[llvm-dev] [assembler + debuginfo] interaction with .file directive and debuginfo

[David Blaikie via llvm-dev]

What does gcc do?

On Fri, Aug 24, 2018 at 9:05 AM via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> There are two forms of the .file directive.  One is simply
>
>                .file "path"
>
> and the other is
>
>                .file <file-number> [ "dir"] "path" [ …. other stuff ]
>
>
>
> The latter is sometimes referred to as the "DWARF" form of the .file
> directive.  I don't know the history about the different forms. I have
> certainly messed with the latter form to accommodate the needs of DWARF v5.
>
>
>
> I think it's pretty certain that the DWARF form of .file means that the .s
> file was generated with debug info, and therefore the correct behavior of
> "clang –g foo.s" in that case is to ignore the –g and assume the assembler
> source has all the right stuff.
>
>
>
> If the older form of .file can mean nothing more than "emit a file symbol"
> and not imply that the assembler source file itself already contains debug
> info, then seeing that form should not cause the assembler to ignore –g.
> But as I said, I don't know the history and I don't understand how the
> first form is generally used.
>
>
>
> I'm quite willing to believe I did not understand the (largely unstated)
> specification for the non-DWARF form of .file, and if others agree that it
> should not imply that debug info already exists, then reporting that as a
> bug is appropriate.  I can't say I'd be able to find time to try to *fix*
> it before October, as I'm already over-committed, but I'd anticipate
> getting to it around that time.
>
> --paulr
>
>
>
> *From:* bcain at codeaurora.org [mailto:bcain at codeaurora.org]
> *Sent:* Thursday, August 23, 2018 3:18 PM
> *To:* llvm-dev at lists.llvm.org
> *Cc:* Robinson, Paul
> *Subject:* [assembler + debuginfo] interaction with .file directive and
> debuginfo
>
>
>
> [ I discussed this briefly with Paul off list and he suggested it would be
> better here. ]
>
>
>
> Among the changes in 7.0.0 are some that were added to support DWARFv5’s
> .debug_line.
>
>
>
> The presence of “.file” mutes debug info generated by the assembler as of
> r328208 – logic in AsmParser::enabledGenDwarfForAssembly().
>
>
>
> The comment there states: “If we haven't encountered any .file directives
> (which would imply that the assembler source was produced with debug info
> already) then emit one describing the assembler source file itself.”
>
>
>
> Does .file imply that the assembly source was already produced with debug
> info?  Perhaps not in all cases.  If you wanted to emit an ELF::STT_FILE
> into your object, for example, you might add ```.file "foo.s"```.
>
>
>
> I think we should change this behavior to the previous – check for “-g”
> but ignore “.file” directives.   Or maybe there’s a compromise to be found
> among the different versions of “.file” with various args?  Thoughts?
>
>
>
> -Brian
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