DMD generates bad DWARF for static array return values

[dlangBugzillaToGithub]

Vladimir Panteleev (@CyberShadow) reported this on 2024-11-28T07:22:49Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=24888

Description

If an extern(C) function in a D program has a return value larger than 16 bytes, trying to call it and print its return value from gdb results in the error message "Too few arguments in function call.".

Reproducer:

cat > bug.d <<EOF
extern(C) auto a() { return (ubyte[16]).init; }
extern(C) auto b() { return (ubyte[17]).init; }
void main(){}
EOF

dmd -g bug

gdb -ex start \
	-ex 'echo === a() ===
' \
	-ex 'p a()' \
	-ex 'echo === b() ===
' \
	-ex 'p b()' \
	-ex 'stop' \
	-ex 'quit' \
	./bug

liushuyu on the gdb bugtracker ( https://sourceware.org/bugzilla/show_bug.cgi?id=30576 ) provided the following explanation:

> (In reply to Vladimir Panteleev from comment #2)
> > It still happens here. GDB 15.1, DMD v2.109.1.
> 
> Oh, I see now, you were using the DMD compiler.
> 
> After some investigations, I think it's a DMD bug. Here's the DWARF dump of
> the function bug.b():
> 
> 0x000000a9:   DW_TAG_subprogram
>                 DW_AT_sibling   (0x000000e7)
>                 DW_AT_MIPS_linkage_name ("b")
>                 DW_AT_type      (0x00000095 "ubyte[17]")
>                 DW_AT_external  (0x01)
>                 DW_AT_pure      (0x01)
>                 DW_AT_name      ("bug.b")
>                 DW_AT_decl_file ("/tmp/./bug.d")
>                 DW_AT_decl_line (2)
>                 DW_AT_decl_column       (16)
>                 DW_AT_low_pc    (0x0000000000042728)
>                 DW_AT_high_pc   (0x0000000000042754)
>                 DW_AT_frame_base        (0x0000004c:
>                    [0x0000000000042728, 0x0000000000042729): DW_OP_breg7
> RSP+8
>                    [0x0000000000042729, 0x000000000004272b): DW_OP_breg7
> RSP+16
>                    [0x000000000004272b, 0x0000000000042754): DW_OP_breg6
> RBP+16)
> 
> 0x000000d3:     DW_TAG_formal_parameter
>                   DW_AT_name    ("__HID1")
>                   DW_AT_type    (0x000000a4 "ubyte (*)[17]")
>                   DW_AT_artificial      (0x00)
>                   DW_AT_decl_file       ("/tmp/./bug.d")
>                   DW_AT_decl_line       (0)
>                   DW_AT_decl_column     (0)
>                   DW_AT_location        (DW_OP_fbreg -24)
> 
> It seems like DMD accidentally included the stack return argument in the
> debug data, which is incorrect (according to the DWARF v5 spec,
> DW_TAG_formal_parameter is only valid for parameters passing into the
> function, not implicit return values defined by ABI).
> 
> If the DWARF dump looks abstract the debug information describes
> the function as:
> 
> extern(C) ubyte[17] b(ubyte[17] __HID1);
> 
> So GDB thought you should pass something to the function.
> 
> In contrast, the GDC compiler wrote the debug information like this:
> 
> 0x00000184:   DW_TAG_subprogram
>                 DW_AT_external  (true)
>                 DW_AT_name      ("b")
>                 DW_AT_decl_file ("/tmp/bug.d")
>                 DW_AT_decl_line (2)
>                 DW_AT_decl_column       (16)
>                 DW_AT_type      (0x00000166 "ubyte[17]")
>                 DW_AT_low_pc    (0x00000000000251c9)
>                 DW_AT_high_pc   (0x00000000000251e6)
>                 DW_AT_frame_base        (DW_OP_call_frame_cfa)
>                 DW_AT_call_all_calls    (true)
> 
> Which is the correct definition for your bug.b function.