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
