Re: [PATCH] slub: reduce total stack usage of slab_err & object_err

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From: Matt Mackall
Date: Tuesday, September 30, 2008 - 11:33 am

On Tue, 2008-09-30 at 12:37 -0500, Matt Mackall wrote:

Ok, on closer inspection, this is part of the x86_64 calling convention.
When calling a varargs function, the caller passes the number of
floating point SSE regs used in rax. The callee then has to save these
away for va_list use. The GCC prologue apparently sets aside space for
xmm0-xmm7 (16 bytes each) all the time (plus rdi, rsi, rdx, rcx, r8, and
r9).

Obviously, we're never passing floating point args in the kernel, so
we're taking about a 40+ byte hit in code size and 128 byte hit in stack
size for every varargs call.

Looks like the gcc people have a patch in progress:

http://gcc.gnu.org/ml/gcc-patches/2008-08/msg02165.html

So I think we should assume that x86_64 will sort this out eventually.

-- 
Mathematics is the supreme nostalgia of our time.

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Re: [PATCH] slub: reduce total stack usage of slab_err & o ..., Matt Mackall, (Tue Sep 30, 11:33 am)