Loop optimizations

[Ian Lance Taylor]

Aurelien Buhrig <aurelien.buhrig.gcc@gmail.com> writes:

> Is there some resources explaining how GCC generates the first RTL
> from Gimple, why some alternatives fail, and what should be done to
> get what we expect ?

Nothing very specific, no.

> For instance, a concrete example with this code:
>
> uint32_t M1[16], M2[16];
>
> int i;
> for (i=0, i<16; i++)
>  M1[i] = M2[i];
>
> The generated code is quite horrible... GCC is messed up with
> SI/PSI/HI trunc and extend, and keeps 2 variables, 1 incremented by 1
> and shifted 2 for addresses, 1 decremented from 16 to 0 for the
> loop...
> I would like GCC to do what it does before I extend Pmode, i.e. extend
> "i" from HI to PSI, use it to address M1 and M2, increment by
> sizeof(uint32_t), and test if i != 16*sizeof(uint32_t)...
>
> So is this problem related to md file?
> What should I look at to find such optimization problems?

I don't see offhand why this would be related to RTL expansion.  Look at
the dump files.  What does the code look like in the last tree dump?
Look at the dump files for the loop passes in particular; what kind of
optimizations do they see or fail to see?

Ian