On 10/03/2016 02:26 PM, Nathan Sidwell wrote:
> On 10/03/16 08:13, Martin Liška wrote:
>> On 08/18/2016 05:53 PM, Jeff Law wrote:
>>> On 08/18/2016 09:51 AM, Andi Kleen wrote:
>>>>> I'd prefer to make updates atomic in multi-threaded applications.
>>>>> The best proxy we have for that is -pthread.
>>>>>
>>>>> Is it slower, most definitely, but odds are we're giving folks
>>>>> garbage data otherwise, which in many ways is even worse.
>>>>
>>>> It will likely be catastrophically slower in some cases.
>>>>
>>>> Catastrophically as in too slow to be usable.
>>>>
>>>> An atomic instruction is a lot more expensive than a single increment. Also
>>>> they sometimes are really slow depending on the state of the machine.
>>> And for those cases there's a way to override.
>>>
>>> The default should be set for correctness.
>>>
>>> jeff
>>
>> I would to somehow resolve the discussion related to default value selection.
>> Is the prevailing consensus that we should set -fprofile-update=atomic when
>> -pthread is set? If so, I'll prepare a patch. I tend to do it this way.
>
> This is my preference.
>
> nathan
Ok, this is final version of patch which implements both the warning and
appending -fprofile-update
to a command line options.
Patch can bootstrap on ppc64le-redhat-linux and survives regression tests.
Ready to be installed?
Martin
>From 343d64a3c6b515053459a8ece6f9b0ad6ce86273 Mon Sep 17 00:00:00 2001
From: marxin <mli...@suse.cz>
Date: Tue, 4 Oct 2016 10:46:48 +0200
Subject: [PATCH] Add -fprofile-update option juggling
gcc/ChangeLog:
2016-10-04 Martin Liska <mli...@suse.cz>
* gcc.c: Set -fprofile-update=atomic when profiling is
enabled and -pthread is set. Warn when one combines
-pthread and -fprofile-update=single for an app using
profiling code.
---
gcc/gcc.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/gcc/gcc.c b/gcc/gcc.c
index fd2b182..5213cb0 100644
--- a/gcc/gcc.c
+++ b/gcc/gcc.c
@@ -1141,7 +1141,14 @@ static const char *cc1_options =
%{-help=*:--help=%*}\
%{!fsyntax-only:%{S:%W{o*}%{!o*:-o %b.s}}}\
%{fsyntax-only:-o %j} %{-param*}\
- %{coverage:-fprofile-arcs -ftest-coverage}";
+ %{coverage:-fprofile-arcs -ftest-coverage}\
+ %{fprofile-arcs|fprofile-generate*|coverage:\
+ %{!fprofile-update=single:\
+ %{pthread:-fprofile-update=atomic}}}\
+ %{fprofile-update=single:\
+ %{fprofile-arcs|fprofile-generate*|coverage:\
+ %{pthread:%n-fprofile-update=atomic should be used\
+ for a multithreaded application}}}";
static const char *asm_options =
"%{-target-help:%:print-asm-header()} "
--
2.9.2
