Bug#131454: gcc -O2 produces wrong code on PPC (gnuchess example)

[Matthias Klose <doko@cs.tu-berlin.de>]

Please could you recheck with gcc-3.0, and if this version doesn't fix
it, with the recently uploaded gcc-snapshot package?

Thanks, Matthias

Lukas Geyer writes:
> Package: gcc
> Version: 2:2.95.4-9
> Severity: important
> 
> Compiling the current gnuchess source fails with this gcc (current woody)
> and optimization set to -O2. The reason seems to be that statements are
> not correctly serialized. Unfortunately I am not an expert in PPC
> assembler but here is the code which breaks (modified from gnuchess source
> file src/move.c, somewhat shortened). Removing the "inline" keyword
> generates correct code with -O2. However, in the original source,
> clear_and_set_bit() is just part of UnmakeMove(), as well as of
> MakeMove(). So, the following should be (even more than) the relevant
> part.
> 
> typedef unsigned long long BitBoard;
> 
> #define SETBIT(b,i)   ((b) |= BitPosArray[i])
> #define CLEARBIT(b,i) ((b) &= NotBitPosArray[i])
> #define TOSQ(a)       ((a) & 0x003F)
> #define FROMSQ(a)     ((a >> 6) & 0x003F)
> 
> inline void clear_and_set_bit(BitBoard *a, short f, short t)
> {
>         CLEARBIT (*a, f);
>         SETBIT (*a, t);
> }
> 
> void UnmakeMove (short side, int *move)
> {
>    BitBoard *a;
>    short f, t, fpiece, cpiece;
>    short xside;
> 
>    side = 1^side;
>    xside = 1^side;
>    f = FROMSQ(*move);
>    t = TOSQ(*move);
>    fpiece = cboard[t];
>    a = &board.b[side][fpiece];
>    clear_and_set_bit(a, t, f);
> 
>    /* continues... */
> }
> 
> So MakeMove() has about the same structure, except of course it switches
> the roles of t and f. Now the gnuchess move generation algorithm generates
> all possible moves, makes them, and unmakes them again. With inlining and
> -O2 however, it seems that making and unmaking of moves is not serialized,
> so that after trying out all pawn moves from the beginning position, white
> has a pawn on every square between the second and fourth row. If someone
> really wants, I can supply the assembler files move.s for -O1 (correct
> code) and -O2 (incorrect), together with the C source.
> 
> Lukas