cmd/compile: wrong pcln entry for instruction

[aarzilli]

go version devel go1.18-9e6ad46bcc Sun Nov 7 04:57:22 2021 +0000 linux/amd64

Given https://play.golang.org/p/wsCMMD9Jk9J compiled with -gcflags='-N -l' the method PushBack compiles to:

TEXT main.(*List[go.shape.int_0]).PushBack(SB) ./buggy.go
...
  buggy.go:22		0x454a90		488b00			MOVQ 0(AX), AX					
  buggy.go:31		0x454a93		8400			TESTB AL, 0(AX)					
  buggy.go:22		0x454a95		c6400801		MOVB $0x1, 0x8(AX)				
...

The instruction 0x454a93 gets assigned to a line outside of the PushBack function, in this case it's the call to PushBack but in more complex examples can be something completely unrelated. Can't make this happen without embedding and type parameters.

cc @dr2chase

[hanchaoqun]

The error seems occurred in the stenciling, when transformDot is doing the XDOT to DOT conversion, the NewSelectorExpr called in AddImplicitDots passed the wrong Pos parameter, that should use n.X.Pos instead of base.Pos n.Pos instead of base.Pos.

go/src/cmd/compile/internal/typecheck/subr.go

Line 163 in 9e6ad46

dot := ir.NewSelectorExpr(base.Pos, ir.ODOT, n.X, path[c].field.Sym)

after noder2 (*List["".List.PushBack.V]).PushBack
...
.   .   .   AS tc(1) # pcln.go:22:22
.   .   .   .   XDOT main.Circular bool tc(1) # pcln.go:22:12
.   .   .   .   .   XDOT main.root PTR-*ExtNode["".List.PushBack.V] tc(1) # pcln.go:22:7
.   .   .   .   .   .   NAME-main.list Class:PPARAM Offset:0 OnStack PTR-*List["".List.PushBack.V] tc(1) # pcln.go:19:7
.   .   .   .   LITERAL-true bool tc(1) # pcln.go:22:24
...

stenciled (*List[%2eshape.int_0]).PushBack
...
.   .   .   AS tc(1) # pcln.go:22:22
.   .   .   .   DOT main.Circular bool tc(1) # pcln.go:22:12
.   .   .   .   .   DOTPTR main.Node Implicit main.Node tc(1) # pcln.go:31:2
.   .   .   .   .   .   DOTPTR main.root PTR-*ExtNode[%2eshape.int_0] tc(1) # pcln.go:22:7
.   .   .   .   .   .   .   NAME-main.list Class:PPARAM DictIndex:2 Offset:0 OnStack Used PTR-*List[%2eshape.int_0] tc(1) # pcln.go:19:7
.   .   .   .   LITERAL-true bool tc(1) # pcln.go:22:24
...

[gopherbot]

Change https://golang.org/cl/361917 mentions this issue: cmd/compile: NewSelectorExpr used use n.X.Pos instead of base.Pos

[cagedmantis]

/cc @randall77 @griesemer

[gopherbot]

Change https://golang.org/cl/362714 mentions this issue: cmd/compile: add line number test for #49436