linux-kernel - Re: [PATCH] gcov: support GCC 7.1

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Message-Id: <3e73df5c-f253-576d-22c3-4f05d96113a0@linux.vnet.ibm.com>
Date:   Thu, 8 Dec 2016 13:52:45 +0100
From:   Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
To:     Martin Liška <mliska@...e.cz>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] gcov: support GCC 7.1

On 24.11.2016 13:46, Martin Liška wrote:
> On 11/09/2016 10:21 AM, Peter Oberparleiter wrote:
>> On 31.10.2016 10:35, Martin Liška wrote:
>>> Starting from GCC 7.1, __gcov_exit is a new symbol expected
>>> to be implemented in a profiling runtime.
>>
>> I tested your patch with kernel 4.9-rc4 compiled with GCC 7.0.0 20161107
>> (experimental) and validated that it fixes the build error due to a
>> missing __gcov_exit symbol.
>>
>> An attempt to read from one of the .gcda files generated by a kernel
>> compiled this way resulted in a crash though. It appears that the number
>> of GCOV_COUNTERS has changed again for GCC 7.x and must be adjusted in
>> the kernel as well. Could you add that to your patch as well?
> 
> Hello.
> 
> Sorry, this is fixed in second version of the patch.
> 
>> Given that GCC 7.1 has not yet been released, I'm wondering if it is
>> safe to program against interfaces that have not yet been fully
>> finalized. Can you provide an indication on whether these gcov-related
>> aspects of GCC might still be changed until release?
> 
> Well, we just flipped to stage3 which means that any new features should
> be added to current master. If you prefer we can commit the patch after
> a release of 7.1 will be done?

I've given this some more thought: if your patch is not applied, GCOV
kernel profiling will definitely not work with GCC 7.x. If it is
applied, there is a high probability that it will work, and a small
chance that additional patches will be required. Overall I would say
this justifies applying the patch now.

>> As a side note, please post your patches inline instead of as attachment
>> as that helps quoting them in replies. See also
>> linux/Documentation/SubmittingPatches.
> 
> Sure, thanks for info. I'm attaching new version:
> 
> Starting from GCC 7.1, __gcov_exit is a new symbol expected
> to be implemented in a profiling runtime. Update number of profile
> counters.
>
> Signed-off-by: Martin Liska <mliska@...e.cz>

Looks good, thanks! Tested successfully with GCC 7.0.0 20161208
(experimental) on Linux kernel 4.9-rc8 (compiles successfully and
produces valid kernel .gcda files).

Andrew, could you pick this change up via your tree?

Reviewed-by: Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
Tested-by: Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>

> ---
>  kernel/gcov/base.c    | 6 ++++++
>  kernel/gcov/gcc_4_7.c | 4 +++-
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/gcov/base.c b/kernel/gcov/base.c
> index 2f9df37..85dca5d 100644
> --- a/kernel/gcov/base.c
> +++ b/kernel/gcov/base.c
> @@ -98,6 +98,12 @@ void __gcov_merge_icall_topn(gcov_type *counters, unsigned int n_counters)
>  }
>  EXPORT_SYMBOL(__gcov_merge_icall_topn);
> 
> +void __gcov_exit (void)
> +{
> +	/* Unused. */
> +}
> +EXPORT_SYMBOL(__gcov_exit);
> +
>  /**
>   * gcov_enable_events - enable event reporting through gcov_event()
>   *
> diff --git a/kernel/gcov/gcc_4_7.c b/kernel/gcov/gcc_4_7.c
> index 6a5c239..46a18e7 100644
> --- a/kernel/gcov/gcc_4_7.c
> +++ b/kernel/gcov/gcc_4_7.c
> @@ -18,7 +18,9 @@
>  #include <linux/vmalloc.h>
>  #include "gcov.h"
> 
> -#if (__GNUC__ > 5) || (__GNUC__ == 5 && __GNUC_MINOR__ >= 1)
> +#if (__GNUC__ >= 7)
> +#define GCOV_COUNTERS			9
> +#elif (__GNUC__ > 5) || (__GNUC__ == 5 && __GNUC_MINOR__ >= 1)
>  #define GCOV_COUNTERS			10
>  #elif __GNUC__ == 4 && __GNUC_MINOR__ >= 9
>  #define GCOV_COUNTERS			9
> 


-- 
Peter Oberparleiter
Linux on z Systems Development - IBM Germany