[PATCH] Fix for inlining decision affected by debug intrinsics

[Eric Christopher]

I'd rather ignore all non-code generating heuristics myself -
including the lifetime heuristics. If they're not going to affect the
size of the generated code then they shouldn't be considered. Also,
debug shouldn't affect optimization decisions. :)

-eric

On Fri, Jan 31, 2014 at 12:06 PM, Chandler Carruth <chandlerc at google.com> wrote:
>
> On Fri, Jan 31, 2014 at 10:55 AM, Dario Domizioli
> <dario.domizioli at gmail.com> wrote:
>>
>> 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 don't think this bug has anything to do with debug info intrinsics, and so
> I don't think this is the correct patch.
>
> I think the bug is that instructions which we simplify away are counted
> toward the vector bonus. There should be no difference between debug info
> intrinsics and lifetime intrinsics. So the fix should be general and apply
> to the the basic cost.
>
>
> However, as a meta point, the "vector bonus" is the single worst aspect of
> our inline heuristics. To an extent, I would rather *remove* the vector
> bonus than make it more correct... but perhaps that battle is best fought
> another day.
>
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