Messages in this thread |  | | Date | Wed, 6 Feb 2013 19:19:05 -0600 | From | Kim Phillips <> | Subject | Re: [RFC] arm: use built-in byte swap function |
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On Wed, 6 Feb 2013 09:02:04 +0000 "Woodhouse, David" <david.woodhouse@intel.com> wrote:
> On Tue, 2013-02-05 at 21:04 -0600, Kim Phillips wrote: > > gcc -Os emits calls to __bswapsi2 on those platforms to save space > > because they don't have the single rev byte swap instruction. > > Is that the right thing for GCC to do in that situation?
if it saves space, why wouldn't it be?
"Many of these functions are only optimized in certain cases; if they are not optimized in a particular case, a call to the library function is emitted." [1]
I see "(arm_arch6 || !optimize_size)" in gcc's define_expand "bswapsi2" source, so GCC considers size optimization as a legitimate one of those cases.
> If so, perhaps we should be *providing* __bswap[sd]i2 functions for it > to use?
either that, or link with libgcc - why does arch/arm64 do this and arch/arm not? It's not obvious from git log.
> If not, perhaps there should be a PR filed? > > Or is our use case justifiably different to the general case of '-Os'? > If so, why?
shouldn't be - a patch, such as this, that claims to reduce code size, and that only turns on the new built-in when CC_OPTIMIZE_FOR_SIZE is off, is generally not good :)
OTOH, the target here is armv6+ performance - not armv4,5 code density - the OPTIMIZE_FOR_SIZE protection prevents armv4,5 build breakage.
Kim
[1] http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html#Other-Builtins
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