Commit Graph

4 Commits

Author SHA1 Message Date
Rachel Craik 1f55420065 [LoopCacheAnalysis]: Add support for negative stride
LoopCacheAnalysis currently assumes the loop will be iterated over in
a forward direction. This patch addresses the issue by using the
absolute value of the stride when iterating backwards.

Note: this patch will treat negative and positive array access the
same, resulting in the same cost being calculated for single and
bi-directional access patterns. This should be improved in a
subsequent patch.

Reviewed By: jdoerfert

Differential Revision: https://reviews.llvm.org/D73064
2020-02-10 13:22:35 -05:00
Rachel Craik f897d087d0 [LoopCacheAnalysis]: Fix assertion failure during cost computation
Ensure the stride and trip count have the same type before multiplying them during reference cost calculation

Reviewed By: jdoefert

Differential Revision: https://reviews.llvm.org/D70192
2019-11-15 14:56:26 -05:00
Whitney Tsang 3caf9af6c6 Title: Improve Loop Cache Analysis LIT tests.
Summary: Make LIT tests unsensitive to analysis output order.
Authored By: etiotto

llvm-svn: 368450
2019-08-09 16:18:22 +00:00
Whitney Tsang dd3b6498b0 Title: Loop Cache Analysis
Summary: Implement a new analysis to estimate the number of cache lines
required by a loop nest.
The analysis is largely based on the following paper:

Compiler Optimizations for Improving Data Locality
By: Steve Carr, Katherine S. McKinley, Chau-Wen Tseng
http://www.cs.utexas.edu/users/mckinley/papers/asplos-1994.pdf
The analysis considers temporal reuse (accesses to the same memory
location) and spatial reuse (accesses to memory locations within a cache
line). For simplicity the analysis considers memory accesses in the
innermost loop in a loop nest, and thus determines the number of cache
lines used when the loop L in loop nest LN is placed in the innermost
position.

The result of the analysis can be used to drive several transformations.
As an example, loop interchange could use it determine which loops in a
perfect loop nest should be interchanged to maximize cache reuse.
Similarly, loop distribution could be enhanced to take into
consideration cache reuse between arrays when distributing a loop to
eliminate vectorization inhibiting dependencies.

The general approach taken to estimate the number of cache lines used by
the memory references in the inner loop of a loop nest is:

Partition memory references that exhibit temporal or spatial reuse into
reference groups.
For each loop L in the a loop nest LN: a. Compute the cost of the
reference group b. Compute the 'cache cost' of the loop nest by summing
up the reference groups costs
For further details of the algorithm please refer to the paper.
Authored By: etiotto
Reviewers: hfinkel, Meinersbur, jdoerfert, kbarton, bmahjour, anemet,
fhahn
Reviewed By: Meinersbur
Subscribers: reames, nemanjai, MaskRay, wuzish, Hahnfeld, xusx595,
venkataramanan.kumar.llvm, greened, dmgreen, steleman, fhahn, xblvaOO,
Whitney, mgorny, hiraditya, mgrang, jsji, llvm-commits
Tag: LLVM
Differential Revision: https://reviews.llvm.org/D63459

llvm-svn: 368439
2019-08-09 13:56:29 +00:00