[PATCH] Fix for inlining decision affected by debug intrinsics

[Eric Christopher]

Agreed with what Manman says. Let's split it in two and then either of
us can commit for you.

Thanks!

-eric

On Fri, Jan 31, 2014 at 11:05 AM, Manman Ren <manman.ren at gmail.com> wrote:
>
> LGTM. Thanks for the detailed comments.
>
> I noticed a few debugging-related changes, it may be better to separate
> those.
> +  DEBUG_PRINT_STAT(Cost);
> +  DEBUG_PRINT_STAT(Threshold);
> +  DEBUG_PRINT_STAT(VectorBonus);
>
> Manman
>
>
> On Fri, Jan 31, 2014 at 10:55 AM, Dario Domizioli
> <dario.domizioli at gmail.com> wrote:
>>
>> Hello LLVM.
>>
>> I have come across a subtle and rare interaction between debug
>> information, vector instructions, and the cost heuristic used by the
>> SimpleInliner (i.e. the InlineCost analysis).
>>
>> It appears that debug intrinsics are counted in the number of instructions
>> taken into consideration by the heuristic. They are also later discounted as
>> being "simplified", so they do not directly affect the inline Cost, but they
>> are taken into account when evaluating the fraction of instructions that are
>> vector instructions. This fraction is used to determine whether to apply a
>> hidden "VectorBonus" to the inlining Threshold, and therefore there is a
>> subtle interaction which means that debug intrinsics can affect the inlining
>> decision.
>>
>> The attached patch fixes the problem by making the inline cost heuristic
>> skip debug intrinsics altogether when examining instructions.
>> I have also added a couple of debug prints to the already existing ones,
>> which I think are generally useful to debug the inline cost heuristic.
>>
>> I have included a test in which I have manufactured a situation where
>> vector instructions would be most of the function's code but they are
>> outnumbered by debug intrinsics when debug information is present. The test
>> exposes the problem in the current trunk and passes with my fix.
>>
>> Please submit for me if you accept this patch, as I do not have commit
>> access.
>>
>> Cheers,
>>     Dario Domizioli
>>     SN Systems - Sony Computer Entertainment
>>
>>
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>
>
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