[PATCH] Fix for inlining decision affected by debug intrinsics

[Chandler Carruth]

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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