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Loop opts should not be recomputing pred lists from scratch #49030

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AndyAyersMS opened this issue Mar 2, 2021 · 11 comments · Fixed by #80769
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Loop opts should not be recomputing pred lists from scratch #49030

AndyAyersMS opened this issue Mar 2, 2021 · 11 comments · Fixed by #80769
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area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI
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8.0.0

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@AndyAyersMS
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AndyAyersMS commented Mar 2, 2021 •
edited by BruceForstall
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Several loop opts end up calling fgUpdateChangedFlowGraph when done, and this calls fgComputePreds, leaking previous pred lists and losing the profile weights carefully calculated earlier.

This seems wrong-headed; I think those optimizations should maintain pred lists and not rebuild them.

Removing the call to fgComputePreds shows a number of places in loop opts where pred lists are not properly updated, and more downstream failures from places unknown where BBF_JMP_TARGET is not properly set, so cleaning this up may be nontrivial.

category:cq
theme:loop-opt
skill-level:expert
cost:medium
impact:medium
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See notes in #48850 for some context.

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cc @BruceForstall

@AndyAyersMS AndyAyersMS removed the untriaged New issue has not been triaged by the area owner label Mar 2, 2021
@AndyAyersMS AndyAyersMS added the area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI label Mar 2, 2021
@BruceForstall BruceForstall self-assigned this Mar 3, 2021
@BruceForstall BruceForstall added this to the 6.0.0 milestone Mar 3, 2021
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We used to always need to rebuild pred lists after renumbering blocks due to pred list ordering being dependent on block number, so presumably phases knew they could be sloppy and things would get cleaned up later. But with the new ensurePredListOrder()/reorderPredList(), that's no longer required, as renumbering also keeps preds lists properly ordered.

Maybe we need a pred list checker that runs fgComputePreds but doesn't update the IR, and compares the results against the existing flowgraph (does such a thing exist already?), then fgUpdateChangedFlowGraph can call the checker to verify that no preds need to be updated, and if that check fails, we know which phase caused the problem.

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I get post-phase check failures about missing preds if I remove the call to fgComputePreds so I think the current checking may be sufficient?

The missing jump target flags might be harder to track down; it's not clear if we have agreement about when the jump target bit needs to be set, and we don't seem to check this post phase. It is checked in codegen.

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BruceForstall commented Apr 13, 2021 •
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I'm thinking we should get rid of BBF_JMP_TARGET entirely and replace its use by BBF_HAS_LABEL for marking blocks that need labels. Then, add a pass over the blocks just before codegen that sets BBF_HAS_LABEL on all necessary blocks, like genPrepForEHCodegen does for EH constructs. Then, we wouldn't need to set and maintain the BBF_JMP_TARGET throughout compilation. We may lose a couple uninteresting asserts but would greatly simplify block maintenance.

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Sounds good to me.

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BBF_JMP_TARGET was removed with #51208

BruceForstall added a commit to BruceForstall/runtime that referenced this issue Apr 23, 2021
@BruceForstall
Don't recompute preds lists during loop cloning
46b4b6d 
Instead, add and modify the appropriate preds when the mechanical
cloning is performed. This will preserve existing profile data
on the edges.

Contributes to dotnet#49030

No x86/x64 SPMI asm diffs.
@BruceForstall BruceForstall mentioned this issue Apr 23, 2021
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BruceForstall added a commit that referenced this issue Apr 24, 2021
@BruceForstall
Don't recompute preds lists during loop cloning (#51757)
51a116d 
Instead, add and modify the appropriate preds when the mechanical
cloning is performed. This will preserve existing profile data
on the edges.

Contributes to #49030

No x86/x64 SPMI asm diffs.
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@BruceForstall Please link this issue with your Loop Optimization user story #43549.

@BruceForstall BruceForstall mentioned this issue Apr 29, 2021
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@EgorBo EgorBo mentioned this issue May 8, 2021
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Loop cloning stopped recomputing preds lists after optimization with #51757

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BruceForstall commented May 10, 2021 •
edited by AndyAyersMS
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Remaining phases calling fgUpdateChangedFlowGraph that recompute preds lists:

@BruceForstall BruceForstall modified the milestones: 6.0.0, 7.0.0 Jul 6, 2021
@BruceForstall BruceForstall mentioned this issue Jul 6, 2021
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@BruceForstall BruceForstall mentioned this issue Feb 15, 2022
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@BruceForstall BruceForstall removed this from the 7.0.0 milestone May 26, 2022
@BruceForstall BruceForstall added this to the 8.0.0 milestone May 26, 2022
@AndyAyersMS AndyAyersMS mentioned this issue Jan 4, 2023
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AndyAyersMS added a commit to AndyAyersMS/runtime that referenced this issue Jan 13, 2023
@AndyAyersMS
JIT: maintain pred lists during loop unrolling
6bebcb1 
We now update pred lists during loop unrolling, rather than recomputing
them from scratch.

There are several parts to the fix: first, `optRedirectBlock' now has
a new ability to add pred references for the flow from a newly cloned
block, be it either to a remapped successor or a non-remapped successor.
Along with this we no longer copy over the block ref count in `CloneBlockState`.
These changes allow us to create the right pred links and ref counts in the
interior of a cloned subgraph.

Second, we now scrub block references from the original loop body blocks
instead of just setting their ref counts to zero.

Finally, we fix up references for exterior flow into and out of the unroll
complex.

Addresses one of the cases mentioned in dotnet#49030.
@AndyAyersMS AndyAyersMS mentioned this issue Jan 13, 2023
Merged
AndyAyersMS added a commit that referenced this issue Jan 18, 2023
@AndyAyersMS
JIT: maintain pred lists during loop unrolling (#80625)
e2d192a 
We now update pred lists during loop unrolling, rather than recomputing
them from scratch.

There are several parts to the fix: first, `optRedirectBlock' now has
a new ability to add pred references for the flow from a newly cloned
block, be it either to a remapped successor or a non-remapped successor.
Along with this we no longer copy over the block ref count in `CloneBlockState`.
These changes allow us to create the right pred links and ref counts in the
interior of a cloned subgraph.

Second, we now scrub block references from the original loop body blocks
instead of just setting their ref counts to zero.

Finally, we fix up references for exterior flow into and out of the unroll
complex.

Addresses one of the cases mentioned in #49030.
AndyAyersMS added a commit to AndyAyersMS/runtime that referenced this issue Jan 18, 2023
@AndyAyersMS
JIT: fix remaining phases that were rebuilding pred edges
63ff3a1 
Loop canonicalization now maintains pred edges. GC poll insertion was already
maintaining the edges but was rebuilding them anyways.

Now pred lists are never rebuilt.

Also revise `fgUpdateChangedFlowGraph` so that it no longer has the
ability to remove or rebuild.

Fixes dotnet#49030.
@AndyAyersMS AndyAyersMS mentioned this issue Jan 18, 2023
Merged
@ghost ghost added the in-pr There is an active PR which will close this issue when it is merged label Jan 18, 2023
AndyAyersMS added a commit that referenced this issue Jan 18, 2023
@AndyAyersMS
JIT: fix remaining phases that were rebuilding pred edges (#80769)
7f3f5ef 
Loop canonicalization now maintains pred edges. GC poll insertion was already
maintaining the edges but was rebuilding them anyways.

Now pred lists are never rebuilt.

Also revise `fgUpdateChangedFlowGraph` so that it no longer has the
ability to remove or rebuild.

Fixes #49030.
Also fixes #80772.
@ghost ghost removed the in-pr There is an active PR which will close this issue when it is merged label Jan 18, 2023
mdh1418 pushed a commit to mdh1418/runtime that referenced this issue Jan 24, 2023
@AndyAyersMS @mdh1418
JIT: fix remaining phases that were rebuilding pred edges (dotnet#80769)
416ca81 
Loop canonicalization now maintains pred edges. GC poll insertion was already
maintaining the edges but was rebuilding them anyways.

Now pred lists are never rebuilt.

Also revise `fgUpdateChangedFlowGraph` so that it no longer has the
ability to remove or rebuild.

Fixes dotnet#49030.
Also fixes dotnet#80772.
@ghost ghost locked as resolved and limited conversation to collaborators Feb 18, 2023
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area-CodeGen-coreclr CLR JIT compiler in src/coreclr/src/jit and related components such as SuperPMI
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JIT: fix remaining phases that were rebuilding pred edges AndyAyersMS/runtime
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@AndyAyersMS @BruceForstall @JulieLeeMSFT