x/tools/gopls: buildssa analyzer is slow on github.com/aws/aws-sdk-go-v2/service/ec2

[adonovan]

This session involves a gopls that has been modified to enable nilness.Analyzer, which itself has been modified to declare a dummy Fact that it doesn't import or export, so as to cause nilness (and thus buildssa) to be run on all dependencies. The gopls analysis driver has also been instrumented to display the wall time of each buildssa unit. Observe how expensive SSA construction is for the ec2 package, which has an extraordinary number of methods.

(This is a minimization of a performance problem reported by a user running staticcheck (whose buildir is almost identical to buildssa) on github.com/hashiform/terraform-provider-aws/internal/provider.) See #61178.

w$ git clone https://github.com/aws/aws-sdk-go-v2
w$ cd service/ec2
ec2$ (cd ~/w/xtools && go build -o x ./gopls ) && time ~/w/xtools/x --profile.cpu=prof check ./internal/customizations/unit_test.go
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/internal/timeconv 383.709µs
2023/07/21 11:18:39 exec buildssa@net/http/internal/testcert 285.916µs
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/internal/strings 291.541µs
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/internal/sdk 1.026167ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/protocol/ec2query 527.208µs
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/ratelimit 1.357458ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/internal/sync/singleflight 1.268208ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/internal/rand 360.208µs
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/protocol/query 10.287959ms
2023/07/21 11:18:39 exec buildssa@net/http/httptest 14.392416ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws 16.287958ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/defaults 2.928167ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/internal/configsources 4.141917ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/internal/endpoints/v2 3.565167ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/signer/internal/v4 6.881791ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/middleware 7.872875ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/service/ec2/internal/endpoints 3.785583ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/retry 5.897459ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/transport/http 8.835125ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/aws/signer/v4 8.82425ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/service/ec2/types 25.772458ms
2023/07/21 11:18:39 exec buildssa@github.com/aws/aws-sdk-go-v2/service/internal/presigned-url 8.336917ms
2023/07/21 11:18:41 exec buildssa@github.com/aws/aws-sdk-go-v2/service/ec2 1.056296292s  <--- note
2023/07/21 11:18:41 exec buildssa@github.com/aws/aws-sdk-go-v2/service/ec2/internal/customizations_test [github.com/aws/aws-sdk-go-v2/service/ec2/internal/customizations.test] 25.270583ms

real	0m3.619s
user	0m10.746s
sys	0m1.532s

The task of this issue is to bring down the cost of buildssa so that it is proportional to the size of the package and the exportdata and factdata sizes of its direct imports, not to its transitive size. go/ssa eagerly constructs methods for every type reachable via reflection, which seems quite unnecessary.

@findleyr

[adonovan]

The cost of buildssa is primarily:

• (a) that it calls CreatePackage for every package reachable through the types.Packages.Imports graph, which is far more than we need to construct ssa.Packages for; and
• (b) that ssa.Package.build gathers runtime types aggressively for all exported members, and creates and builds all their methods. Such aggressive reachability makes sense for whole program analysis but shouldn't be built into "separate analysis" clients such as buildssa.

I think it should be possible to alter buildssa to compute a more precise set of necessary reachable things (even if that should cost an extra pass over the syntax) and then make go/ssa build only these things (possibly requiring a new internal API function, at least temporarily).

Longer term, we should audit go/ssa through the lens of separate analysis and identify all the places where it deals with cross-package dependencies so that we can reduce to a minimum the amount of stuff from other packages (direct and indirect imports) that needs to be created before building.

[gopherbot]

Change https://go.dev/cl/512136 mentions this issue: go/analysis/passes/buildssa: reduce package creation

[gopherbot]

Change https://go.dev/cl/511715 mentions this issue: go/ssa: construct methods lazily

[findleyr]

Since we can't fix this until the equivalent change is ported to staticcheck, we're not going to land this for the next release. Moving to subsequent milestone.

[findleyr]

This unfortunately seems unlikely to get fixed for v0.14; bumping to v0.15.0.

[gopherbot]

Change https://go.dev/cl/537478 mentions this issue: go/analysis/passes/buildssa: reduce package creation

[adonovan]

I've made a number of changes to go/ssa to achieve this goal, that I forgot to link to this issue. Here they are:

• https://go.dev/cl/538357 - go/ssa: clarify Program.RuntimeTypes
• https://go.dev/cl/538358 - go/ssa: build methods of RuntimeTypes lazily
• https://go.dev/cl/538435 - go/ssa: TestStdlib: report count of syntactic functions too
• https://go.dev/cl/538763 - go/ssa: create methods on demand
• https://go.dev/cl/539255 - go/ssa: make isParameterized thread-safe
• https://go.dev/cl/539256 - go/ssa: hang instances off generic Function, not Program
• https://go.dev/cl/539257 - go/ssa: remove Function.Syntax optimization
• https://go.dev/cl/539555 - go/ssa: update builder doc comment
• https://go.dev/cl/539659 - go/analysis/passes/nilness: fix bug in method value
• https://go.dev/cl/539675 - go/ssa: don't treat _ specially in Prog.{Func,Const,Var}Value
• https://go.dev/cl/540216 - go/ssa: two minor cleanups