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

[David Laight]

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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