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cmd/asm: misassembles MOVD #65193

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clausecker opened this issue Jan 21, 2024 · 3 comments
Closed

cmd/asm: misassembles MOVD #65193

clausecker opened this issue Jan 21, 2024 · 3 comments
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compiler/runtime Issues related to the Go compiler and/or runtime.

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@clausecker
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Go version

go version go1.20.12 freebsd/amd64

Output of go env in your module/workspace:

GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/usr/home/fuz/.cache/go-build"
GOENV="/usr/home/fuz/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="freebsd"
GOINSECURE=""
GOMODCACHE="/usr/home/fuz/src/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="freebsd"
GOPATH="/usr/home/fuz/src/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go120"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go120/pkg/tool/freebsd_amd64"
GOVCS=""
GOVERSION="go1.20.12"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="cc"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
CGO_CFLAGS="-I$HOME/include -I/usr/local/include"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-L$HOME/lib -L/usr/local/lib"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3711417635=/tmp/go-build -gno-record-gcc-switches"

What did you do?

I have tried to assemble an amd64 assembly file that contains an instruction like

MOVD (AX), X0

This instruction is found in clausecker/pospop. As specified by Intel, it is supposed to perform a 32 bit load from (SI) into X0.

What did you see happen?

I observe that the instruction was assembled to the instruction

a.s:10		0x331			f30f7e00		MOVQ 0(AX), X0                       // movq (%rax),%xmm0

which performs a 64 bit load from memory. When (AX) is right before an unmapped page, this causes a crash.

What did you expect to see?

I expect the MOVD mnemonic to assembly into a 32 bit move instruction, not a 64 bit one.
@gopherbot gopherbot added the compiler/runtime Issues related to the Go compiler and/or runtime. label Jan 21, 2024
@randall77
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MOVD is a 64-bit float load. MOVL is the 32-bit int load.
(The Go assembler syntax is different in this respect than one you might be used to, like at&t.)

@clausecker
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That sounds reasonable. Still, one heck of a confusion. Perhaps this could be documented better. A lot of the SIMD stuff has Intel-like mnemonics as opposed to the Plan 9-like mnemonics one would expect, so it's kind of a tossup which one you need to use and what behaviour you get.

@randall77
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Yeah I agree it is kind of a mishmash. I think at some point we decided there were too many intel instructions to make a new plan9 syntax version of each one.

We could add more documentation to https://go.dev/doc/asm if that would help. CLs appreciated.

I'm going to close this issue, as it isn't a bug.

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@clausecker @randall77 @gopherbot