Query regarding relocatable stuff in DWO.

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1

Hello Everyone,

Consider this asm snippet for a moment –

section .debug_info.dwo
…
.long .Ldebug_macinfo0 # DW_AT_macros

…

.section .debug_macinfo.dwo,“e”,@progbits

.Ldebug_macinfo0:
.byte 0x1 # Define macro

…
When compiling this with clang, produces
fatal error: error in backend: A dwo section may not contain relocations – seems fair,
Since we don’t want relocations in DWO file. Please note here GCC{trunk} has no problem with this /Okay with relocations in DWO/ Why??

Now the real problem – Since DW_AT_macros/macinfo can appear in both split/non-split case.{Only in one or another}. Pointing to macro/macro.dwo section.
Now clang won’t allow me to use above approach. So only option left to avoid relocations, emitting DW_AT_macros attribute as a difference of labels.
i.e
.long .Lcu_macro_begin1-.debug_macro.dwo # DW_AT_macro_info

This worked great, clang is fine, GDB also happy. But that was manual assembly hack I opted. I didn’t find any API in /MCStreamer/ that can provide me a difference of 2 labels as label.
This is needed here –
TheCU.addSectionLabel(TheCU.getUnitDie(),dwarf::DW_AT_macros,
/Label here?/, TLOF.getDwarfMacroDWOSection()->getBeginSymbol()); – requires a label to add in.

Theirs analogous EmitLabelDifference(Sym, Sym, Size) API – but that’s for emitting + return type is void, So can’t plug it in in here.

Any hints, how to overcome this ?

Thanks in anticipation!
Sourabh.

2

Hello Everyone,

Consider this asm snippet for a moment –

section .debug_info.dwo
…
.long .Ldebug_macinfo0 # DW_AT_macros

…

.section .debug_macinfo.dwo,“e”,@progbits

.Ldebug_macinfo0:
.byte 0x1 # Define macro

…
When compiling this with clang, produces
fatal error: error in backend: A dwo section may not contain relocations – seems fair,
Since we don’t want relocations in DWO file. Please note here GCC{trunk} has no problem with this /Okay with relocations in DWO/ Why??

Now the real problem – Since DW_AT_macros/macinfo can appear in both split/non-split case.{Only in one or another}. Pointing to macro/macro.dwo section.
Now clang won’t allow me to use above approach. So only option left to avoid relocations, emitting DW_AT_macros attribute as a difference of labels.
i.e
.long .Lcu_macro_begin1-.debug_macro.dwo # DW_AT_macro_info

Judging by GCC’s behavior, and other cases of similar features (consider the abbrev table offset in the CU header in Split DWARF - similarly, it isn’t relocated, but is a constant value (computed, potentially by a difference of two labels - but in LLVM’s case it’s probably just to always emit it as zero, not sure how we actually implement it though))

This worked great, clang is fine, GDB also happy. But that was manual assembly hack I opted. I didn’t find any API in /MCStreamer/ that can provide me a difference of 2 labels as label.

This is needed here –
TheCU.addSectionLabel(TheCU.getUnitDie(),dwarf::DW_AT_macros,
/Label here?/, TLOF.getDwarfMacroDWOSection()->getBeginSymbol()); – requires a label to add in.

Yeah, there’s lots of other cases in the DWARF where a difference of two labels is needed as an attribute value - consider DW_AT_high_pc for instance (in DWARFv4 and above), it’s emitted as a difference of two labels, like this:

if (DD->getDwarfVersion() < 4)
addLabelAddress(D, dwarf::DW_AT_high_pc, End);
else
addLabelDelta(D, dwarf::DW_AT_high_pc, End, Begin);

Oh, an even more analogous example, it seems, is DWARFv4 extension split DWARF support for ranges:

if (isDwoUnit())
addSectionDelta(ScopeDIE, dwarf::DW_AT_ranges, List.Label,
RangeSectionSym);
else
addSectionLabel(ScopeDIE, dwarf::DW_AT_ranges, List.Label,
RangeSectionSym);

So it looks like you might be able to use the same logic for the DW_AT_macros attribute. (looks like the only difference between the two options (addLabelDelta and addSectionLabel) is the DW_FORM chosen, and the “addSectionLabel” one uses sec_offset where available, which is what GCC uses for its DW_AT_macro, etc))