cmd/compile: exponential behavior for deeply nested structs #65540
Description
Go version
tip
Output of go env in your module/workspace:
GO111MODULE=""
GOARCH="arm64"
GOBIN=""
GOCACHE="/Users/khr/Library/Caches/go-build"
GOENV="/Users/khr/Library/Application Support/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="arm64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/khr/gopath/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/khr/gopath"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/Users/khr/go1.20.6"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/Users/khr/go1.20.6/pkg/tool/darwin_arm64"
GOVCS=""
GOVERSION="go1.20.6"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -arch arm64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/z9/dty110711l9cr9w3ktv1_2380000gn/T/go-build2801687852=/tmp/go-build -gno-record-gcc-switches -fno-common"What did you do?
test.go:
package a
type S0 struct{ a, b S1 }
type S1 struct{ a, b S2 }
type S2 struct{ a, b S3 }
type S3 struct{ a, b S4 }
type S4 struct{ a, b S5 }
type S5 struct{ a, b S6 }
type S6 struct{ a, b S7 }
type S7 struct{ a, b S8 }
type S8 struct{ a, b S9 }
type S9 struct{ a, b S10 }
type S10 struct{ a, b S11 }
type S11 struct{ a, b S12 }
type S12 struct{ a, b S13 }
type S13 struct{ a, b S14 }
type S14 struct{ a, b S15 }
type S15 struct{ a, b S16 }
type S16 struct{ a, b S17 }
type S17 struct{ a, b S18 }
type S18 struct{ a, b S19 }
type S19 struct{ a, b S20 }
type S20 struct{ a, b S21 }
type S21 struct{ a, b S22 }
type S22 struct{ a, b S23 }
type S23 struct{ a, b int }
Compile with
go tool compile -cpuprofile=foo.prof ~/gowork/test.go
It takes a surprisingly long time. The pprof output points to cmd/compile/internal/types/alg.go:AlgType as the culprit. It is walking the exponential-sized object to determine its algorithm class. It should probably memoize the computation somehow so it isn't exponential.
Add more lines to the example to make it even more slower.
Found during discussion in #65495.
What did you see happen?
flat flat% sum% cum cum%
2330ms 28.14% 28.14% 7470ms 90.22% cmd/compile/internal/types.AlgType
2040ms 24.64% 52.78% 2070ms 25.00% cmd/compile/internal/types.(*Type).Fields
1010ms 12.20% 64.98% 3850ms 46.50% cmd/compile/internal/types.IsPaddedField
640ms 7.73% 72.71% 790ms 9.54% cmd/compile/internal/types.PtrDataSize
What did you expect to see?
Fast compilation.
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