RE: Odd performance regression with -Os

["Weddington, Eric" <eweddington at cso dot atmel dot com>]

> -----Original Message-----
> From: Mark Mitchell [mailto:mark@codesourcery.com] 
> Sent: Monday, December 29, 2008 11:51 AM
> To: Andrew Haley
> Cc: Eric Botcazou; gcc@gcc.gnu.org; Georg-Johann Lay
> Subject: Re: Odd performance regression with -Os
> 
> Andrew Haley wrote:
> > Eric Botcazou wrote:
> >>> Thanks.  Are you holding this because we're in Stage 3?
> >> The patch was written very recently so I wanted to let it 
> go through a good 
> >> deal of internal testing.  Moveover I haven't measured its 
> impact on anything 
> >> else than Ada benchmarks (and on a patched 4.3 branch).  
> If people think that
> >> it would be worth having for the 4.4 release, I can port 
> it and conduct basic 
> >> testing with it on the mainline, but that's pretty much it.
> > 
> > Well, it's a fairly nasty regression on embedded targets with no
> > multiplier, where people are likely to use -Os.  Sounds to me like
> > it qualifies for 4.4
> 
> I agree.

I just tried Eric's patch <http://gcc.gnu.org/ml/gcc/2008-12/msg00330.html> for the AVR on 4.3.2 (patch slightly modified to patch against the 4.3.2 release) and tested with Andrew's original test case. The AVR is a perfect target to test this as it is an 8-bit embedded processor, there are a number of variants that do not have multiply instructions, and almost all applications are compiled with -Os.

I compiled the original test case using -mmcu=at90usb82 -Os. It compiled to 15 instructions at 32 bytes, with a call to __mulhi3. With the patch, the test case compiled to 10 instructions at 20 bytes, and no call to libgcc.

I haven't regtested the patch yet, but so far I like what I see.

Thanks,
Eric Weddington