cmd/compile: optimise away deferred calls to empty functions

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

Does this issue reproduce with the latest release?

go version devel +48c7973 Fri Jul 20 20:08:15 2018 +0000 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/ainar/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/ainar/go"
GOPROXY=""
GORACE=""
GOROOT="/home/ainar/go/gotip"
GOTMPDIR=""
GOTOOLDIR="/home/ainar/go/gotip/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
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 -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build664294769=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/TbjFF3fe8Wu

$ go run -gcflags '-S' assertopt.go

What did you expect to see?

Lines 12, 13, and 14 skipped by the compiler.

What did you see instead?

Line 12 is skipped, but defers on 13 and 14 are still there. The Go compiler leaves those in, despite the fact that they are NOOPs.

This makes it hard to create assertions (in this case, postconditions) that don't hurt the performance of production builds using build tags.