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base repository: rust-lang/llvm-project
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head repository: rust-lang/llvm-project
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compare: bbf96c9f18aa47770c4f571f714d8641ddbe86e9
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  • 4 commits
  • 6 files changed
  • 1 contributor

Commits on Mar 5, 2021

  1. [Loads] Extract helper frunction for available load/store (NFC) 

    This contains the logic for extracting an available load/store
from a given instruction, to be reused in a following patch.

(cherry picked from commit 7c706aa)
    @nikic
    nikic committed Mar 5, 2021
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  2. [Loads] Add optimized FindAvailableLoadedValue() overload (NFCI) 

    FindAvailableLoadedValue() accepts an iterator by reference. If no
available value is found, then the iterator will either be left
at a clobbering instruction or the beginning of the basic block.
This allows using FindAvailableLoadedValue() across multiple blocks.

If this functionality is not needed, as is the case in InstCombine,
then we can use a much more efficient implementation: First try
to find an available value, and only perform clobber checks if
we actually found one. As this function only looks at a very small
number of instructions (6 by default) and usually doesn't find an
available value, this saves many expensive alias analysis queries.

(cherry picked from commit e0615bc)
    @nikic
    nikic committed Mar 5, 2021
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  3. [LICM] Make promotion faster 

    Even when MemorySSA-based LICM is used, an AST is still populated
for scalar promotion. As the AST has quadratic complexity, a lot
of time is spent in this step despite the existing access count
limit. This patch optimizes the identification of promotable stores.

The idea here is pretty simple: We're only interested in must-alias
mod sets of loop invariant pointers. As such, only populate the AST
with loop-invariant loads and stores (anything else is definitely
not promotable) and then discard any sets which alias with any of
the remaining, definitely non-promotable accesses.

If we promoted something, check whether this has made some other
accesses loop invariant and thus possible promotion candidates.

This is much faster in practice, because we need to perform AA
queries for O(NumPromotable^2 + NumPromotable*NumNonPromotable)
instead of O(NumTotal^2), and NumPromotable tends to be small.
Additionally, promotable accesses have loop invariant pointers,
for which AA is cheaper.

This has a signicant positive compile-time impact. We save ~1.8%
geomean on CTMark at O3, with 6% on lencod in particular and 25%
on individual files.

Conceptually, this change is NFC, but may not be so in practice,
because the AST is only an approximation, and can produce
different results depending on the order in which accesses are
added. However, there is at least no impact on the number of promotions
(licm.NumPromoted) in test-suite O3 configuration with this change.

Differential Revision: https://reviews.llvm.org/D89264

(cherry picked from commit 3d8f842)
    @nikic
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  4. [AST] Remove unused Loop member (NFC) 

    To fix some build bots after D89264.

(cherry picked from commit 29034f3)
    @nikic
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