Re: const volatile behaviour change in GCC 7

[Andreas Schwab <schwab at suse dot de>]

On Sep 22 2016, Sebastian Huber <sebastian.huber@embedded-brains.de> wrote:

> for RTEMS we use linker sets to initialize the system. The following code
> worked up to GCC 6, but no longer in GCC 7:
>
> typedef void ( *rtems_sysinit_handler )( void );
>
> typedef struct {
>   rtems_sysinit_handler handler;
> } rtems_sysinit_item;
>
> rtems_sysinit_item volatile const _Linker_set__Sysinit_begin[0]
> __attribute__((__section__(".rtemsroset." "_Sysinit" ".begin")))
> __attribute__((__used__));
>
> rtems_sysinit_item volatile const _Linker_set__Sysinit_end[0]
> __attribute__((__section__(".rtemsroset." "_Sysinit" ".end")))
> __attribute__((__used__));
>
> void rtems_initialize_executive(void)
> {
>   const volatile rtems_sysinit_item *cur = _Linker_set__Sysinit_begin;
>   const volatile rtems_sysinit_item *end = _Linker_set__Sysinit_end;
>
>   while ( cur != end ) {
>     ( *cur->handler )();
>     ++cur;
>   }
> }
>
> The corresponding GNU ld linker script section is:
>
>     .rtemsroset : ALIGN_WITH_INPUT {
>         KEEP (*(SORT(.rtemsroset.*)))
>     } > REGION_RODATA AT > REGION_RODATA_LOAD
>
> In GCC 7, the compiler deduces that "cur != end" is always true and
> generates an infinite loop.
>
> Up to GCC 6 the "volatile const" seemed to prevent this optimization.

These qualifiers do not say anything about the variable cur and end
themselves, only about the values they point to.  As such I don't see
how they can have any influence on the value of "cur != end".

Andreas.

-- 
Andreas Schwab, SUSE Labs, schwab@suse.de
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