[PATCH] Debug info: Support fragmented variables

[Adrian Prantl]

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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