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

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Message-ID: <f918ecb0b6bf43f3bf0f526084d8467b@AcuMS.aculab.com>
Date:   Wed, 20 Mar 2019 11:18:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.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>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "luto@...nel.org" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "dvlasenk@...hat.com" <dvlasenk@...hat.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "dvyukov@...gle.com" <dvyukov@...gle.com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>
Subject: RE: [PATCH 02/25] tracing: Improve "if" macro code generation

From: > Peter Zijlstra
> Sent: 18 March 2019 15:39
> 
> 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.
> 
> Make it easier for GCC to optimize by keeping the conditional as a
> conditional rather than converting it to an integer.  This shrinks the
> generated code quite a bit, and also makes the code sane enough for
> objtool to understand.
...
>  include/linux/compiler.h |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -67,7 +67,7 @@ void ftrace_likely_update(struct ftrace_
>  				.line = __LINE__,			\
>  			};						\
>  		______r = !!(cond);					\

	Is that (or maybe just the !!) needed any more??

> -		______f.miss_hit[______r]++;					\
> +		______r ? ______f.miss_hit[1]++ : ______f.miss_hit[0]++;\
>  		______r;						\
>  	}))
>  #endif /* CONFIG_PROFILE_ALL_BRANCHES */
> 

	David

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