cmd/compile: handle 64-bit masks better on 32-bit systems

[rsc]

On a 32-bit system, an expression like uint64value & constantUint64Mask could easily be decomposed two 32-bit mask operations. That does not seem to happen (see below, especially the OR instructions). It seems like probably it should.

$ cat x.go
package p

func f(x uint64) uint64 {
	return x & (1<<60 - 1)
}
$ GOARCH=arm go build -gcflags=-S x.go
# command-line-arguments
"".f STEXT size=40 args=0x10 locals=0x0 leaf
	0x0000 00000 (/Users/rsc/x.go:3)	TEXT	"".f(SB), LEAF|NOFRAME|ABIInternal, $-4-16
	0x0000 00000 (/Users/rsc/x.go:3)	FUNCDATA	$0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (/Users/rsc/x.go:3)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (/Users/rsc/x.go:3)	FUNCDATA	$2, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x0000 00000 (/Users/rsc/x.go:4)	PCDATA	$0, $0
	0x0000 00000 (/Users/rsc/x.go:4)	PCDATA	$1, $0
	0x0000 00000 (/Users/rsc/x.go:4)	MOVW	"".x(FP), R0
	0x0004 00004 (/Users/rsc/x.go:4)	SRL	$28, R0, R1
	0x0008 00008 (/Users/rsc/x.go:4)	MOVW	"".x+4(FP), R2
	0x000c 00012 (/Users/rsc/x.go:4)	ORR	R2<<4, R1, R1
	0x0010 00016 (/Users/rsc/x.go:4)	BFXU	$28, R0, $0, R0
	0x0014 00020 (/Users/rsc/x.go:4)	ORR	R1<<28, R0, R0
	0x0018 00024 (/Users/rsc/x.go:4)	MOVW	R0, "".~r1+8(FP)
	0x001c 00028 (/Users/rsc/x.go:4)	SRL	$4, R1, R0
	0x0020 00032 (/Users/rsc/x.go:4)	MOVW	R0, "".~r1+12(FP)
	0x0024 00036 (/Users/rsc/x.go:4)	JMP	(R14)
...
$ GOARCH=386 go build -gcflags=-S x.go
# command-line-arguments
"".f STEXT size=62 args=0x10 locals=0x0
	0x0000 00000 (/Users/rsc/x.go:3)	TEXT	"".f(SB), ABIInternal, $0-16
	0x0000 00000 (/Users/rsc/x.go:3)	MOVL	(TLS), CX
	0x0007 00007 (/Users/rsc/x.go:3)	CMPL	SP, 8(CX)
	0x000a 00010 (/Users/rsc/x.go:3)	JLS	55
	0x000c 00012 (/Users/rsc/x.go:3)	FUNCDATA	$0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x000c 00012 (/Users/rsc/x.go:3)	FUNCDATA	$1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x000c 00012 (/Users/rsc/x.go:3)	FUNCDATA	$2, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB)
	0x000c 00012 (/Users/rsc/x.go:4)	PCDATA	$0, $0
	0x000c 00012 (/Users/rsc/x.go:4)	PCDATA	$1, $0
	0x000c 00012 (/Users/rsc/x.go:4)	MOVL	"".x+8(SP), AX
	0x0010 00016 (/Users/rsc/x.go:4)	SHLL	$4, AX
	0x0013 00019 (/Users/rsc/x.go:4)	MOVL	"".x+4(SP), CX
	0x0017 00023 (/Users/rsc/x.go:4)	MOVL	CX, DX
	0x0019 00025 (/Users/rsc/x.go:4)	SHRL	$28, CX
	0x001c 00028 (/Users/rsc/x.go:4)	ORL	AX, CX
	0x001e 00030 (/Users/rsc/x.go:4)	SHLL	$4, DX
	0x0021 00033 (/Users/rsc/x.go:4)	SHRL	$4, DX
	0x0024 00036 (/Users/rsc/x.go:4)	MOVL	CX, AX
	0x0026 00038 (/Users/rsc/x.go:4)	SHLL	$28, CX
	0x0029 00041 (/Users/rsc/x.go:4)	ORL	DX, CX
	0x002b 00043 (/Users/rsc/x.go:4)	MOVL	CX, "".~r1+12(SP)
	0x002f 00047 (/Users/rsc/x.go:4)	SHRL	$4, AX
	0x0032 00050 (/Users/rsc/x.go:4)	MOVL	AX, "".~r1+16(SP)
	0x0036 00054 (/Users/rsc/x.go:4)	RET
	0x0037 00055 (/Users/rsc/x.go:4)	NOP
	0x0037 00055 (/Users/rsc/x.go:3)	PCDATA	$1, $-1
	0x0037 00055 (/Users/rsc/x.go:3)	PCDATA	$0, $-1
	0x0037 00055 (/Users/rsc/x.go:3)	CALL	runtime.morestack_noctxt(SB)
	0x003c 00060 (/Users/rsc/x.go:3)	JMP	0

[cherrymui]

Emmm. I'm pretty sure it decomposes 64-bit mask to two 32-bit masks when I wrote the code a few years ago. https://go.googlesource.com/go/+/6d1aaf143c879ae7acbd20a1e60bf74681bf6055/src/cmd/compile/internal/ssa/gen/dec64.rules#88

This particular case is interesting, as the generic opt pass (runs before decompose pass) changes the AND to a pair of shifts (x<<4>>4), and the decomposing shifts is more complex.

If it is a different mask, which cannot be rewritten to shifts, it works as expected.

func f(x uint64) uint64 {
	return x & 0x1234567890abcdef
}

	0x0000 00000 (x64.go:4)	MOVW	"".x(FP), R0
	0x0004 00004 (x64.go:4)	AND	$-1867788817, R0, R0
	0x000c 00012 (x64.go:4)	MOVW	R0, "".~r1+8(FP)
	0x0010 00016 (x64.go:4)	MOVW	"".x+4(FP), R0
	0x0014 00020 (x64.go:4)	AND	$305419896, R0, R0
	0x001c 00028 (x64.go:4)	MOVW	R0, "".~r1+12(FP)
	0x0020 00032 (x64.go:4)	JMP	(R14)

Maybe we should either not do the mask-to-shift rewriting for 64-bit ops on 32-bit architectures, or somehow recognizes this pattern and do better in decomposing.

[gopherbot]

Change https://golang.org/cl/183957 mentions this issue: cmd/compile: not rewrite 64-bit AND to shifts on 32-bit machines