Re: [PATCH libatomic/arm] avoid warning on constant addresses (PR 101379)

[Thomas Schwinge <thomas@codesourcery.com>]

Hi Martin!

On 2021-07-09T17:11:25-0600, Martin Sebor via Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
> The attached tweak avoids the new -Warray-bounds instances when
> building libatomic for arm. Christophe confirms it resolves
> the problem (thank you!)

As Abid has just reported in
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101374#c16>, similar
problem with GCN target libgomp build:

    In function ‘gcn_thrs’,
        inlined from ‘gomp_thread’ at [...]/source-gcc/libgomp/libgomp.h:803:10,
        inlined from ‘GOMP_barrier’ at [...]/source-gcc/libgomp/barrier.c:34:29:
    [...]/source-gcc/libgomp/libgomp.h:792:10: error: array subscript 0 is outside array bounds of ‘__lds struct gomp_thread * __lds[0]’ [-Werror=array-bounds]
      792 |   return *thrs;
          |          ^~~~~

    gcc/config/gcn/gcn.h:  c_register_addr_space ("__lds", ADDR_SPACE_LDS);                   \

    libgomp/libgomp.h-static inline struct gomp_thread *gcn_thrs (void)
    libgomp/libgomp.h-{
    libgomp/libgomp.h-  /* The value is at the bottom of LDS.  */
    libgomp/libgomp.h:  struct gomp_thread * __lds *thrs = (struct gomp_thread * __lds *)4;
    libgomp/libgomp.h-  return *thrs;
    libgomp/libgomp.h-}

..., plus a few more.  Work-around:

       struct gomp_thread * __lds *thrs = (struct gomp_thread * __lds *)4;
    +# pragma GCC diagnostic push
    +# pragma GCC diagnostic ignored "-Warray-bounds"
       return *thrs;
    +# pragma GCC diagnostic pop

..., but it's a bit tedious to add that in all that the other places,
too.  (So I'll consider some GCN-specific '-Wno-array-bounds' if we don't
get to resolve this otherwise, soon.)

> As we have discussed, the main goal of this class of warnings
> is to detect accesses at addresses derived from null pointers
> (e.g., to struct members or array elements at a nonzero offset).

(ACK, and thanks for that work!)

> Diagnosing accesses at hardcoded addresses is incidental because
> at the stage they are detected the two are not distinguishable
> from each another.
>
> I'm planning (hoping) to implement detection of invalid pointer
> arithmetic involving null for GCC 12, so this patch is a stopgap
> solution to unblock the arm libatomic build without compromising
> the warning.  Once the new detection is in place these workarounds
> can be removed or replaced with something more appropriate (e.g.,
> declaring the objects at the hardwired addresses with an attribute
> like AVR's address or io; that would enable bounds checking at
> those addresses as well).

Of course, we may simply re-work the libgomp/GCN code -- but don't we
first need to answer the question whether the current code is actually
"bad"?  Aren't we going to get a lot of similar reports from
kernel/embedded/other low-level software developers, once this is out in
the wild?  I mean:

> PR bootstrap/101379 - libatomic arm build failure after r12-2132 due to -Warray-bounds on a constant address
>
> libatomic/ChangeLog:
>       * /config/linux/arm/host-config.h (__kernel_helper_version): New
>       function.  Adjust shadow macro.
>
> diff --git a/libatomic/config/linux/arm/host-config.h b/libatomic/config/linux/arm/host-config.h
> index 1520f237d73..777d08a2b85 100644
> --- a/libatomic/config/linux/arm/host-config.h
> +++ b/libatomic/config/linux/arm/host-config.h
> @@ -39,8 +39,14 @@ typedef void (__kernel_dmb_t) (void);
>  #define __kernel_dmb (*(__kernel_dmb_t *) 0xffff0fa0)
>
>  /* Kernel helper page version number.  */
> -#define __kernel_helper_version (*(unsigned int *)0xffff0ffc)

Are such (not un-common) '#define's actually "bad", and anyhow ought to
be replaced by something like the following?

> +static inline unsigned*
> +__kernel_helper_version ()
> +{
> +  unsigned *volatile addr = (unsigned int *)0xffff0ffc;
> +  return addr;
> +}
>
> +#define __kernel_helper_version (*__kernel_helper_version())

(No 'volatile' in the original code, by the way.)


Grüße
 Thomas
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