cmd/compile: excessive time spent in line numbering

[DanielMorsing]

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.

[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

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[prattmic]

cc @golang/compiler

The source getting compiled is in https://github.com/valkey-io/valkey-go/blob/main/internal/cmds/gen_search_test.go, I believe.

[prattmic]

In triage, our theory is that most of the function calls in these functions are inlined, which results in a huge number of positions since each one has its own inline tree.