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Date: Tue, 11 Dec 2012 09:49:00 -0000
Message-ID: <CAFiYyc000_e9R4HDpjVUpJuooDbnk6NVH3BkUTU3jZRwEj=dpw@mail.gmail.com>
Subject: Re: [PATCH i386]: Enable push/pop in pro/epilogue for modern CPUs
From: Richard Biener <richard.guenther@gmail.com>
To: Mike Stump <mikestump@comcast.net>
Cc: Xinliang David Li <davidxl@google.com>, Uros Bizjak <ubizjak@gmail.com>, 	GCC Patches <gcc-patches@gcc.gnu.org>
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On Mon, Dec 10, 2012 at 10:07 PM, Mike Stump <mikestump@comcast.net> wrote:
> On Dec 10, 2012, at 12:42 PM, Xinliang David Li <davidxl@google.com> wrot=
e:
>> I have not measured the CFI size impact -- but conceivably it should
>> be larger -- which is unfortunate.
>
> Code speed and size are preferable to optimizing dwarf size=85  :-)  I'd =
let dwarf 5 fix it!

Well, different to debug info, CFI data has to be in memory to make
unwinding work.
These days most Linux distributions enable asyncronous unwind tables so any
size savings due to shorter push/pop epilogue/prologue sequences has to be
offsetted by the increase in CFI data.  I'm not sure there is really a
speed difference
between both variants (well, maybe due to better icache footprint of
the push/pop
variant).

That said - I'd prefer to have more data on this before making the switch f=
or
the generic model.  What was your original motivation?  Just "theory" or was
it a real case?

Thanks,
Richard.