[PATCH] D82881: [DEBUGINFO]Fix debug info for packed bitfields.

[Alexey Bataev via Phabricator via llvm-commits]

ABataev marked an inline comment as done.
ABataev added a comment.

In D82881#2123970 <https://reviews.llvm.org/D82881#2123970>, @aprantl wrote:

> I see that you are modifying the DWARF v2 codepath. The output emitted in DWARF v2 for bitfields on x86 is peculiar unintuitive because the spec was written with big-endian machines in mind. Have you verified (e.g., on https://godbolt.org) that GCC does the same thing?


gcc acts the same way just like llvm does. But seems to me, it is incorrect. In llvm, the bitfield consumes 2 bytes, but debug info says that it consumes only one bytes and emits negative offset. It may break debug info for some targets, like NVPTX.
I checked the new debug info, it works correctly with gdb.



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Comment at: llvm/test/DebugInfo/X86/packed_bitfields.ll:11
 ; Note that DWARF 2 counts bit offsets backwards from the high end of
 ; the storage unit to the high end of the bit field.
 
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aprantl wrote:
> Actually, this is weird. The comment mentions DWARF 2, but the RUN line seems to use the default, which is 4 at the moment. We might want to make sure to test both.
I will add the run line for both.


Repository:
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https://reviews.llvm.org/D82881