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Subject: Re: [EXT] Ideas on debug constexpr functions or other functions that have been 100% removed from the EXE
From: Paul Hampson (p_hampson_at_[hidden])
Date: 2019-02-19 05:22:31


I had a quick play with debugging information output, but neither of the two compilers I had immediately at hand would produce the DW_AT_const_expr tag when told to use Dwarf 4 (the version that added constexpr support), although I did get DW_AT_const_value for the variable holding the result value. Both compilers (clang 4 and gcc 6) were pretty insistent on disappearing the constexpr function entirely. Clang disappeared it even in the non-contexpr case, although I didn’t check the assembly to see if it had *actually* disappeared it, or was just leaving it out of the Dwarf data.

I was using https://github.com/eliben/pyelftools to dump Dwarf, and it doesn’t support Dwarf 5; this version of clang was happy to produce Dwarf 5; gcc silently downgraded to Dwarf 4.

Anyway, it does look like the Dwarf container can hold the information I think it’ll need, as it’s able to point at chunks of code and say “This is a particular inlined method”, with a tag marking whether that was compile-time or run-time evaluated (and the result), which means the real machine code for that inline method is available, unless it is totally elided from the object.

However, I’m not sure if I was asking the wrong questions, or the compilers just don’t bother storing in the Dwarf data when a function was completely elided due to inlining, constexpr or not.

I did think it was interesting that Dwarf considers constexpr functions a subset of inlined functions, and they store them as ‘skeleton’ + ‘instance’. So I guess they were thinking along the same lines as me, that what we’re talking about is a narrower sub-case of inlined-into-incomprehensibility function call trees.

I don’t know what MSVC’s debugging approach is for inline functions, but if it’s similar to Dwarf, then we may have a good starting place to put together a discussion with an achievable outcome.

Anyway, punting this exploration until I get around to setting up a real, up-to-date ELF compilation environment. I suspect ‘if constexpr’ and newer compilers might have more luck getting a DW_AT_const_expr-tagged inline function entry in a Dwarf DB.

--
Paul “TBBle” Hampson

From: SG14 <sg14-bounces_at_[hidden]> On Behalf Of Scott Wardle
Sent: Monday, 18 February 2019 2:05 PM
To: Low Latency:Game Dev/Financial/Trading/Simulation/Embedded Devices <sg14_at_[hidden]>
Subject: Re: [SG14] [EXT] Ideas on debug constexpr functions or other functions that have been 100% removed from the EXE

Keeping an eye on what dwarf is doing is a great idea. I have added my self to their mailing list and will lurk for a bit. Looks like the data they have is parameter and return values and the you could get the text of the function I think.

On Feb 17, 2019, at 11:01 AM, Edward Catmur <ecatmur_at_[hidden]<mailto:ecatmur_at_[hidden]>> wrote:

In the context of tooling for debuggers, I think you might want to look at the progress (if any) that DWARF are making in adding constexpr support[3] (note:may be very out of date) to that standard (used extensively in the Linux world, by gcc, llvm and gdb among others).

3. http://dwarfstd.org/ShowIssue.php?issue=090107.1
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