hash/maphash: Comparable[string] causes unnecessary allocation

[aclements]

Go version

go version devel go1.24-fb5fa2a839 Mon Nov 25 18:39:27 2024 +0000 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='/usr/bin/gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/austin/.cache/go-build'
GODEBUG=''
GOENV='/home/austin/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build1361679833=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/home/austin/r/go/pkg/mod'
GONOPROXY=''
GONOSUMDB=''
GOOS='linux'
GOPATH='/home/austin/r/go'
GOPRIVATE=''
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/home/austin/go.tmp'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/austin/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/home/austin/go.tmp/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='devel go1.24-fb5fa2a839 Mon Nov 25 18:39:27 2024 +0000'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

I ran the following using go test -bench . comparable_test.go: https://play.golang.org/p/jv23SKNWLy6

What did you see happen?

I saw the following results:

goos: linux
goarch: amd64
cpu: 11th Gen Intel(R) Core(TM) i7-1185G7 @ 3.00GHz
BenchmarkMaphashString-8   	30859322	        38.03 ns/op	       2 B/op	       1 allocs/op
BenchmarkMapString-8       	55735060	        20.58 ns/op	       0 B/op	       0 allocs/op
BenchmarkMaphashInt-8      	173806796	         7.706 ns/op	       0 B/op	       0 allocs/op
PASS

Notably, BenchmarkMaphashString has one allocation per op.

What did you expect to see?

I expected to see zero allocations per iteration in any of these benchmarks because the result of hashing a string is independent of that string's address. I discussed this issue at length in #54670 (comment) (possibly at too much length!). While the current implementation deals with the issue of stack pointers by unconditionally escaping them, it doesn't currently handle the more subtle case of strings, as demonstrated by this benchmark.

cc @randall77 @golang/compiler

[gabyhelp]

Related Issues

• hash/maphash: Hash always escapes #35636 (closed)
• cmd/compile, runtime: runtime.slicebytetostringtmp is NOT emitted for map[string][]byte m[string(byteslice)] in a certain case #47698 (closed)
• runtime: reuse map string key instead of allocating #55930
• runtime: Benchmarking reports unexpected memory allocation when using range with map structure #37757
• maps: Equal should not allocate on the heap #65329 (closed)
• testing: Wrong number of allocations #54041 (closed)
• runtime: map key hashing is slow for compound types (array/struct) #13271 (closed)
• reflect: map iteration does unnecessary excessive allocation for non-pointer types #32424 (closed)

Related Code Changes

• cmd/compile: use memhash for hashing strings in arrays and structs
• cmd/compile: avoid string allocations when map key is struct or array literal

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[gopherbot]

Change https://go.dev/cl/632715 mentions this issue: hash/maphash, cmd/compile: make Comparable[string] not escape its argument

[cherrymui]

Discussed with @dmitshur , polishing a new feature (to make it more conformant to the proposal) is within the scope of the freeze. So we can target 1.24 for this.