ARM GCC pessimizer

[Paulo Matos]

On 18/03/14 21:03, Paul D. DeRocco wrote:
> I'm using arm-none-eabi-gcc 4.7.3, compiling for an M4 with the following
> options:
>
> -mcpu=cortex-m4
> -mthumb
> -mfloat-abi=hard
> -mfpu=fpv4-sp-d16
> -g3
> -gdwarf-2
> -gstrict-dwarf
> -O3
> -ffunction-sections
> -fdata-sections
> -std=gnu99
> -fsigned-char
> -D__VFPV4__
> -specs=ewl_c_noio.specs
>
> Here's a small code fragment, part of a FIR filter:
>
> i = T00 + T10 + T20 + T30 + T40 + T50 + T60 + T70
>          + T80 + T90 + TA0 + TB0 + TC0 + TD0 + TE0 + TF0
>          - ((2 * T00) & -((s >>  0) & 1))
>          - ((2 * T10) & -((s >>  2) & 1))
>          - ((2 * T20) & -((s >>  4) & 1))
>          - ((2 * T30) & -((s >>  6) & 1))
>          - ((2 * T40) & -((s >>  8) & 1))
>          - ((2 * T50) & -((s >> 10) & 1))
>          - ((2 * T60) & -((s >> 12) & 1))
>          - ((2 * T70) & -((s >> 14) & 1))
>          - ((2 * T80) & -((s >> 16) & 1))
>          - ((2 * T90) & -((s >> 18) & 1))
>          - ((2 * TA0) & -((s >> 20) & 1))
>          - ((2 * TB0) & -((s >> 22) & 1))
>          - ((2 * TC0) & -((s >> 24) & 1))
>          - ((2 * TD0) & -((s >> 26) & 1))
>          - ((2 * TE0) & -((s >> 28) & 1))
>          - ((2 * TF0) & -((s >> 30) & 1));
>
> s is an unsigned int containing bits to be filtered. The T* symbols are
> #defined constants. The compiler cleverly compiles "-((s >> #) & 1" into a
> signed bit-field extract instruction, which picks out the bit, right
> justifies it, and propagates it through all 32 bits. For a while, it was
> sane enough to load the initial constant (the sum of all the T* symbols)
> into a register, then for each bit, compute the mask, AND each one with
> the corresponding constant, and subtract it from the register. Then, all
> of a sudden, some other change prompted it to compute each mask and store
> it into a local variable on the stack, and then use it later. Since there
> are actually eight pieces of code like this, the result is huge,
> memory-intensive, and slow.
>
> What mechanism would prompt the compiler to do such a dumb thing? Is there
> any optimization option that relates to this? I've tried -O3 and -O2,
> tried various "register" declarations, dipped my toe into a few of the
> specific optimizations listed in the docs, but there are a ton of them.
> Any ideas?
>


Could you please post some code that we can compile and give it a try 
ourselves. At least for me, it's hard to understand what's going on if I 
can't compile the example. And from the small snippet you show it's hard 
to guess not knowing the rest of the code, types of each variable, etc.

-- 
PMatos