Golang cmd/compile: excessive time spent in line numbering

Go version

go version go1.26-devel_1ea7594c12 Tue Aug 12 17:15:55 2025 +0100 linux/amd64

Output of go env in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/daniel/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/daniel/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build2048561827=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/home/daniel/src/valkey-go/go.mod'
GOMODCACHE='/home/daniel/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/daniel/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/daniel/src/godev'
GOSUMDB='sum.golang.org'
GOTELEMETRY='local'
GOTELEMETRYDIR='/home/daniel/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/daniel/src/godev/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.26-devel_1ea7594c12 Tue Aug 12 17:15:55 2025 +0100'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

While investigating #74974, I found that compiling github.com/valkey-io/valkey-go/internal/cmds was very slow, even without the offending change. I took a profile and found most of the time was spent in nilcheckelim, clearing the cached line numberings

excerpt from `-d ssa/number_lines/stats=1':

/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:8:23:   number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   1774    MAXFILE 17506   NFILES  7580    search0
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:111:23:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   1821    MAXFILE 21575   NFILES  4060    search1
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:214:23:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   4077    MAXFILE 28287   NFILES  6712    search2
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:317:20:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   3776    MAXFILE 30555   NFILES  2269    search3
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:420:20:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   3703    MAXFILE 32161   NFILES  1607    search4
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:523:20:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   6803    MAXFILE 34554   NFILES  2384    search5
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:626:20:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   6700    MAXFILE 38035   NFILES  3474    search6
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:729:20:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   6597    MAXFILE 40677   NFILES  2636    search7
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:832:20:     number_lines    SUM_LINE_RANGE  100 MAXMIN_LINE_RANGE   6494    MAXFILE 42164   NFILES  1482    search8
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:935:20:     number_lines    SUM_LINE_RANGE  500 MAXMIN_LINE_RANGE   7399    MAXFILE 43111   NFILES  944 search9
/home/daniel/src/valkey-go/internal/cmds/gen_search_test.go:1038:20:    number_lines    SUM_LINE_RANGE  350 MAXMIN_LINE_RANGE   7620    MAXFILE 43536   NFILES  423 search10

These generated tests, where every expression is an inlined call, results in the line numbering jumping between files, making Func.cachedLineStarts very large, and since we clear it on every Block during nilcheckelim, a significant time is spent here.

pprof excerpt

What did you see happen?

21.51s compile time

What did you expect to see?

Much lower compile times.

Comment From: gabyhelp

Related Issues

• cmd/compile: increased line number churn #20367 (closed)
• cmd/go: test reported as cached, but not actually cached #41593
• cmd/go: add option to output timestamps along with command tracing #22526 (closed)
• cmd/compile: compile time increased greatly in 1.25rc1 with commit ed24bb4e60 #74615 (closed)

Related Code Changes

• cmd/compile: index line number tables by source file to improve sparsity
• cmd/compile: caching result of AlgType
• cmd/compile,sync: make accessing address of zero offset struct field inline cost 0
• [dev.inline] cmd/internal/src: replace src.Pos with syntax.Pos
• cmd/compile: attempt to deflake debug_test.go
• cmd/compile: link source lines to blocks

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