Loop vectorizer missed opt remarks are sometimes uninformatively terse

[TiborGY]

When a loop cannot be vectorized by the loop vectorizer for some reason, it should leave an informative message in the optimization remarks log.
But quite often this is not the case and only a "loop not vectorized" message is given as a missed-optimization remark.
See https://godbolt.org/z/41Yxvcsn9

I believe the cause is that these messages are emitted by LoopVectorizeHints::emitRemarkWithHints()

llvm-project/llvm/lib/Transforms/Vectorize/LoopVectorizationLegality.cpp

Line 215 in b01cdde

void LoopVectorizeHints::emitRemarkWithHints() const {

This is called from multiple locations in LoopVectorize.cpp, therefore multiple causes of vectorization failure collapse into the non-specific "loop not vectorized" message.

[fhahn]

cc @tobias-stadler

[tobias-stadler]

The vectorization failure reason is emitted in the form of "Analysis" Remarks (see the console output of https://godbolt.org/z/x7TjW67PT), because there can be multiple reasons why a loop fails to vectorize. The "Missed" Remark itself just reports the final decision. I'm not familiar with how the opt-viewer in compiler explorer invokes clang, but it might need to enable -Rpass-analysis=loop-vectorize.

[TiborGY]

The vectorization failure reason is emitted in the form of "Analysis" Remarks (see the console output of https://godbolt.org/z/x7TjW67PT), because there can be multiple reasons why a loop fails to vectorize. The "Missed" Remark itself just reports the final decision. I'm not familiar with how the opt-viewer in compiler explorer invokes clang, but it might need to enable -Rpass-analysis=loop-vectorize.

Interesting! When the optimization report is enabled in Compiler Explorer, it runs Clang with -fsave-optimization-record and expects to read the remarks (missed, passed and analysis remarks) from the resulting file. Indeed, all three types of remarks are represented in the Opt Viewer tool at the link I have given.

So I would guess that for some reason these analysis remarks are not making their way into the YAML file. The Clang manual says "If -foptimization-record-passes is not used, all the passes are included in the optimization record." This option is not used by CE, so it should get all pass remarks.

-Rpass-analysis=loop-vectorize does get me these remarks into the console output, but even when both that and -fsave-optimization-record are set, these remarks are only written to the console.

[tobias-stadler]

I think this is an issue with how compiler explorer filters for analysis remarks. The remark is in the YAML (see https://godbolt.org/z/bKq3fY8hM), but it has the remark type AnalysisFPCommute. My guess is that compiler explorer filters for Analysis, so it misses the important remark here. Currently, we rarely use those "subclasses" of remarks, so I'm not surprised that noone noticed this before. I'm in the process of reworking the infrastructure for this. For now, I would suggest fixing the filter in compiler explorer to match Analysis* (Existing remark types:

llvm-project/llvm/include/llvm/Remarks/Remark.h

Line 66 in 983c8b6

enum class Type {

)

[TiborGY]

For now, I would suggest fixing the filter in compiler explorer to match Analysis*

Thanks, will do