linux-kernel - Re: [PATCH 02/25] tracing: Improve "if" macro code generation

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Date:   Thu, 9 May 2019 11:48:43 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Laight <David.Laight@...lab.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "hpa@...or.com" <hpa@...or.com>,
        "julien.thierry@....com" <julien.thierry@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "luto@...capital.net" <luto@...capital.net>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "catalin.marinas@....com" <catalin.marinas@....com>,
        "james.morse@....com" <james.morse@....com>,
        "valentin.schneider@....com" <valentin.schneider@....com>,
        "brgerst@...il.com" <brgerst@...il.com>,
        "luto@...nel.org" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "dvlasenk@...hat.com" <dvlasenk@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>
Subject: Re: [PATCH 02/25] tracing: Improve "if" macro code generation

On 5/9/19 11:47 AM, Josh Poimboeuf wrote:
> On Thu, May 09, 2019 at 01:45:31PM -0500, Josh Poimboeuf wrote:
>> On Thu, May 09, 2019 at 02:29:02PM -0400, Steven Rostedt wrote:
>>> On Thu, 9 May 2019 09:51:59 -0700
>>> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>>
>>>> On Thu, May 9, 2019 at 6:01 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>>>>>
>>>>> This patch works. Can I get your Signed-off-by for it?  
>>>>
>>>> Yes. Please write some kind of comprehensible commit log for it, but
>>>
>>> How's this:
>>>
>>> "Peter Zijlstra noticed that with CONFIG_PROFILE_ALL_BRANCHES, the "if"
>>> macro converts the conditional to an array index.  This can cause GCC
>>> to create horrible code.  When there are nested ifs, the generated code
>>> uses register values to encode branching decisions.
>>>
>>> Josh Poimboeuf found that replacing the define "if" macro from using
>>> the condition as an array index and incrementing the branch statics
>>> with an if statement itself, reduced the asm complexity and shrinks the
>>> generated code quite a bit.
>>>
>>> But this can be simplified even further by replacing the internal if
>>> statement with a ternary operator.
>>>
>>> Reported-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>>> Reported-by: Josh Poimboeuf <jpoimboe@...hat.com>
>>
>> Actually, my original fix already went in:
>>
>>   37686b1353cf ("tracing: Improve "if" macro code generation")
>>
>> But it introduced a regression:
>>
>>   https://lkml.kernel.org/r/201905040509.iqQ2CrOU%lkp@intel.com
>>
>> which Linus' patch fixes for some reason.
> 
> /me curses URL encoding
> 
> https://lkml.kernel.org/r/201905040509.iqQ2CrOU%25lkp@intel.com
> 

Still fails for me.

-- 
~Randy