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cmd/compile: shorten code to load a pointer to a global #20452

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josharian opened this issue May 22, 2017 · 2 comments

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commented May 22, 2017

package p

var s string = "abc"

func f() {
	g(&s)
}

//go:noinline
func g(s *string) {}

With NOPs and the like removed, f compiles to:

"".f STEXT size=62 args=0x0 locals=0x10
	0x0000 00000 (t.go:5)	TEXT	"".f(SB), $16-0
	0x0000 00000 (t.go:5)	MOVQ	(TLS), CX
	0x0009 00009 (t.go:5)	CMPQ	SP, 16(CX)
	0x000d 00013 (t.go:5)	JLS	55
	0x000f 00015 (t.go:5)	SUBQ	$16, SP
	0x0013 00019 (t.go:5)	MOVQ	BP, 8(SP)
	0x0018 00024 (t.go:5)	LEAQ	8(SP), BP
	0x001d 00029 (t.go:5)	LEAQ	"".s(SB), AX
	0x0024 00036 (t.go:6)	MOVQ	AX, (SP)
	0x0028 00040 (t.go:6)	CALL	"".g(SB)
	0x002d 00045 (t.go:7)	MOVQ	8(SP), BP
	0x0032 00050 (t.go:7)	ADDQ	$16, SP
	0x0036 00054 (t.go:7)	RET
	0x0037 00055 (t.go:5)	CALL	runtime.morestack_noctxt(SB)
	0x003c 00060 (t.go:5)	JMP	0
	rel 5+4 t=16 TLS+0
	rel 32+4 t=15 "".s+0
	rel 41+4 t=8 "".g+0
	rel 56+4 t=8 runtime.morestack_noctxt+0

The instructions at 29 and 36 look inefficient. "".s(SB) is a linker-resolved relocation, so seems like we should be able to do this in a single MOVQ instruction: MOVQ "".s(SB), (SP).

@randall77 is that correct?

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commented May 22, 2017

I don't think this would work. Such an instruction would need two modR/M fields, which isn't allowed.

foo:
	movq	100(%rip),(%rsp)
 gcc -c tmp1.s
tmp1.s:2:17: error: invalid operand for instruction
 movq 100(%rip),(%rsp)
                ^~~~~~
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commented May 22, 2017

Ahh. Thanks.

@josharian josharian closed this May 22, 2017

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