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cmd/compile: PGO compiler diagnostics #58153

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prattmic opened this issue Jan 30, 2023 · 3 comments
Open

cmd/compile: PGO compiler diagnostics #58153

prattmic opened this issue Jan 30, 2023 · 3 comments
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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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Today, the compiler has some limited diagnostics of behavior with PGO using the -d=pgoinline=1 flag. These are really just ad-hoc logging points. We want more robust diagnostics, for reasons @josharian lays out well in #55022 (comment):

The more opaque the toolchain and its inputs are, the harder it is to (a) write safety checks that detect performance problems before they make it to production and (b) diagnose performance issues after they make it to production.

Plus (c) of making it easier for us to investigate compilers issues filed here.

Our current thinking for initial work here is to provide more complete/structured output of all optimization decisions, rather than just "decisions due to PGO", because the latter is ambiguous. e.g., if we decide to do an inline because of PGO and that affects the non-PGO heuristics further up the call tree, does that count as a "PGO" decision.

cc @cherrymui @aclements
@prattmic prattmic added this to the Go1.21 milestone Jan 30, 2023
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jan 30, 2023
@prattmic prattmic mentioned this issue Jan 30, 2023
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@mknyszek mknyszek added the NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one. label Jan 30, 2023
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prattmic commented Jun 5, 2023

Bumping to 1.22. There is more structural work to be done.

For 1.21, PGO devirtualization does include structured JSON output with -d=pgodebug=3 detailing every single call with its type (direct, indirect interface, etc) and whether it was devirtualized. That can then be used for external analysis (e.g., with a tool like https://go.dev/cl/494717).

@prattmic prattmic modified the milestones: Go1.21, Go1.22 Jun 5, 2023
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Change https://go.dev/cl/528400 mentions this issue: cmd/compile: add pgohash for debugging/bisecting PGO optimizations

gopherbot pushed a commit that referenced this issue Sep 19, 2023
@cherrymui
cmd/compile: add pgohash for debugging/bisecting PGO optimizations
3fb86fb 
When a PGO build fails or produces incorrect program, it is often
unclear what the problem is. Add pgo hash so we can bisect to
individual optimization decisions, which often helps debugging.

Related to #58153.

Change-Id: I651ffd9c53bad60f2f28c8ec2a90a3f532982712
Reviewed-on: https://go-review.googlesource.com/c/go/+/528400
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
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Minor change: -m differentiates PGO vs static devirtualization, but doesn't note PGO-enabled inlines (-d=pgodebug=1 is required for that).

@cherrymui cherrymui modified the milestones: Go1.22, Backlog Nov 27, 2023
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compiler/runtime Issues related to the Go compiler and/or runtime. NeedsInvestigation Someone must examine and confirm this is a valid issue and not a duplicate of an existing one.
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@mknyszek @prattmic @gopherbot @cherrymui