cmd/compile: elide call to runtime.growslice with provable capacity availability

[dsnet]

Using go1.12

Consider the following snippet:

/*12*/	func Append(b []byte) []byte {  
/*13*/		if cap(b)-len(b) > 3 {      
/*14*/			b = append(b, 0, 1, 2)  
/*15*/		}                           
/*16*/		return b                    
/*17*/	}

This compiles to the following snippet:

 "".Append STEXT size=192 args=0x30 locals=0x48
 	0x0000 00000 (/tmp/sandbox188310960/main.go:12)	TEXT	"".Append(SB), ABIInternal, $72-48
 	0x0000 00000 (/tmp/sandbox188310960/main.go:12)	MOVQ	(TLS), CX
 	0x0009 00009 (/tmp/sandbox188310960/main.go:12)	CMPQ	SP, 16(CX)
 	0x000d 00013 (/tmp/sandbox188310960/main.go:12)	JLS	182
 	0x0013 00019 (/tmp/sandbox188310960/main.go:12)	SUBQ	$72, SP
 	0x0017 00023 (/tmp/sandbox188310960/main.go:12)	MOVQ	BP, 64(SP)
 	0x001c 00028 (/tmp/sandbox188310960/main.go:12)	LEAQ	64(SP), BP
 	0x0021 00033 (/tmp/sandbox188310960/main.go:13)	MOVQ	"".b+96(SP), AX
 	0x0026 00038 (/tmp/sandbox188310960/main.go:13)	MOVQ	"".b+88(SP), CX
 	0x002b 00043 (/tmp/sandbox188310960/main.go:13)	MOVQ	AX, DX
 	0x002e 00046 (/tmp/sandbox188310960/main.go:13)	SUBQ	CX, AX
 	0x0031 00049 (/tmp/sandbox188310960/main.go:13)	CMPQ	AX, $3
 	0x0035 00053 (/tmp/sandbox188310960/main.go:13)	JLE	172
- 	0x0037 00055 (/tmp/sandbox188310960/main.go:14)	LEAQ	3(CX), AX
- 	0x003b 00059 (/tmp/sandbox188310960/main.go:14)	CMPQ	AX, DX
- 	0x003e 00062 (/tmp/sandbox188310960/main.go:14)	JHI	105
 	0x0040 00064 (/tmp/sandbox188310960/main.go:14)	MOVQ	"".b+80(SP), BX
 	0x0045 00069 (/tmp/sandbox188310960/main.go:14)	MOVW	$256, (BX)(CX*1)
 	0x004b 00075 (/tmp/sandbox188310960/main.go:14)	MOVB	$2, 2(BX)(CX*1)
 	0x0050 00080 (/tmp/sandbox188310960/main.go:16)	MOVQ	BX, "".~r1+104(SP)
 	0x0055 00085 (/tmp/sandbox188310960/main.go:16)	MOVQ	AX, "".~r1+112(SP)
 	0x005a 00090 (/tmp/sandbox188310960/main.go:16)	MOVQ	DX, "".~r1+120(SP)
 	0x005f 00095 (/tmp/sandbox188310960/main.go:16)	MOVQ	64(SP), BP
 	0x0064 00100 (/tmp/sandbox188310960/main.go:16)	ADDQ	$72, SP
 	0x0068 00104 (/tmp/sandbox188310960/main.go:16)	RET
- 	0x0069 00105 (/tmp/sandbox188310960/main.go:14)	LEAQ	type.uint8(SB), BX
- 	0x0070 00112 (/tmp/sandbox188310960/main.go:14)	MOVQ	BX, (SP)
- 	0x0074 00116 (/tmp/sandbox188310960/main.go:14)	MOVQ	"".b+80(SP), BX
- 	0x0079 00121 (/tmp/sandbox188310960/main.go:14)	MOVQ	BX, 8(SP)
- 	0x007e 00126 (/tmp/sandbox188310960/main.go:14)	MOVQ	CX, 16(SP)
- 	0x0083 00131 (/tmp/sandbox188310960/main.go:14)	MOVQ	DX, 24(SP)
- 	0x0088 00136 (/tmp/sandbox188310960/main.go:14)	MOVQ	AX, 32(SP)
- 	0x008d 00141 (/tmp/sandbox188310960/main.go:14)	CALL	runtime.growslice(SB)
- 	0x0092 00146 (/tmp/sandbox188310960/main.go:14)	MOVQ	40(SP), BX
- 	0x0097 00151 (/tmp/sandbox188310960/main.go:14)	MOVQ	48(SP), AX
- 	0x009c 00156 (/tmp/sandbox188310960/main.go:14)	MOVQ	56(SP), DX
- 	0x00a1 00161 (/tmp/sandbox188310960/main.go:14)	ADDQ	$3, AX
- 	0x00a5 00165 (/tmp/sandbox188310960/main.go:14)	MOVQ	"".b+88(SP), CX
- 	0x00aa 00170 (/tmp/sandbox188310960/main.go:14)	JMP	69
 	0x00ac 00172 (/tmp/sandbox188310960/main.go:16)	MOVQ	CX, AX
 	0x00af 00175 (/tmp/sandbox188310960/main.go:16)	MOVQ	"".b+80(SP), BX
 	0x00b4 00180 (/tmp/sandbox188310960/main.go:16)	JMP	80
 	0x00b6 00182 (/tmp/sandbox188310960/main.go:16)	NOP
 	0x00b6 00182 (/tmp/sandbox188310960/main.go:12)	CALL	runtime.morestack_noctxt(SB)
 	0x00bb 00187 (/tmp/sandbox188310960/main.go:12)	JMP	0
 	0x0000 64 48 8b 0c 25 00 00 00 00 48 3b 61 10 0f 86 a3  dH..%....H;a....
 	0x0010 00 00 00 48 83 ec 48 48 89 6c 24 40 48 8d 6c 24  ...H..HH.l$@H.l$
 	0x0020 40 48 8b 44 24 60 48 8b 4c 24 58 48 89 c2 48 29  @H.D$`H.L$XH..H)
 	0x0030 c8 48 83 f8 03 7e 75 48 8d 41 03 48 39 d0 77 29  .H...~uH.A.H9.w)
 	0x0040 48 8b 5c 24 50 66 c7 04 0b 00 01 c6 44 0b 02 02  H.\$Pf......D...
 	0x0050 48 89 5c 24 68 48 89 44 24 70 48 89 54 24 78 48  H.\$hH.D$pH.T$xH
 	0x0060 8b 6c 24 40 48 83 c4 48 c3 48 8d 1d 00 00 00 00  .l$@H..H.H......
 	0x0070 48 89 1c 24 48 8b 5c 24 50 48 89 5c 24 08 48 89  H..$H.\$PH.\$.H.
 	0x0080 4c 24 10 48 89 54 24 18 48 89 44 24 20 e8 00 00  L$.H.T$.H.D$ ...
 	0x0090 00 00 48 8b 5c 24 28 48 8b 44 24 30 48 8b 54 24  ..H.\$(H.D$0H.T$
 	0x00a0 38 48 83 c0 03 48 8b 4c 24 58 eb 99 48 89 c8 48  8H...H.L$X..H..H
 	0x00b0 8b 5c 24 50 eb 9a e8 00 00 00 00 e9 40 ff ff ff  .\$P........@...
 	rel 5+4 t=16 TLS+0
 	rel 108+4 t=15 type.uint8+0
 	rel 142+4 t=8 runtime.growslice+0
 	rel 183+4 t=8 runtime.morestack_noctxt+0

The regions marked in red seem superfluous. At a high-level, we already know that cap(b)-len(b) > 3 so the check at 0x0037 is entire redundant. Also since there is guaranteed capacity, the code at 0x0069 and on is entirely unnecessary.

[bcmills]

CC @randall77 @josharian @martisch for proving things.
(Please let me know if I've got that list wrong — the prove pass doesn't quite correspond to a source path in https://dev.golang.org/owners.)

[martisch]

Note sure how common the check (cap(b)-len(b) > 3) with subtraction is.
Just for hand optimisation: When using an addition (cap(b) > len(b)+3), which is the way append does the capacity check, the runtime.growslice call is removed in go1.11. https://godbolt.org/z/-2qtCX
(godbolt shows a call to growslice for go tip, I did not check go1.12 so there might be a regression).

For prove and any elimination of dead code the more general optimisation seems to be to infer from x-y > z that x > z+y where x & y & z >= 0 and eliminate branches accordingly. I did not check if the >= 0 condition is needed and this also always works with negative numbers and overflow.

[josharian]

Yeah, the owners list for the compiler is unfortunate, in that there isn’t always a nice source path correspondence. For prove, I also think of @aclements, @rasky, and @zdjones.