x/arch: incorrect disassembly for x86 FMA instruction

[zhangyoufu]

What version of Go are you using (go version)?

$ go version
go version go1.15 darwin/amd64

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/Users/zhangyoufu/Library/Caches/go-build"
GOENV="/Users/zhangyoufu/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/zhangyoufu/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/zhangyoufu/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.15/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.15/libexec/pkg/tool/darwin_amd64"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD=""
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 -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/vf/10556_c97vj8qmvw000tgh5w0000gn/T/go-build228934958=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Sample:

package main

import (
    "log"
    "math"
)

func main() {
    log.Print(math.FMA(0, 0, 0))
}

$ go tool compile -S a.go | head -n 20
"".main STEXT size=183 args=0x0 locals=0x38
    0x0000 00000 (a.go:8)   TEXT    "".main(SB), ABIInternal, $56-0
    0x0000 00000 (a.go:8)   MOVQ    (TLS), CX
    0x0009 00009 (a.go:8)   CMPQ    SP, 16(CX)
    0x000d 00013 (a.go:8)   PCDATA  $0, $-2
    0x000d 00013 (a.go:8)   JLS 173
    0x0013 00019 (a.go:8)   PCDATA  $0, $-1
    0x0013 00019 (a.go:8)   SUBQ    $56, SP
    0x0017 00023 (a.go:8)   MOVQ    BP, 48(SP)
    0x001c 00028 (a.go:8)   LEAQ    48(SP), BP
    0x0021 00033 (a.go:8)   FUNCDATA    $0, gclocals·69c1753bd5f81501d95132d08af04464(SB)
    0x0021 00033 (a.go:8)   FUNCDATA    $1, gclocals·568470801006e5c0dc3947ea998fe279(SB)
    0x0021 00033 (a.go:8)   FUNCDATA    $3, "".main.stkobj(SB)
    0x0021 00033 (a.go:8)   MOVBQZX runtime.x86HasFMA(SB), AX
    0x0029 00041 (a.go:8)   TESTQ   AX, AX
    0x002c 00044 (a.go:9)   JEQ 136
    0x002e 00046 (a.go:9)   XORPS   X0, X0
    0x0031 00049 (a.go:9)   VFMADD231SD X0, X0, X0
    0x0036 00054 (a.go:9)   XORPS   X1, X1
    0x0039 00057 (a.go:9)   MOVUPS  X1, ""..autotmp_1+32(SP)
go tool compile: signal: broken pipe
$ go build a.go
$ go tool objdump -s main.main a | head -n13
TEXT main.main(SB) /Users/zhangyoufu/a.go
  a.go:8        0x10a8760       65488b0c2530000000  MOVQ GS:0x30, CX
  a.go:8        0x10a8769       483b6110        CMPQ 0x10(CX), SP
  a.go:8        0x10a876d       0f869a000000        JBE 0x10a880d
  a.go:8        0x10a8773       4883ec38        SUBQ $0x38, SP
  a.go:8        0x10a8777       48896c2430      MOVQ BP, 0x30(SP)
  a.go:8        0x10a877c       488d6c2430      LEAQ 0x30(SP), BP
  a.go:8        0x10a8781       480fb6054b710f00    MOVZX runtime.x86HasFMA(SB), AX
  a.go:8        0x10a8789       4885c0          TESTQ AX, AX
  a.go:9        0x10a878c       745a            JE 0x10a87e8
  a.go:9        0x10a878e       0f57c0          XORPS X0, X0
  a.go:9        0x10a8791       c4e2f9b9c00f57c9    MOVL $-0x36a8f040, CX
  a.go:9        0x10a8799       0f114c2420      MOVUPS X1, 0x20(SP)

What did you expect to see?

objdump shows FMA instruction VFMADD231SD X0, X0, X0.

What did you see instead?

objdump shows MOVL $-0x36a8f040, CX

[cagedmantis]

/cc @cherrymui

[gopherbot]

Change https://go.dev/cl/618036 mentions this issue: x86: allow decoding of AVX instructions