[llvm-dev] AttrBuilder API change

Philip Reames via llvm-dev
Thu, 06 Jan 2022 17:05:50 -0000

+1 from me.

As an aside, has anyone looked at memoizing the strings used for string
attributes?  In practice, there tend to be a very small number of unique
strings which are heavily reused.  If we had a string table for the
attribute strings, we could hold the index in the Attribute itself, and
reduce storage fairly significantly. (Well, assuming we implemented the
small string optimization to handle the case where the attribute name is
shorted than the index size.)

We could either do this implicitly, or even expose it in the public
API.  (i.e. LLVMContext has a method along the lines of
"memorizeAttributeName" which returns a spare slot in the Attributes
enum.)

Philip

On 1/6/22 7:00 AM, Serge Guelton via llvm-dev wrote:
> Hi folks,
>
> the AttrBuilder class has been showing off a lot on performance report,
> impacting compile time in a negative and unnecessery way. Several commits
[1] by
> Nikita Popov already improved the situation by avoid unnecesseray calls to
this
> builder, and more recenly, the AttributeMask class got introduced [2] as a
way to
> split the logic between adding and removing attributes.
>
> The final step [3] (that sometimes give a 1% performance increase! [4])
avoid
> using an std::map of SmallString to store target dependent attributes, and
uses
> a SmallVector of Attribute instead.
>
> This implies changing all AttrBuilder constructors to take an LLVMContext
as
> parameter. Given how widely used this class is, I thought I'd advertise
the
> change here and let it mature a few extra days before it gets merged!
>
> In [3], rnk is suggesting I'd land a temporary commit that introduces
extra
> constructors that accept an extra LLVM Context just to ease the transition.
> Would that be useful to someone here?
>
> [1] eg 6e30cb7673df293fd294acef7eadca8050b5a71e
> [2] 9290ccc3c1a17a7874de020656db38183a20f6b0
> [3] https://reviews.llvm.org/D116599
> [4]
https://llvm-compile-time-tracker.com/compare.php?from=5de322295f4ade692dc4f1823ae4450ad3c48af2&to=05bc480bf641a9e3b466619af43a2d123ee3f71d&stat=instructions
>
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