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Date: Sun, 14 Jan 2018 18:21:09 -0600
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Arnd Bergmann <arnd@arndb.de>
Cc: "open list:HARDWARE RANDOM NUMBER GENERATOR CORE"
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Subject: Re: [cryptodev:master 130/134] aes_generic.c:undefined reference to
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On Sun, Jan 14, 2018 at 10:40:36PM +0100, Arnd Bergmann wrote:
> Right. I've done some more investigation anyway, starting over with the
> analysis of the gcc options that change it. I've found now that turning
> off '-fcode-hoisting' but leaving on the other options I had suspected
> earlier (-O2 instead of -Os, -ftree-sra, -ftree-pre) also fixes the
> stack problem, and appears to result in the best performance so
> far.

Oh nice!

> I need to rerun the whole test matrix, but that seems rather
> promising, and the result may also help debug what's really happening.

-fcode-hoisting moves all expression evaluation to as early as possible;
for this AES code that means it will increase register pressure a lot,
causing a lot of spilling (well, that is my guess).  If that is so, then
we need to dial down -fcode-hoisting a bit, maybe make it aware of
register pressure.

Glad you found a smoking gun,


Segher