cmd/compile: mangled DWARF location for incoming string parameter

[thanm]

What version of Go are you using (go version)?

This is with Go 1.17/tip.

Does this issue reproduce with the latest release?

Yes.

What operating system and processor architecture are you using (go env)?

linux/amd64

What did you do?

Build this program:

package main

var G int

//go:noinline
func docall(f func()) {
	f()
}

//go:noinline
func another(x int) {
	println(G)
}

//go:noinline
func livestring(s string) {
	docall(func() {
		println("s is", s)
	})
	G++
	another(0)
}

//go:noinline
func livestring2(s string) {
	docall(func() {
		println("s is", s)
	})
	G++
	another(int(s[0]))
}

func main() {
	livestring("foo")
	livestring2("bar")
}

If you stop in 'livestring2", the DWARF location expression emitted for the parameter 's' is damaged/incorrect.

$ gdb livestring
...
(gdb) b main.livestring2
Breakpoint 1 at 0x455680: file /usr/local/google/home/thanm/livestring.go, line 25.
(gdb) run
Starting program: /usr/local/google/home/thanm/livestring 
...
Thread 1 "livestring" hit Breakpoint 1, main.livestring2 (s=<error reading variable: access outside bounds of object referenced via synthetic pointer>) at /usr/local/google/home/thanm/livestring.go:25
25	func livestring2(s string) {
(gdb) p s
access outside bounds of object referenced via synthetic pointer
(gdb) info address s
Symbol "s" is multi-location:
  Base address 0x455680  Range 0x455680-0x4556c8: a variable in $rax [8-byte piece], and a variable in $rax [8-byte piece], and a complex DWARF expression:
     0: DW_OP_fbreg 8
    [8-byte piece], and a complex DWARF expression:
     0: DW_OP_fbreg 8
    [8-byte piece]
  Range 0x4556c8-0x45571b: a complex DWARF expression:
     0: DW_OP_call_frame_cfa
    [8-byte piece], and a complex DWARF expression:
     0: DW_OP_call_frame_cfa
    [8-byte piece], and a complex DWARF expression:
     0: DW_OP_fbreg 8
    [8-byte piece], and a complex DWARF expression:
     0: DW_OP_fbreg 8
    [8-byte piece]
.
(gdb) 

What did you expect to see?

Sane location expression

What did you see instead?

Malformed location expression. We should not be seeing 4 pieces, this is only a 2-piece variable. Something is going wrong somewhere along the line.

[thanm]

There are two things going wrong here.

The first problem is the same as issue 46845, the problem with zero-width types (I've already sent a fix, it is being reviewed).

The second problem is that the SSA name table is getting messed up as a result of inconsistency in the compiler has to how the internals of a Go string are represented. In most places a string looks like "{ *uint8, int }", however there are also a few places where it winds up looking like "{ unsafe.Pointer, uintptr }". What happens is that the same value gets entered into f.NamedValues twice, once for each type. Here's an example excerpt from the GOSSAFUNC+ output for the test case in question, this is just after the expand_calls phase:

name s[string]: []
name s[*uint8]: [v44]
name s+8[int]: [v31]
name s[unsafe.Pointer]: [v44]
name s+8[uintptr]: [v31]

Here "s" is the incoming string param. This should be fairly easy to take care of by tracking down the oddball types and canonicalizing the type on "{ *uint8, int }".

[gopherbot]

Change https://golang.org/cl/362715 mentions this issue: cmd/compile: use canonical string representation in abiutils

[gopherbot]

Change https://golang.org/cl/364037 mentions this issue: cmd/link: add some better DWARF location expression tests

[gopherbot]

Change https://golang.org/cl/364674 mentions this issue: dwarf_testing: add new pseudo-pacakge with DWARF location expression tests