From: Richard Earnshaw <Richard.Earnshaw@foss.arm.com>
To: Andrea Corallo <andrea.corallo@arm.com>
Cc: gcc-help@gcc.gnu.org, nd@arm.com
Subject: Re: Correct way to express to the compiler "this does not get clobbered"?
Date: Thu, 3 Dec 2020 13:06:45 +0000 [thread overview]
Message-ID: <1632b7ed-dd89-a53e-8367-a8e29d34bdf4@foss.arm.com> (raw)
In-Reply-To: <gkrtut3gjjz.fsf@arm.com>
On 03/12/2020 12:28, Andrea Corallo wrote:
> Richard Earnshaw <Richard.Earnshaw@foss.arm.com> writes:
>
>> On 03/12/2020 11:47, Andrea Corallo via Gcc-help wrote:
>>> Hi all,
>>>
>>> I've a piece of code that reduced looks like this:
>>>
>>> #+begin_src C
>>> typedef struct {
>>> void (*fun_ptr)(void);
>>> } x_t;
>>>
>>> x_t *x;
>>>
>>> void
>>> f (void)
>>> {
>>> const x_t const *y = x;
>>> for (int i = 0; i < 1000; ++i)
>>> y->fun_ptr ();
>>> }
>>> #+end_src
>>>
>>> What is the correct way (if any) to express to the compiler that the
>>> value of y->fun_ptr does not get clobbered by the function call itself
>>> so the corresponding load to obtain its value can be moved out of the
>>> loop?
>>>
>>> My understanding is that the const qualifier is more for diagnostic
>>> reasons and is not sufficient for GCC to make this assumption. OTOH I
>>> cannot give 'fun_ptr' the attribute pure as it's not.
>>>
>>> Thanks
>>>
>>> Andrea
>>>
>>
>> Why not just put the function pointer in a local variable? Then the
>> compiler will know that the value can't change.
>
> Hi Richard,
>
> yeah that would do the job.
>
> This is generated code and changing the code generator that way might
> not be completely trivial (e.g. where is the best position to perform
> the assignments to each local variable?). Therefore before going for
> this solution I'd like to be convinced there's no way to express this
> directly to GCC.
>
> Andrea
>
Perhaps you could declare y with the restrict qualifier? But that would
need to apply to all of *y, not just fun_ptr.
R.
next prev parent reply other threads:[~2020-12-03 13:06 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-03 11:47 Andrea Corallo
2020-12-03 12:04 ` Richard Earnshaw
2020-12-03 12:28 ` Andrea Corallo
2020-12-03 13:06 ` Richard Earnshaw [this message]
2020-12-03 13:40 ` Andrea Corallo
2020-12-03 18:24 ` David Brown
2020-12-03 23:00 ` Andrea Corallo
2020-12-04 7:23 ` stefan
2020-12-04 9:52 ` Andrea Corallo
2020-12-04 17:27 ` Segher Boessenkool
2020-12-04 18:16 ` Andrea Corallo
2020-12-04 18:33 ` Segher Boessenkool
2020-12-04 18:45 ` Andrea Corallo
[not found] ` <017701d6ca6d$7dc8dc90$795a95b0$@bebbosoft.de>
2020-12-04 18:57 ` AW: " Stefan Franke
2020-12-04 22:52 ` Andrea Corallo
2020-12-05 14:01 ` David Brown
2020-12-05 14:17 ` AW: " Stefan Franke
2020-12-05 18:10 ` Andrea Corallo
2020-12-07 22:27 ` Segher Boessenkool
2020-12-05 19:03 ` David Brown
2020-12-06 11:29 ` AW: " Stefan Franke
2020-12-05 17:29 ` Andrea Corallo
2020-12-05 19:07 ` David Brown
2020-12-06 9:22 ` Jonathan Wakely
2020-12-04 8:27 ` David Brown
2020-12-04 10:05 ` Andrea Corallo
2020-12-05 8:06 ` Liu Hao
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