On Fri, Dec 4, 2020 at 7:01 PM Alexander Yermolovich <ayerm...@fb.com>
wrote:
>
>
> ------------------------------
> *From:* Richard Biener <richard.guent...@gmail.com>
> *Sent:* Friday, December 4, 2020 12:36 AM
> *To:* David Blaikie <dblai...@gmail.com>
> *Cc:* Alexander Yermolovich <ayerm...@fb.com>; Jakub Jelinek <
> ja...@redhat.com>; Mark Wielaard <m...@klomp.org>; gcc@gcc.gnu.org <
> gcc@gcc.gnu.org>; ikud...@accesssoftek.com <ikud...@accesssoftek.com>;
> mask...@google.com <mask...@google.com>
> *Subject:* Re: DWARF64 gcc/clang flag discussion
>
> On Thu, Dec 3, 2020 at 6:52 PM David Blaikie <dblai...@gmail.com> wrote:
>
>
>
> On Wed, Dec 2, 2020 at 5:10 PM Alexander Yermolovich <ayerm...@fb.com>
> wrote:
>
>
>
> ------------------------------
> *From:* David Blaikie <dblai...@gmail.com>
> *Sent:* Wednesday, December 2, 2020 1:12 PM
> *To:* Alexander Yermolovich <ayerm...@fb.com>
> *Cc:* Richard Biener <richard.guent...@gmail.com>; Jakub Jelinek <
> ja...@redhat.com>; Mark Wielaard <m...@klomp.org>; gcc@gcc.gnu.org <
> gcc@gcc.gnu.org>; ikud...@accesssoftek.com <ikud...@accesssoftek.com>;
> mask...@google.com <mask...@google.com>
> *Subject:* Re: DWARF64 gcc/clang flag discussion
>
>
>
> On Wed, Dec 2, 2020 at 10:44 AM Alexander Yermolovich <ayerm...@fb.com>
> wrote:
>
>
>
> ------------------------------
> *From:* David Blaikie <dblai...@gmail.com>
> *Sent:* Tuesday, December 1, 2020 10:33 AM
> *To:* Alexander Yermolovich <ayerm...@fb.com>
> *Cc:* Richard Biener <richard.guent...@gmail.com>; Jakub Jelinek <
> ja...@redhat.com>; Mark Wielaard <m...@klomp.org>; gcc@gcc.gnu.org <
> gcc@gcc.gnu.org>; ikud...@accesssoftek.com <ikud...@accesssoftek.com>;
> mask...@google.com <mask...@google.com>
> *Subject:* Re: DWARF64 gcc/clang flag discussion
>
>
>
> On Mon, Nov 30, 2020 at 5:04 PM Alexander Yermolovich <ayerm...@fb.com>
> wrote:
>
>
>
> ------------------------------
> *From:* David Blaikie <dblai...@gmail.com>
> *Sent:* Monday, November 30, 2020 12:09 PM
> *To:* Alexander Yermolovich <ayerm...@fb.com>
> *Cc:* Richard Biener <richard.guent...@gmail.com>; Jakub Jelinek <
> ja...@redhat.com>; Mark Wielaard <m...@klomp.org>; gcc@gcc.gnu.org <
> gcc@gcc.gnu.org>; ikud...@accesssoftek.com <ikud...@accesssoftek.com>;
> mask...@google.com <mask...@google.com>
> *Subject:* Re: DWARF64 gcc/clang flag discussion
>
> On Mon, Nov 30, 2020 at 11:36 AM Alexander Yermolovich <ayerm...@fb.com>
> wrote:
>
> Thank you David for driving the conversation, sorry I was on vacation.
>
>
> All good - really appreciate everyone chipping in whenever/however they
> can!
>
>
>
> I guess discussion is from perspective of having both flags
> gdwarf32/gdwarf64. In which case it's a valid question on whether they
> should imply -g like -gdwarf-#.
> But can this be viewed as only a -gdwarf64 flag, that is a qualifier to
> other debug flags that enable debug information? DWARF spec says that 32bit
> should be a default, and 64bit should be used rarely (paraphrasing). So
> when user enabled debug information the default expectation is that it will
> be 32bit. There is no real need for a flag to say "I want debug
> information, and I want it 32bit".
>
>
> I'm not quite with you here, I think. I believe it's important to be able
> to opt into and out of things at any point on the command line - because of
> how complex build systems build up command lines. You might have a
> -gdwarf64 set as a project default, but for some reason want to opt into
> -gdwarf32 in other parts (perhaps you're building the debug info for your
> interface library you intend to ship to clients who might only have DWARF32
> support, but your library is big and needs DWARF64 for the rest). A general
> architectural principle of most command line arguments to the compiler is
> that they can be opted into/out of fairly symmetrically (hence all the
> -*no-* variant flags).
>
> [Alex] Ah I see, good point.
>
>
> On the other hand, 64bit DWARF format must be enabled. So from users
> perspective it's "I want debug information enabled for particular DWARF
> version and level, oh and I want it to be 64bit".
>
>
> But there's also the possibility of wanting to turn on DWARF64 for any
> debug info in your build, but not necessarily wanting to turn on debug info
> while doing so. Eg: you have a big build system, with a variety of users
> and flags all over the place - maybe users opting in to -g2 on some files
> and -g1 on others, and/or under different build modes. And the project as a
> whole is reaching the DWARF64 tipping point and you'd like to say "if we're
> generating DWARF, make it DWARF64". We've recently encountered this sort of
> situation with -gsplit-dwarf and with -gdwarf-N (in switching to DWARFv5 we
> had this situation where there's a big code base/build system with many
> users, many uses of different -gN-type flags and it'd be impractical to go
> and add -gdwarf-5 to all the -gN uses to make them all use DWARFv5, so we
> added -fdebug-default-version=N (I might be misremembering the spelling) to
> Clang to support this use case of "If we're generating DWARF, make it
> DWARFv5")
>
> [Alex] I think we are saying similar thing. The -gdwarf64 doesn't enable
> debug generation, but if it is enabled it will be 64bit. A "qualifier" of
> sorts.
>
>
> OK. My concern there, though I think it's the preferable semantics for the
> -gdwarf64 flag when considered in isolation, is now -gdwarf64 and -gdwarf-5
> would have some fairly subtly different semantics (the latter enables debug
> info and the former does not) in contrast to how close their spelling is.
> (in addition to their general similarly maybe being a source of confusion -
> even if the numbers aren't close to each other)
> [Alex] That is a good point, but I guess broader question is if precedence
> is not good, should we follow it? If it's clearly documented, might be OK.
> As my professor Brian Harvey said many, many, many moons ago during my
> undergrad: "RTFM". If my understanding of exchange on this thread is
> correct, there doesn't seem to be a consensus on this point.
>
>
> Re: lacking consensus: yep. That's my understanding.
>
> Admittedly my sort of wedge take on this is - why not use -f flags that
> don't have the ambiguity/overlap with "maybe it enables debug info too". (&
> maybe we could use -fdwarf32/64 in Clang, to avoid implementing
> -gdwarf32/64 with a difference in behavior in terms of
> also-enables-emission - then implement whatever semantics GCC ended up
> picking) - well, "why not" because of the general take that debug info
> flags should be "-g", fair enough.
>
> So, yeah, I'm still pretty undecided about how Clang should move forward
> with this in the immediate future.
> [Alex] Well if there is no consensus maybe clang can trail blaze. 🙂
>
>
> My concern with doing so is both that GCC might not make the same
> decision, leading to differences in flag semantics (and my other concerns
> about the available options that lean towards the -g* style naming and the
> ambiguities around composability or implied -g in general and in particular
> with the likely names here and how close they are to existing names that do
> imply -g, hence my personal preference towards the -f names). Might end up
> going with the -f names out of a combination of my desire for clear
> designation of composability, and sidestepping the issue of GCC
> compatibility in the future if GCC's going to pick a -g name and semantics
> (implied -g or otherwise) anyway. This does mean I/we/clang loses a chance
> to perhaps somewhat more heavily (than just my words here) influence GCC's
> flag choice (eg: by Clang choosing the -g* naming and making it composable,
> for instance), but avoids the chance of divergent behavior between the two
> compilers, or breaking backwards compatibility to maintain compatibility.
>
> All up I just don't have great feelings about any of the paths forward,
> really. I think -fdwarf32/64 is the better option for users, that
> -gdwarf32/64 implying -g would be unfortunate (composability is valuable)
> and -gdwarf32/64 being composable would be pretty confusing to users given
> -gdwarf-N isn't composable & they are very similar flags/spellings
> (similarly for -g32/64 compared to -gN).
>
>
> Sorry, just going around in circles a bit, I guess this may be a better
> summary:
> If I had to pick a -g flag/semantic for this, I guess I'd pick
> -gdwarf32/64 without implied -g. I'd pick that if I knew GCC would
> implement it to match - but if GCC might go either way on implied -g, I
> think I'd rather avoid changing the semantics of the flag later on & use a
> different name GCC isn't likely to define different semantics for (and I'd
> probably pick -fdwarf32/64).
>
>
> There's an approved patch to add -gdwarf{32,64} not implying -g
> [Alex] Do you happen to have a link on hand to it?
>
https://gcc.gnu.org/pipermail/gcc-patches/2020-December/560734.html
> Richard.
>
>
>
> - Dave
>
>
