cmd/compile: cleanup code paths that aren't needed now that -p is required

[mdempsky]

Since CL 391014 (a987aaf), cmd/compile now requires the -p flag. There's a lot of code to handle code paths that aren't needed any more.

At least a few cleanups I can think of:

1. Initialize types.LocalPkg with base.Ctxt.Pkgpath instead of "".
2. Change types.TypeHash to use t.LinkString instead of fmtTypeIDHash.
3. Get rid of "package height" and just sort method sets on identifier and package path. (We can just write package height as 0 in the iexport data format; public importers ignore it anyway.)
4. Change cmd/link (or cmd/internal/obj?) to Fatalf if it sees "". in any linker symbols.

Perhaps other opportunities.

The iexport file format uses "" for the current package too, which probably needs to be retained for compatibility with legacy importers.

/cc @randall77 @dr2chase @cuonglm

[randall77]

I think point 2 will be the right fix to revert/improve what CL 389474 did.

[cuonglm]

I think point 2 will be the right fix to revert/improve what CL 389474 did.

I'm going to send a CL for this.

[cuonglm]

I think point 2 will be the right fix to revert/improve what CL 389474 did.

I'm going to send a CL for this.

Hmm, simply changing types.TypeHash to use t.LinkString() make go run run.go panic with:

panic: interface conversion: reflect.Type is *reflect.rtype, not *reflect.rtype (types from different scopes)

goroutine 1 [running]:
reflect.PointerTo(...)
	/home/cuonglm/sources/go/src/reflect/type.go:1442
encoding/json.newTypeEncoder({0x566c30?, 0x516440?}, 0x1?)
	/home/cuonglm/sources/go/src/encoding/json/encode.go:422 +0x3e5
encoding/json.typeEncoder({0x566c30?, 0x516440})
	/home/cuonglm/sources/go/src/encoding/json/encode.go:404 +0x16b
encoding/json.typeFields({0x566c30, 0x528440})
	/home/cuonglm/sources/go/src/encoding/json/encode.go:1385 +0x668
encoding/json.cachedTypeFields({0x566c30?, 0x528440})
	/home/cuonglm/sources/go/src/encoding/json/encode.go:1417 +0xab
encoding/json.(*decodeState).object(0xc0000022a8, {0x5123c0?, 0xc00002e5c0?, 0xc000061980?})
	/home/cuonglm/sources/go/src/encoding/json/decode.go:654 +0x21e
encoding/json.(*decodeState).value(0xc0000022a8, {0x5123c0?, 0xc00002e5c0?, 0xc0000619d0?})
	/home/cuonglm/sources/go/src/encoding/json/decode.go:373 +0x45
encoding/json.(*decodeState).unmarshal(0xc0000022a8, {0x5123c0?, 0xc00002e5c0?})
	/home/cuonglm/sources/go/src/encoding/json/decode.go:180 +0x1de
encoding/json.(*Decoder).Decode(0xc000002280, {0x5123c0, 0xc00002e5c0})
	/home/cuonglm/sources/go/src/encoding/json/stream.go:73 +0x177
main.glob..func1()
	/home/cuonglm/sources/go/test/run.go:68 +0x1a5
main.init()
	/home/cuonglm/sources/go/test/run.go:75 +0x3f9
exit status 2

It seems to need more works than I expect, so feel free to go ahead if you have one, otherwise, I will take another look when I'm back with my laptop.

cc @mdempsky @randall77

[mdempsky]

@cuonglm At least step 1 needs to happen before we can do step 2. :)

[gopherbot]

Change https://go.dev/cl/393715 mentions this issue: cmd/compile: set LocalPkg.Path to -p flag

[gopherbot]

Change https://go.dev/cl/393716 mentions this issue: cmd/compile/internal/types: use Type.LinkString in TypeHash

[gopherbot]

Change https://go.dev/cl/393896 mentions this issue: test: remove obsolete test cases that misuse -p

[gopherbot]

Change https://go.dev/cl/403757 mentions this issue: cmd/compile: combo set LocalPkg.Path to -p flag

[gopherbot]

Change https://go.dev/cl/406059 mentions this issue: cmd/compile/internal/ir: remove PkgFuncName assumption that LocalPkg.Path == ""

[gopherbot]

Change https://go.dev/cl/406056 mentions this issue: cmd/compile/internal/noder: remove unified IR assumptions on LocalPkg.Path == ""

[gopherbot]

Change https://go.dev/cl/406057 mentions this issue: cmd/compile/internal/typecheck: remove iexport assumption of LocalPkg.Path == ""

[gopherbot]

Change https://go.dev/cl/406055 mentions this issue: cmd/compile/internal/gc: parse command-line flags earlier