[LLVMdev] Improving loop vectorizer support for loops with a volatile iteration variable (original) (raw)

Sean Silva chisophugis at gmail.com
Wed Jul 15 18:09:12 PDT 2015

On Wed, Jul 15, 2015 at 5:34 PM, Chandler Carruth <chandlerc at google.com> wrote:

On Wed, Jul 15, 2015 at 12:55 PM Hyojin Sung <hsung at us.ibm.com> wrote:

Hi all,

I would like to propose an improvement of the “almost dead” block elimination in Transforms/Local.cpp so that it will preserve the canonical loop form for loops with a volatile iteration variable. *** Problem statement Nested loops in LCALS Subset B (*https://codesign.llnl.gov/LCALS.php* <https://urldefense.proofpoint.com/v2/url?u=https-3A_codesign.llnl.govLCALS.php&d=AwMGaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4bhmuhMHA&m=aWKfvN4c8lvUSvVn8J0Z2ajTctlBJf0198Au28epBr0&s=4d9dt5ODcDWHHatSrwu5ZYT9ebgVzNEtpOlIR87izCM&e=>) are not vectorized with LLVM -O3 because the LLVM loop vectorizer fails the test whether the loop latch and exiting block of a loop is the same. The loops are vectorizable, and get vectorized with LLVM -O2 I would be interested to know why -O2 succeeds here. and also with other commercial compilers (icc, xlc). *** Details These loops ended up with different loop latch and exiting block after a series of optimizations including loop unswitching, jump threading, simplify-the-CFG, and loop simplify. The fundamental problem here is that the above optimizations cannot recognize a loop with a volatile iteration variable and do not preserve its canonical loop structure. Ok, meta-level question first: Why do we care about performance of loops with a volatile iteration variable? That seems both counter-intuitive and unlikely to be a useful goal. We simply don't optimize volatile operations well in any part of the optimizer, and I'm not sure why we need to start trying to fix that. This seems like an irreparably broken benchmark, but perhaps there is a motivation I don't yet see.

A quick look at the tarball on the linked site suggests that the volatile iteration variable is done on purpose so that the outer "run thing N times and take the average" loop can't be optimized.

-- Sean Silva

Assuming that sufficient motivation arises to try to fix this, see my comments below:

(1) Loop unswitching generates several empty placeholder BBs only with PHI nodes after separating out a shorter path with no inner loop execution from a standard path. (2) Jump threading and simplify-the-CFG passes independently calls TryToSimplifyUnconditionalBranchFromEmptyBlock() in Transforms/Utils/Local.cpp to get rid of almost empty BBs. (3) TryToSimplifyUnconditionalBranchFromEmtpyBlock() eliminates the placeholder BBs after loop unswitching and merges them into subsequent blocks including the header of the inner loop. Before eliminating the blocks, the function checks if the block is a loop header by looking at its PHI nodes so that it can be saved, but the test fails with the loops with a volatile iteration variable. Why does this fail for a volatile iteration variable but not for a non-volatile one? I think understanding that will be key to understanding how it should be fixed.

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