> -----Original Message-----
> From: Paulo Marques [mailto:pmarq...@grupopie.com]
> Sent: Thursday, September 23, 2010 7:57 AM
> To: Weddington, Eric
> Cc: Johannes Bauer; avr-gcc-list@nongnu.org; Praveen, Vidya;
> Singh, Abnikant
> Subject: Re: [avr-gcc-list] Missed optimization or am *I*
> missing something?
Hi Paulo,
>
> Yes, it seems that in the WEIRD=0 case, the compiler thinks the loop
> gets simple enough to be unrolled. If you look at the long version,
> there is no loop at all in it.
Blech. Why on earth does the compiler think that unrolling any loop produces
*smaller* code when the user specifies -Os? We seriously need to look at the
conditions that trigger that optimization.
> One thing I noticed a long time ago with avr-gcc (I haven't
> checked more
> recent versions for this) is that splitting expressions into simpler
> ones helps the compiler detect that it doesn't need to upgrade the
> variables to 16 bit int's.
>
> For instance, if you write the code as:
>
> unsigned char tmp;
> ...
> offset = DMA.CH0.TRFCNT;
> offset -= WEIRD;
> boundedBufferValueSum = 0;
> for (i = 0; i < 4; i++) {
> tmp = offset;
> tmp += i;
> tmp += WEIRD;
> tmp %= FOOBUFSIZE;
> boundedBufferValueSum += fooBoundedBuffer[tmp];
> }
>
> you'll probably get smaller code. Yes, it's ugly, but if that function
> is in some hot path, it might be worth it...
True. However, we're at a point where we want to start fixing the compiler
back-end so the user doesn't have to write code to adapt to the compiler's poor
optimizations.
Eric
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