cmd/compile: maps with type params doesn't recognize map-clearing range idiom

[wgrr]

In go1.18, the compiler output for this program (https://go.dev/play/p/GVufoDzAG5-) on tested targets {openbsd,linux}/{amd64,arm64,386} the generic version does not use runtime.mapclear, instead it iterates over the map deleting each element, I expected both functions generate a very similar code, instead clear generates (roughly):

mov    rbx,rax
lea    rax,type.map[string]bool
xchg   ax,ax
call   runtime.mapclear

clear2 (copied from exp/maps.Clear) generates (roughly):

call   runtime.mapiterinit
jmp    loop
start: mov rcx,QWORD PTR [rdx]
mov    rdi,QWORD PTR [rdx+0x8]
lea    rax,type.go.shape.map[string]bool_0
mov    rbx,QWORD PTR [rsp+0x20]
call   runtime.mapdelete_faststr
lea    rax,[rsp+0x28]
call   runtime.mapiternext
loop:  mov rdx,QWORD PTR [rsp+0x28]
test   rdx,rdx
jne    start

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN="/home/w/bin"
GOCACHE="/home/w/.cache/go-build"
GOENV="/home/w/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/w/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/w/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.18"
GCCGO="/usr/bin/gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
GOWORK=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build896486634=/tmp/go-build -gno-record-gcc-switches"

[randall77]

In clear, at the point where we look for this optimization, we have:

.   .   DELETE-Args
.   .   .   NAME-main.m esc(no) Class:PPARAM Offset:0 OnStack MAP-map[string]bool tc(1) # issue51699.go:4:12
.   .   .   NAME-main.i esc(no) Class:PAUTO Offset:0 OnStack string tc(1) # issue51699.go:5:6

In clear2, we have

.   .   DELETE-Args
.   .   .   NAME-main.m esc(no) Class:PPARAM DictIndex:1 Offset:0 OnStack go.shape.map[string]bool_0 tc(1) # issue51699.go:11:46
.   .   .   CONVNOP Implicit string tc(1) # issue51699.go:13:9
.   .   .   .   NAME-main.i esc(no) Class:PAUTO DictIndex:2 Offset:0 OnStack go.shape.string_1 tc(1) # issue51699.go:12:6

The extra CONVNOP is because i can be anything of string shape, but the type of the map key must be exactly string. I think we can expand the mapclear optimization to allow nop conversions on the arguments.

That said, there may be a larger issue here, which is that I think the type of m should be map[go.shape.string_1]go.shape.bool_2, not go.shape.map[string]bool_0. Something about making shape types out of type parameters that are dependent on other type parameters might need some tweaking.

[cuonglm]

FYI, clear and clear2 are compiled the same with unified IR.

[gopherbot]

Change https://go.dev/cl/393434 mentions this issue: cmd/compile: allow noop conversions when comparing expressions