[LLVMdev] Generating DWARF information that pretends an outparam is the return value

[Josh Matthews]

Unfortunately this is what I'm already doing, and it's not working. I
drew inspiration from a disassembly of a program that showed that

struct foo {
  int a; float b; char buf[80];
}

struct foo get_foo(void) {
  foo f = { 5, 2.5 };
  return f;
}

would turn into

define void @get_foo(%struct.foo* sret %agg.result) nounwind ssp {
  ...
  call void @llvm.memcpy.p0i8.p0i8.i64(i8* %2, i8* %3, i64 88, i32 4,
i1 false), !dbg !24
  ret void, !dbg !25
}

with similar metadata to what you wrote.  I tried adding the sret
attribute to my generated functions, but there was no change - maybe
it doesn't work for non-structural types.

Cheers,
Josh

On 6 December 2011 19:18, Devang Patel <dpatel at apple.com> wrote:
>
> On Dec 6, 2011, at 4:13 PM, Devang Patel wrote:
>
> Hi Josh,
>
> On Dec 4, 2011, at 9:33 PM, Josh Matthews wrote:
>
> I'm working on generating debug information for Rust, and I'm
> currently stumped on how to generate DWARF output via LLVM that will
> correctly represent this (eg. when exiting from foo2(), I'd like to
> see "Value returns is $1 = 5"). I'm generating debug metadata nodes
> directly instead of using DIBuilder since it doesn't seem to
> encapsulate situations like this.
>
>
> So, I assume you figured out how to create metadata nodes for TAG_subprogram
> and corresponding TAG_subroutine_type. If yes, then the zero'th element of
> array of types you supply (subroutine_type is encoded as composite type, see
> http://llvm.org/docs/SourceLevelDebugging.html#format_composite_type) is
> return type.
>
> Let's say you have
>
>      1
>      2 int foo() {
>      3  char c = 'a';
>      4  return 42;
>      5 }
>
> Try generating following metadata nodes (I am not including all of them
> here) to describe 'char' as the return type of function 'foo'.
>
> !1 = metadata !{i32 589870, i32 0, metadata !2, metadata !"foo", metadata
> !"foo", metadata !"", metadata !2, i32 2, metadata !3, i1 false, i1 true,
> i32 0, i32 0, i32 0, i32 0, i1 f\
> alse, i32 ()* @foo, null, null} ; [ DW_TAG_subprogram ]
>
> !3 = metadata !{i32 589845, metadata !2, metadata !"", metadata !2, i32 0,
> i64 0, i64 0, i32 0, i32 0, i32 0, metadata !4, i32 0, i32 0} ; [
> DW_TAG_subroutine_type ]
>
>
>
> !4 = metadata !{metadata !8}
>
>
>
> !8 = metadata !{i32 589860, metadata !0, metadata !"char", null, i32 0, i64
> 8, i64 8, i64 0, i32 0, i32 6} ; [ DW_TAG_base_type ]
>
>
> Next, when you generate metadata nodes for arguments, slide argument numbers
> by 1. (http://llvm.org/docs/SourceLevelDebugging.html#format_variables)
>
> I have not tried this myself, but I hope it helps.
> -
> Devang
>
>
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