cmd/compile: non-constant rotations are not emitted

[rasky]

Please answer these questions before submitting your issue. Thanks!

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

`go version devel +34b455da44 Sat Feb 4 07:24:20 2017 +0000 darwin/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/rasky/Sources/go"
GORACE=""
GOROOT="/Users/rasky/Sources/gosrc"
GOTOOLDIR="/Users/rasky/Sources/gosrc/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/lw/jdbk7p_d4gj6qpydczpbw2080000gn/T/go-build646363960=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

func testRotate_easy(op uint32, cnt uint) uint32 {
	cnt = cnt & 31
	return (op << cnt) | (op << (32 - cnt))
}

func testRotate_complex(op uint32) uint32 {
	rot := uint((op >> 7) & 0x1E)
	return ((op & 0xFF) >> rot) | ((op & 0xFF) << (32 - rot))
}

What did you expect to see?

Rotate opcode emitted

What did you see instead?

Rotate opcode not emitted

[rasky]

/cc @randall77

[randall77]

See also #18616 (addbits.RotateLeft).

[gopherbot]

CL https://golang.org/cl/39711 mentions this issue.

[randall77]

I'm going to close this as done. The easy case is handled now as is, or using bits.RotateLeft.
The hard case seems less pressing, and you can get the rotate behavior by either adding an additional &0x1f or using bits.Rotateleft.