cmd/compile: improve code generation for temporary slice copy and inlining

[rasky]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.8beta2 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="/usr/local/Cellar/go/1.8beta2_1/libexec"
GOTOOLDIR="/usr/local/Cellar/go/1.8beta2_1/libexec/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-build116611858=/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 DebugPerf(mem []byte, x int) {
	op := binary.LittleEndian.Uint32(mem[x:])
	fmt.Println(op)
}

What did you expect to see?

When inlining, the temporary slice could be fully elided. Moreover, a single bound-check on x+3 would be sufficient.

What did you see instead?

	test.go:9	0x1086bc1	488b442470		MOVQ 0x70(SP), AX
	test.go:9	0x1086bc6	488b4c2460		MOVQ 0x60(SP), CX
	test.go:9	0x1086bcb	4839c8			CMPQ CX, AX
	test.go:9	0x1086bce	0f879c000000		JA 0x1086c70
	test.go:9	0x1086bd4	4829c1			SUBQ AX, CX
	test.go:9	0x1086bd7	488b542468		MOVQ 0x68(SP), DX
	test.go:9	0x1086bdc	4829c2			SUBQ AX, DX
	test.go:9	0x1086bdf	48ffca			DECQ DX
	test.go:9	0x1086be2	48c1fa3f		SARQ $0x3f, DX
	test.go:9	0x1086be6	4883f2ff		XORQ $-0x1, DX
	test.go:9	0x1086bea	4821d0			ANDQ DX, AX
	test.go:9	0x1086bed	4883f903		CMPQ $0x3, CX
	test.go:9	0x1086bf1	7676			JBE 0x1086c69
	test.go:9	0x1086bf3	488b4c2458		MOVQ 0x58(SP), CX
	test.go:9	0x1086bf8	8b0401			MOVL 0(CX)(AX*1), AX

Basically, the last line (with AX=x) plus a single bound-check would be sufficient.

[randall77]

Related: #16813. The patch there doesn't seem to help here, unfortunately.

[randall77]

Also related: #15397