go: cmd/compile: it is not possible to prevent FMA with complex values

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

$ go version
go version go1.13.6 linux/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="on"
GOARCH="arm64"
GOBIN="/home/user/bin"
GOCACHE="/home/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/home/user/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/home/user/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="0"
GOMOD="/dev/null"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build515689865=/tmp/go-build -gno-record-gcc-switches"

What did you do?

Examine the assembly generated for the code at https://play.golang.org/p/JuTC-BPAIJN with GOARCH=arm64

What did you expect to see?

No FMA/FMS instructions emitted.

What did you see instead?

	0x00ac 00172 (/home/user/c.go:19)	PCDATA	ZR, $5
	0x00ac 00172 (/home/user/c.go:19)	FMOVD	8(R8), F4
	0x00b0 00176 (/home/user/c.go:19)	FMSUBD	F1, F3, F4, F3
	0x00b4 00180 (/home/user/c.go:19)	SCVTFD	R2, F5
	0x00b8 00184 (/home/user/c.go:19)	FADDD	F5, F3, F3
	0x00bc 00188 (/home/user/c.go:19)	FMOVD	F3, (R0)(R6)
	0x00c0 00192 (/home/user/c.go:19)	FMULD	F4, F0, F0
	0x00c4 00196 (/home/user/c.go:19)	FMADDD	F1, F0, F2, F0
	0x00c8 00200 (/home/user/c.go:19)	FMOVD	ZR, F1
	0x00cc 00204 (/home/user/c.go:19)	FADDD	F0, F1, F0

No amount of wrapping the operands in complex128 prevents this AFAICS.

About this issue

Original URL
State: open
Created 4 years ago
Comments: 16 (15 by maintainers)

Most upvoted comments

The original issues that prompted this have been resolved, in part by changing the compiler to not emit fused operation instructions. The original issues covered the range of differences in precision causing changes in output precision, differences in precision changing gross behaviour and an actual real bug in our code (~50 issues were rolled up in the original problem).

Because of the complexity of the issues we had (and the number of sites where fused operations can be constructed), it would have been very helpful to be able to tell the compiler to not emit fused operation instructions in order to exclude that as a cause. I’ll note that discordance between precision at different sites has historically been a cause of catastrophic failures (I hope no-one is using Go for missile guidance).

kortschak on Feb 22, 2020