal,
although you can already get the same effect by compiling with -msoft-float)

-Andi