[cfe-dev] [Proposal]: Fast non-portable extension to retrieve a type name from a pointer as a GV: __builtin_type_name(T* Ptr, [int Style])
via cfe-dev
cfe-dev at lists.llvm.org
Mon Sep 10 21:00:06 PDT 2018
Hi,
Well, essentially arguments like these and changing semantics of the language is what I was
trying to avoid by having it as a builtin, and besides it's much much simpler in terms of linkage
and all the other related issues. If anything I don't see why go down the complex route and
change language semantics as well as worrying about linking and all the possible test cases
as well as having a feature test for it, a trivial builtin just requires changing a few files. And it does
seem to have genuine uses since just browsing GitHub or even looking at LLVM's own support lib,
there are hacks to do this precise thing.
And hey it's even more useful if you can easily test for whether its available or not, if a customer
does decide to use it in their code that they intend to make portable (though again, I proposed this
as a Clang extension, from which there's fairly graceful fallback mechanisms) and which is already
implemented in my work tree, I'd just have to rename a few things and put the diff up for review and
I'm fine maintaining such extension.
The typeid(,,,) proposal is fine but it does seem to cause additional complications which were just
discussed. With a builtin you have the option of A). Using it if available and falling back to something
like lambda + __PRETTY_FUNCTION_ B). Flat out writing non-portable code if you only want to target
Clang as a C/C++ compiler.
And even taking that into account, static reflection is never coming to C. So that's another issue that
this wouldn't address. So while I know that the proposal got shot down pretty quickly on IRC, I would
urge everyone to take a moment to reconsider, as we already have a couple of Clang-only extensions,
and that's ignoring the point that any other compiler vendor could implement the same thing with likely
very little effort if they wanted to do so.
Thanks.
- Kristina
On 06/09/2018 at 7:04 PM, "Ben Craig" <ben.craig at ni.com> wrote:
>
>Just to be clear, I’m not trying to shoot down anything, just
>trying to make sure I understand what’s happening. I’m totally
>fine with those tradeoffs.
>
>From: James Y Knight <jyknight at google.com>
>Sent: Thursday, September 6, 2018 12:58 PM
>To: Ben Craig <ben.craig at ni.com>
>Cc: Richard Smith <richard at metafoo.co.uk>; kristina at nym.hush.com;
>Clang Dev <cfe-dev at lists.llvm.org>
>Subject: Re: [cfe-dev] [Proposal]: Fast non-portable extension to
>retrieve a type name from a pointer as a GV:
>__builtin_type_name(T* Ptr, [int Style])
>
>Yes -- and again, those are the exact same behaviors you'll see if
>you instead test "throw ClassWithExternalVtable()" today, in GCC
>and Clang.
>
>On Thu, Sep 6, 2018 at 1:46 PM Ben Craig
><ben.craig at ni.com<mailto:ben.craig at ni.com>> wrote:
>What happens if you…
>
> 1. typeid(ClassWithExternalVtable) from a -fno-rtti TU, and the
>vtable is in a -frtti TU?
> 2. typeid in the -frtti, vtable in the -fno-rtti?
>
>Wild guesses…
>
> 1. will cause an extra, weak linkage type_info to be created,
>but the linker or loader will use the one from the -frtti TU.
>Object file size suffers a little bit, but people are generally
>happy.
> 2. I’m expecting an error at link or load time here, because
>the -frtti TU is expecting someone else to provide the type_info,
>but no one does.
>
>From: James Y Knight
><jyknight at google.com<mailto:jyknight at google.com>>
>Sent: Thursday, September 6, 2018 10:49 AM
>To: Ben Craig <ben.craig at ni.com<mailto:ben.craig at ni.com>>
>Cc: Richard Smith
><richard at metafoo.co.uk<mailto:richard at metafoo.co.uk>>;
>kristina at nym.hush.com<mailto:kristina at nym.hush.com>; Clang Dev
><cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
>Subject: Re: [cfe-dev] [Proposal]: Fast non-portable extension to
>retrieve a type name from a pointer as a GV:
>__builtin_type_name(T* Ptr, [int Style])
>
>On Wed, Sep 5, 2018 at 10:32 PM Ben Craig
><ben.craig at ni.com<mailto:ben.craig at ni.com>> wrote:
>Which TU(s) will the type_info object go in, and what will the
>linkage of those objects be? If you call typeid(SomeClass), and
>all of SomeClass’s functions are out-of-lined in another TU, will
>you generate a type_info object of weak_linkage in the current TU?
> Or will you expect the TU that has SomeClass’s functions in it to
>define the type_info object?
>
>I think if you want things to work, then you will need to generate
>a weak linkage type_info in each TU that calls typeid(SomeClass).
>I’m not sure if that’s ok or not though.
>
>Yes, the last. It is what we do already -- only in special cases
>can we assume that there is typeinfo available externally (certain
>standard types, and when RTTI is enabled, types with an externally
>available vtable). In other cases, the typeinfo is already emitted
>where required, with weak linkage.
>
>
>From: Richard Smith
><richard at metafoo.co.uk<mailto:richard at metafoo.co.uk>>
>Sent: Tuesday, September 4, 2018 5:49 PM
>To: Ben Craig <ben.craig at ni.com<mailto:ben.craig at ni.com>>
>Cc: James Y Knight
><jyknight at google.com<mailto:jyknight at google.com>>;
>kristina at nym.hush.com<mailto:kristina at nym.hush.com>; Clang Dev
><cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>>
>Subject: Re: [cfe-dev] [Proposal]: Fast non-portable extension to
>retrieve a type name from a pointer as a GV:
>__builtin_type_name(T* Ptr, [int Style])
>
>On Tue, 4 Sep 2018 at 15:02, Ben Craig via cfe-dev <cfe-
>dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
>If this proves useful, I could amend my in-flight paper (
>http://www.open-
>std.org/jtc1/sc22/wg21/docs/papers/2018/p1105r0.html#rtti<https://u
>rldefense.proofpoint.com/v2/url?u=http-3A__www.open-
>2Dstd.org_jtc1_sc22_wg21_docs_papers_2018_p1105r0.html-
>23rtti&d=DwMFaQ&c=I_0YwoKy7z5LMTVdyO6YCiE2uzI1jjZZuIPelcSjixA&r=y8m
>ub81SfUi-
>UCZRX0Vl1g&m=7aFHQSaTijYTIv4QldkgDVFOtHraMIeWJ9BI91rRi2c&s=DIlVM7Dk
>ikKHU3dBzN5SBhcNaQ5wA2a0k8xuUbDoasE&e=> ) to allow for
>typeid(type) in freestanding, while still allowing
>typeid(variable) to be ill formed. I would need to carefully
>evaluate how much of <typeinfo> and <typeindex> would make sense.
>
>If you go down this path, it would make more sense to only
>disallow the dynamic form of typeid(expr) (that is, when the
>operand is a glvalue of polymorphic class type). There's also
>implementation experience of that: that's what MSVC's /GR- flag
>does.
>
>From: cfe-dev <cfe-dev-bounces at lists.llvm.org<mailto:cfe-dev-
>bounces at lists.llvm.org>> On Behalf Of James Y Knight via cfe-dev
>Sent: Tuesday, September 4, 2018 4:42 PM
>To: kristina at nym.hush.com<mailto:kristina at nym.hush.com>
>Cc: Clang Dev <cfe-dev at lists.llvm.org<mailto:cfe-
>dev at lists.llvm.org>>
>Subject: Re: [cfe-dev] [Proposal]: Fast non-portable extension to
>retrieve a type name from a pointer as a GV:
>__builtin_type_name(T* Ptr, [int Style])
>
>As discussed on IRC, ISTM this would be better spelled as:
> typeid(<typename>).name();
>
>The issue with that at the moment is that typeid() is an error if
>you build with -fno-rtti. However, there appears to be no reason
>why we cannot support `typeid(<typename>)` even with -fno-rtti.
>Unlike `typeid(<variable>)`, it requires no extra data to be
>emitted, since there's no possibility that dynamic dispatch is
>required. Therefore, similarly to how exception support still
>functions with -fno-rtti by emitting the explicitly required
>typeinfo data on demand, so too can typeid(<typename>).
>
>I note also that when .name() is the only value used, the
>remainder of the typeinfo data is already omitted from the output
>when compiling with optimizations.
>
>It looks like supporting this would be only be a few line change
>in clang, since all the underlying infrastructure is there already
>to support the EH with -fno-rtti use-case.
>
>On Mon, Sep 3, 2018 at 10:41 AM via cfe-dev <cfe-
>dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>> wrote:
>The concept itself is pretty trivial, the builtin takes one or two
>arguments, one of which
>has to be a pointer to an arbitrary type, this makes it
>particularly useful with C++
>auto declarations or even printing the type by simply casting a
>null pointer to
>the type in question. It's qualifiers are retained for the sake of
>printing them
>depending on the requested style.
>
>Implementation
>^^^^^^^^^^^^^^
>
>const char* TypeName = __builtin_type_name(T* Ptr, [int Style])
>
>After validating either 1 or 2 argument form (ie. Pointed to type,
>and whether it's a
>record declaration), SemaChecking will set the return type to
>TheCall->setType(
>Context.getPointerType(Context.CharTy.withConst())) leaving it for
>Clang's CodeGen
>to deal with.
>
>Second argument is used to control the output format in form of a
>bitmask
>passed down to PrintingPolicy (as I port this I will decouple the
>values so the
>builtin's behavior isn't dependent on further changes in
>PrintingPolicy. At
>which point the type is retrieved using `getAsString` and stored
>in the CGM
>with `GetAddrOfConstantCString` allowing coalescing of those
>strings later
>during linking. as it's cast to a Int8Ptr.
>
>This is all done in Clang without needing to add anything to the
>LLVM core.
>
>Things left to do before submitting for code review
>^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>* There is no test coverage, so I need to write up a test file,
>comprehensively
> testing cases I haven't considered before like Objective-C
>pointers, any
> pointer to a special Clang type that may behave unexpectedly,
>complex
> C++ templates (or simple ones, was my main use of it). Target is
>IR since
> Clang fully lowers it, so in theory it's platform agnostic
>providing there is
> enough space (the original use was for Embedded C++ with no RTTI.
>* While this is out of scope, a GUID buffer as a a style would
>provide a form
> type IDs in absence of RTTI or alternatively smaller types like
>u32 or u16
> (aimed at single module builds where they can remain unique, ie.
>embedded
> systems with a kernel using those to represent kernel object
>types).
>
>Rationale
>^^^^^^^^^
>It's clear that this functionality is desired outside of embedded
>domain,
>typically with hacks involving a lambda and __PRETTY_FUNCTION__,
>on case
>being in `lib/Support/TypeName.h` in LLVMSupport. Many non-
>portable hacks
>that depend on compiler versions exist. This doesn't aim to be
>portable,
>just to be compact, not have a runtime cost, and provide this
>information
>either for debugging or even for more practical reasons.
>
>I wanted to find out if there was interest in this kind of thing
>since
>I have developed a variety of different useful and not so useful
>extensions,
>originally for embedded use but I want to upstream them for
>general use,
>I want to know what the consensus is on 1) This particular
>extension
>2). Further extensions that were originally developed for embedded
>use
>but have turned out to be useful in a lot of contexts where you
>are willing
>to sacrifice portability (now with `__has_builtin` this is
>extremely easy to
>test for and fallback on something else).
>
>On the other hand it's a lot to do overall so I would prefer to
>get a consensus
>whether each feature (even this small one) is worth cleaning up
>and putting
>up for code review, since I understand that something like builtin
>bloat
>or non portability may be of concern. As for the formal name I'd
>like to call
>it extensions for Embedded C++ 2 gating it behind an opt-in flag
>such as
>`-fecxx2-extensions`.
>
>Other things involve limited reflection retrieving the bare
>minimum that's
>needed, a llvm::formatv style formatter accelerator, getting names
>of record
>scope at different levels with 0 being similar to the desired
>__CLASSNAME__
>macro (at least by some).
>
>Looking forward to any feedback whether positive or negative.
>Thank you for your time.
>- Kristina
>
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