[PATCH] Avoid some -Wunreachable-code-ctrl

[Mikael Morin]

On 30/11/2021 14:25, Richard Biener wrote:
> On Tue, 30 Nov 2021, Mikael Morin wrote:
> 
>> Le 29/11/2021 à 16:03, Richard Biener via Gcc-patches a écrit :
>>> diff --git a/gcc/fortran/frontend-passes.c b/gcc/fortran/frontend-passes.c
>>> index f5ba7cecd54..16ee2afc9c0 100644
>>> --- a/gcc/fortran/frontend-passes.c
>>> +++ b/gcc/fortran/frontend-passes.c
>>> @@ -5229,7 +5229,6 @@ gfc_expr_walker (gfc_expr **e, walk_expr_fn_t exprfn,
>>> void *data)
>>>       case EXPR_OP:
>>>         WALK_SUBEXPR ((*e)->value.op.op1);
>>>         WALK_SUBEXPR_TAIL ((*e)->value.op.op2);
>>> -	    break;
>>>       case EXPR_FUNCTION:
>>>         for (a = (*e)->value.function.actual; a; a = a->next)
>>>           WALK_SUBEXPR (a->expr);
>>
>> I’m uncomfortable with the above change.
>> It makes it look like there is a fall through, but there is not.
>> Maybe inline the macro to make the continue explicit, or use WALK_SUBEXPR
>> instead of WALK_SUBEXPR_TAIL and hope the compiler will do the tail call
>> optimization.
> 
> Ah, it follows the style in tree.c:walk_tree_1 where break was used
> inconsistently after WALK_SUBTREE_TAIL which was then more obvious
> to me to clean up.  I didn't realize the fortran FE only had a
> single WALK_SUBEXPR_TAIL.
> 
> I'm not sure inlining will make the situation more clear, for
> sure using WALK_SUBEXPR would but it might loose the tailcall.
> 
> Would you accept an additional comment after WALK_SUBEXPR_TAIL like
> 
>            case EXPR_OP:
>              WALK_SUBEXPR ((*e)->value.op.op1);
>              WALK_SUBEXPR_TAIL ((*e)->value.op.op2);
>              /* tail-recurse  */
> 
My preference would be a gcc_unreachable() or something similar, but I 
understand it would get a warning as well?

Without better idea, I’m fine with an even more explicit comment:

     /* No fallthru because of the tail recursion above.  */

> ?  Btw, a fallthru would be diagnosed by GCC unless we put
> 
>              /* Fallthru  */
> 
> here.
Sure, but my main concern was misreading from programmers (including 
me), which is not diagnosed by compilers.

>  Maybe renaming WALK_SUBEXPR_TAIL to WALK_SUBEXPR_WITH_CONTINUE
> or WALK_SUBEXPR_BY_TAIL_RECURSING or WALK_SUBEXPR_TAILRECURSE would
> be more obvious?
> 
I think the comment above would be enough.

Thanks.