cmd/compile: emitted DWARF location for complex128 param is incorrect

[thanm]

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

Using tip (linux.amd64)

$ go version
go version devel +565e70fcef Mon Mar 29 08:11:13 2021 +0000 linux/amd64

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?

Compile this program:

https://play.golang.org/p/_v676oJ4LLO

and look at the DWARF generated for the function main.tryme

What did you expect to see?

Correct location expressions for all params

What did you see instead?

For the parameter "z", here is the parameter DIE:

 <2><3c428>: Abbrev Number: 16 (DW_TAG_formal_parameter)
    <3c429>   DW_AT_name        : z
    <3c42b>   DW_AT_variable_parameter: 0
    <3c42c>   DW_AT_decl_line   : 7
    <3c42d>   DW_AT_type        : <0x3e46a>
    <3c431>   DW_AT_location    : 0x5a5a3 (location list)

and here is the relevent .debug_loc section:

    0005a5a3 ffffffffffffffff 000000000045f340 (base address)
    0005a5b3 000000000045f340 000000000045f399 (DW_OP_fbreg: 24; DW_OP_piece: 8; DW_OP_fbreg: 24; DW_OP_piece: 8)
    0005a5cd <End of list>

Note the two pieces are the same (for real and imaginary). If I stop in a debugger and print the value of the param, I get:

(gdb) p z
$1 = 2 + 2i
(gdb)

which is obviously wrong (should be 1 + 2i).

I looked at older versions of Go (1.15, 1.14, and it appears that they get this wrong as well, although in slightly different ways).