Re: gccgo emits GIMPLE with temporaries for boolean expressions unlike gcc, gdc

["Martin Liška" <mliska@suse.cz>]

CCing Go maintainer.

Martin

On 8/3/22 15:25, j wrote:
> 
> Hello,
> 
> I've proposed a patch [1] for condition coverage profiling in gcc, implemented in the middle-end alongside the branch coverage. I've written most of the tests for C and a few for C++ and finally got around to try it with a toy example for D and go and noticed something odd about Go's CFG construction.
> 
> abc.c:
>     int fn (int a, int b, int c) {
>         if (a && (b || c))
>             return a;
>         else
>             return b * c;
>     }
> 
> abc.d:
>     int fn (int a, int b, int c) {
>         if (a && (b || c))
>             return a;
>         else
>             return b * c;
>     }
> 
> abc.go:
>     func fn (a int, b int, c int) int {
>         a_ := a != 0;
>         b_ := b != 0;
>         c_ := c != 0;
> 
>         if a_ && (b_ || c_) {
>             return 1;
>         } else {
>             return 0;
>         }
>     }
> 
> All were built with gcc --coverage -fprofile-conditions (my patch, but it does not affect this) and no optimization. The C and D programs behaved as expected:
> 
> gcov --conditions abc.d:
> 
>         3:    3:int fn (int a, int b, int c) {
>        3*:    4:    if (a && (b || c))
> conditions outcomes covered 3/6
> condition  1 not covered (false)
> condition  2 not covered (true)
> condition  2 not covered (false)
>         1:    5:        return a;
>         -:    6:    else
>         2:    7:        return b * c;
> 
> 
> gcov --conditions abc.go:
>         3:    5:func fn (a int, b int, c int) int {
>         3:    6:        a_ := a != 0;
>         3:    7:        b_ := b != 0;
>         3:    8:        c_ := c != 0;
>         -:    9:
>        3*:   10:        if a_ && (b_ || c_) {
> condition outcomes covered 2/2
> condition outcomes covered 1/2
> condition  0 not covered (true)
> condition outcomes covered 2/2
>         1:   11:            return 1;
>         -:   12:        } else {
>         2:   13:            return 0;
>         -:   14:        }
>         -:   15:}
> 
> So I dumped the gimple gcc -fdump-tree-gimple abc.go:
> 
> int main.fn (int a, int b, int c)
> {
>   int D.2725;
>   int $ret0;
> 
>   $ret0 = 0;
>   {
>     bool a_;
>     bool b_;
>     bool c_;
> 
>     a_ = a != 0;
>     b_ = b != 0;
>     c_ = c != 0;
>     {
>       {
>         GOTMP.0 = a_;
>         if (GOTMP.0 != 0) goto <D.2719>; else goto <D.2720>;
>         <D.2719>:
>         {
>           {
>             GOTMP.1 = b_;
>             _1 = ~GOTMP.1;
>             if (_1 != 0) goto <D.2721>; else goto <D.2722>;
>             <D.2721>:
>             {
>               GOTMP.1 = c_;
>             }
>             <D.2722>:
>           }
>           GOTMP.2 = GOTMP.1;
>           GOTMP.0 = GOTMP.2;
>         }
>         <D.2720>:
>       }
>       if (GOTMP.0 != 0) goto <D.2723>; else goto <D.2724>;
>       <D.2723>:
> 
> 
>       {
>         {
>           $ret0 = 1;
>           D.2725 = $ret0;
>           // predicted unlikely by early return (on trees) predictor.
>           return D.2725;
>         }
>       }
>       <D.2724>:
>       {
>         {
>           $ret0 = 0;
>           D.2725 = $ret0;
>           // predicted unlikely by early return (on trees) predictor.
>           return D.2725;
>         }
>       }
>     }
>   }
> }
> 
> Where as D (and C) is more-or-less as you would expect:
> 
> int fn (int a, int b, int c)
> 
> 
> {
>   int D.7895;
> 
>   if (a != 0) goto <D.7893>; else goto <D.7891>;
>   <D.7893>:
>   if (b != 0) goto <D.7892>; else goto <D.7894>;
>   <D.7894>:
>   if (c != 0) goto <D.7892>; else goto <D.7891>;
>   <D.7892>:
>   D.7895 = a;
>   // predicted unlikely by early return (on trees) predictor.
>   return D.7895;
>   <D.7891>:
>   D.7895 = b * c;
>   // predicted unlikely by early return (on trees) predictor.
>   return D.7895;
> }
> 
> Clearly the decision inference algorithm is unable to properly instrument to Go program for condition coverage because of the use of temporaries in the emitted GIMPLE. The question is: is this intentional and/or required from Go's semantics or could it be considered a defect? Is emitting the GIMPLE without the use of temporaries feasible at all?
> 
> Thanks,
> Jørgen
> 
> [1] https://gcc.gnu.org/pipermail/gcc-patches/2022-July/598165.html