[PATCH] Debug info: Support fragmented variables

[David Blaikie]

Hi Adrian,

Thanks for taking a look at this issue - it's certainly something we'll
need.

It might be easier to review if you posted to Phabricator? (Chandler should
really look at the SROA parts of this and strongly prefers Phab - and
looking at this there's a few first-blush comments I'd like to make that'd
be much easier in that format)


On Mon, Feb 3, 2014 at 10:45 AM, Adrian Prantl <aprantl at apple.com> wrote:

> Debug info: Support fragmented variables.
>
> Currently, LLVM cannot represent locations for aggregate variables that
> are fragmented across multiple Values. This situation arises, e.g.,
> during SROA, or even as part of the x86_64 calling convention when a struct
> is passed by value. This patch adds the functionality to emit
> DWARF location expressions using DW_OP(_bit)_piece to express partial
> values.
>
> This is implemented by adding a new operation type OpPiece to complex
> DIVariables which accepts an offset and a size.
>
> The canonical example would be something like this:
>
> > typedef struct { long int a; int b;} S;
> >
> > int foo(S s) {
> >         return s.b;
> > }
>
> which at -O1 is now codegen'd into:
>
> > ; Function Attrs: nounwind readnone ssp uwtable
> > define i32 @foo(i64 %s.coerce0, i32 %s.coerce1) #0 {
> > entry:
> >   tail call void @llvm.dbg.value(metadata !{i64 %s.coerce0}, i64 0,
> metadata !20)
> >   tail call void @llvm.dbg.value(metadata !{i32 %s.coerce1}, i64 0,
> metadata !21)
> >   ret i32 %s.coerce1, !dbg !24
> > }
>
>
> with this patch we'd emit the following DWARF:
> >
> > 0x00000047:         TAG_formal_parameter [3]
> >                      AT_name( "s" )
> >                      AT_decl_file( "struct.c" )
> >                      AT_decl_line( 3 )
> >                      AT_type( {0x00000069} ( S ) )
> >                      AT_location( 0x00000000
> >                         0x0000000000000000 - 0x0000000000000008: rdi,
> piece 0x00000008
> >                         0x0000000000000000 - 0x0000000000000006: rsi,
> bit-piece 32 64
> >                         0x0000000000000006 - 0x0000000000000008: rax,
> bit-piece 32 64  )
> >
> >
>
>
> cheers,
> adrian
>
>
>
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