Re: How avoid bad optimization in gcc >= 4.3?

[Georg-Johann Lay <avr at gjlay dot de>]

Andrew Haley schrieb:
> Georg-Johann Lay wrote:
>
>
> > is there a way to prevent gcc 4.3.x from doing bad optimizations?
>
>
> We don't have a -fno-bad-optimizations flag; patches welcome!

I could patch it in the avr backend, but IMHO it is not good to
clutter up backend code because the middle end is on a trip ;-)

> > I attached an example that shows the performance degradation for
> > a source compiled with -Os (-O2 is similar) for target avr.
> >
> > The code from the new compiler increased by at about 30%
> > with respect to good old 3.4.6 and needs more as double
> > of stack slots (13 instead of 6).
> >
> >   text    data     bss     dec     hex filename
> >    156       0       0     156      9c foo-3.4.6.o
> >    206       0       0     206      ce foo-4.3.2.o
> >
> > Moreover, it expands a rather trivial address computation into a
> > multiplication on a target without hardware multiplier, so
> > there is additional, unacceptable time and code penalty
> > caused by a call to __mulhi3 from libgcc2.
> >
> > The multiplication is by -2, i.e. trivial
>
>
> This is strange.  Can you provide an example of that address
> computation as a test case?
>
> Make it just a few lines long, please.


I compiled with
avr-gcc -mmcu=attiny85 -S foo2.c -dp -save-temps -fverbose-asm -Os 
-fno-keep-inline-functions -fno-common -fno-inline-small-functions 
-fno-tree-scev-cprop -fno-split-wide-types

Georg-Johann

typedef unsigned int uint8_t __attribute__((__mode__(__QI__)));
typedef unsigned int uint16_t __attribute__ ((__mode__ (__HI__)));

typedef struct
{
    uint8_t timer_val;
    uint16_t puls[10];
    uint8_t codes[];
} powercode_t;

#define pgm_read_word(addr) \
(__extension__({                            \
    uint16_t __addr16 = (uint16_t)(addr);   \
    uint16_t __result;                      \
    __asm__  ("lpm %A0,%a1+\n\tlpm %B0,%a1" : "=r" (__result), "=z" (__addr16) : "1" (__addr16));                                      \
    __result;                               \
}))

void foo2 (uint8_t on_off, powercode_t * code)
{
    uint16_t offtime;

    do
    {
        offtime = pgm_read_word(& code->puls[on_off-1]);
    } while (offtime != 0);
}