cmd/compile: optimize reflect.TypeOf(x)==reflect.TypeOf(y)

[adonovan]

I notice that the code generated for a comparison of rtypes is surprisingly poor, involving a ton of spills and a call to an intrinsic:

func sameType(x, y any) bool {
	return reflect.TypeOf(x) != reflect.TypeOf(y)
}

golang.org/x/tools/gopls/internal/golang.sameType STEXT size=176 args=0x20 locals=0x48 funcid=0x0 align=0x0
	0x0000 00000 	L0505	TEXT	golang.org/x/tools/gopls/internal/golang.sameType(SB), ABIInternal, $80-32
	0x0000 00000 	L0505	MOVD	16(g), R16
	0x0004 00004 	L0505	PCDATA	$0, $-2
	0x0004 00004 	L0505	CMP	R16, RSP
	0x0008 00008 	L0505	BLS	144
	0x000c 00012 	L0505	PCDATA	$0, $-1
	0x000c 00012 	L0505	MOVD.W	R30, -80(RSP)
	0x0010 00016 	L0505	MOVD	R29, -8(RSP)
	0x0014 00020 	L0505	SUB	$8, RSP, R29
	0x0018 00024 	L0505	MOVD	R0, golang.org/x/tools/gopls/internal/golang.x(FP)
	0x001c 00028 	L0505	MOVD	R1, golang.org/x/tools/gopls/internal/golang.x+8(FP)
	0x0020 00032 	L0505	MOVD	R2, golang.org/x/tools/gopls/internal/golang.y+16(FP)
	0x0024 00036 	L0505	MOVD	R3, golang.org/x/tools/gopls/internal/golang.y+24(FP)
	0x0028 00040 	L0505	FUNCDATA	$0, gclocals·emgMVsd5mo9g0HGy8lQYQA==(SB)
	0x0028 00040 	L0505	FUNCDATA	$1, gclocals·lV4l69IdnTMCVxlqSaJ25Q==(SB)
	0x0028 00040 	L0505	FUNCDATA	$2, golang.org/x/tools/gopls/internal/golang.sameType.stkobj(SB)
	0x0028 00040 	L0505	FUNCDATA	$5, golang.org/x/tools/gopls/internal/golang.sameType.arginfo1(SB)
	0x0028 00040 	L0505	FUNCDATA	$6, golang.org/x/tools/gopls/internal/golang.sameType.argliveinfo(SB)
	0x0028 00040 	L0505	PCDATA	$3, $1
	0x0028 00040 	     	NOP
	0x0028 00040 	L1303	MOVD	R0, internal/abi.a-16(SP)
	0x002c 00044 	L1303	MOVD	R1, internal/abi.a-8(SP)
	0x0030 00048 	L0179	MOVD	internal/abi.a-16(SP), R4
	0x0034 00052 	     	NOP
	0x0034 00052 	     	NOP
	0x0034 00052 	     	NOP
	0x0034 00052 	L1303	MOVD	R2, internal/abi.a-32(SP)
	0x0038 00056 	L1303	MOVD	R3, internal/abi.a-24(SP)
	0x003c 00060 	L0179	MOVD	internal/abi.a-32(SP), R3
	0x0040 00064 	     	NOP
	0x0040 00064 	     	NOP
	0x0040 00064 	L2719	CMP	$0, R4
	0x0044 00068 	L0510	MOVD	$go:itab.*reflect.rtype,reflect.Type(SB), R5
	0x004c 00076 	L0510	CSEL	NE, R5, ZR, R0
	0x0050 00080 	L2719	CMP	$0, R3
	0x0054 00084 	L0510	CSEL	NE, R5, ZR, R5
	0x0058 00088 	L0510	CMP	R5, R0
	0x005c 00092 	L2719	BEQ	104
	0x0060 00096 	L2719	MOVD	$1, R3
	0x0064 00100 	L2719	JMP	128
	0x0068 00104 	L2719	CMP	$0, R4
	0x006c 00108 	L0510	CSEL	NE, R4, ZR, R1
	0x0070 00112 	L2719	CMP	$0, R3
	0x0074 00116 	L0510	CSEL	NE, R3, ZR, R2
	0x0078 00120 	L0510	PCDATA	$1, $1
	0x0078 00120 	L0510	CALL	runtime.ifaceeq(SB)
	0x007c 00124 	L0510	EOR	$1, R0, R3
	0x0080 00128 	L0510	MOVD	R3, R0
	0x0084 00132 	L0510	MOVD	-8(RSP), R29
	0x0088 00136 	L0510	MOVD.P	80(RSP), R30
	0x008c 00140 	L0510	RET	(R30)
	0x0090 00144 	L0510	NOP
	0x0090 00144 	L0505	PCDATA	$1, $-1
	0x0090 00144 	L0505	PCDATA	$0, $-2
	0x0090 00144 	L0505	STP	(R0, R1), 8(RSP)
	0x0094 00148 	L0505	STP	(R2, R3), 24(RSP)
	0x0098 00152 	L0505	MOVD	R30, R3
	0x009c 00156 	L0505	CALL	runtime.morestack_noctxt(SB)
	0x00a0 00160 	L0505	PCDATA	$0, $-1
	0x00a0 00160 	L0505	LDP	8(RSP), (R0, R1)
	0x00a4 00164 	L0505	LDP	24(RSP), (R2, R3)
	0x00a8 00168 	L0505	JMP	0

Is there any reason it couldn't be recognized as a special case and compiled as if written using unsafe Go code like this:

func sameType(x, y any) bool {
	type eface struct {
		t, v unsafe.Pointer
	}
	return ((*eface)(unsafe.Pointer(&x))).t == ((*eface)(unsafe.Pointer(&y))).t
}

which boils down to a pair of loads and a comparison:

golang.org/x/tools/gopls/internal/golang.sameType STEXT size=48 args=0x20 locals=0x0 funcid=0x0 align=0x0 leaf
	0x0000 00000 	L0505	TEXT	golang.org/x/tools/gopls/internal/golang.sameType(SB), LEAF|NOFRAME|ABIInternal, $0-32
	0x0000 00000 	L0505	FUNCDATA	$0, gclocals·emgMVsd5mo9g0HGy8lQYQA==(SB)
	0x0000 00000 	L0505	FUNCDATA	$1, gclocals·acF1O9X4FQHZUTLQivBEZA==(SB)
	0x0000 00000 	L0505	FUNCDATA	$2, golang.org/x/tools/gopls/internal/golang.sameType.stkobj(SB)
	0x0000 00000 	L0505	FUNCDATA	$5, golang.org/x/tools/gopls/internal/golang.sameType.arginfo1(SB)
	0x0000 00000 	L0505	MOVD	R0, golang.org/x/tools/gopls/internal/golang.x(FP)
	0x0004 00004 	L0505	MOVD	R1, golang.org/x/tools/gopls/internal/golang.x+8(FP)
	0x0008 00008 	L0505	MOVD	R2, golang.org/x/tools/gopls/internal/golang.y+16(FP)
	0x000c 00012 	L0505	MOVD	R3, golang.org/x/tools/gopls/internal/golang.y+24(FP)
	0x0010 00016 	L0509	MOVD	golang.org/x/tools/gopls/internal/golang.x(FP), R1
	0x0014 00020 	L0509	MOVD	golang.org/x/tools/gopls/internal/golang.y+16(FP), R2
	0x0018 00024 	L0509	CMP	R1, R2
	0x001c 00028 	L0509	CSET	EQ, R0
	0x0020 00032 	L0509	RET	(R30)

[gabyhelp]

Related Issues

• cmd/compile: Devirtualize calls when concrete type behind interface is statically known #19361 (closed)
• cmd/compile: (dev.typeparams) assembly generated for a simple generic function is massive #46998 (closed)
• cmd/compile: silly amounts of code generated for simple struct equality tests #15303 (closed)
• cmd/go: unnecessary stdlib recompilation when passing -m flag #22527 (closed)
• cmd/compile: unexpected memequal call in short string comparison #24765
• cmd/compile: elide unnecessary copies #54613
• cmd/compile: use branchless instructions for simple conditionals #11813 (closed)
• cmd/compile: elide write barriers for pointer swaps #16765 (closed)

Related Code Changes

• cmd/compile: prioritize non-CALL struct member comparisons
• reflect: simplify TypeOf implementation

_{(Emoji vote if this was helpful or unhelpful; more detailed feedback welcome in this discussion.)}

[gopherbot]

Change https://go.dev/cl/635435 mentions this issue: cmd/compile: avoid ifaceeq call if we know the interface is direct