[PATCH] D63626: [clang][NewPM] Remove exception handling before loading pgo sample profile data

[Leonard Chan via Phabricator via cfe-commits]

leonardchan added a comment.

In D63626#1553054 <https://reviews.llvm.org/D63626#1553054>, @chandlerc wrote:

> See inline comment, but I think we should just drop the testing of the function attribute bit here rather than adjusting the pipeline.


Ok. Removed the pipeline change for now.

In D63626#1553646 <https://reviews.llvm.org/D63626#1553646>, @davidxl wrote:

> Is there a bug reference somewhere?


No bug has been filed for this, but I'm not sure if this is a bug if there aren't really any issues with PGO under the new PM.



================
Comment at: llvm/lib/Passes/PassBuilder.cpp:665-668
+    // We must also remove exception handling before attaching sample profile
+    // data.
+    MPM.addPass(
+        createModuleToPostOrderCGSCCPassAdaptor(PostOrderFunctionAttrsPass()));
----------------
tejohnson wrote:
> chandlerc wrote:
> > Does the this need to go before the EarlyFPM as well as before the sample profile loader?
> > 
> > Also, this is ... a pretty expensive thing to do... Do we really need this? We've been using ThinLTO and sample PGO with the new PM for a long time and haven't seen any real issue. I think we can probably just leave this difference between the two pass managers and remove the test for this pass running rather than adding it.
> > 
> > CC-ing some others just to double check, but I'm not aware of any issues we've seen with PGO that were because of this discrepancy.
> We don't tend to use exception handling so might not be hitting the issue described in the headline.
> 
> If this is fixing an issue with SamplePGO and EH cleanup interactions, can you add a test that tests for that (i.e. is there a code optimization issue currently?).
I'm not entirely familiar with the expected codegen for SamplePGO or PruneEH and don't really know if not adding these 2 passes breaks anything.

Under the legacy PM, exception handling was removed via `PruneEH` before creating the sample profile loader, so this change was meant to match the behavior since `function-attrs` and `function(simplify-cfg)` (the new PM equivalent for `-prune-eh`) did not initially run before `SampleProfileLoaderPass` was added to the pipeline.

The original patch that added `CodeGen/pgo-sample.c` (D21197) doesn't seem to have any codegen tests either and only checks that PruneEH runs before sample profile loader creation, so I'm also unsure of what code optimizations are meant to happen.


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  https://reviews.llvm.org/D63626/new/

https://reviews.llvm.org/D63626