[llvm-dev] [PATCH] D14227: Add a new attribute: norecurse

Thu Nov 5 08:26:41 PST 2015

Ah I see.

I can't think of a way that would help users in any particular way offhand.
I hadn't considered exposing it to clang users - do you think there is
merit in that?

James

On Thu, 5 Nov 2015 at 16:08 Aaron Ballman <aaron at aaronballman.com> wrote:

> On Thu, Nov 5, 2015 at 11:06 AM, James Molloy <james at jamesmolloy.co.uk>
> wrote:
> > Hi Aaron,
> >
> > I think it must necessarily be exposed to users - clang must add it in
> > certain circumstances for example. I don't think this is particularly
> > different to many other attributes such as nocapture/nounwind, that are
> > exposed to users and can be set by users in exceptional circumstances but
> > are primarily inferred by the midend.
>
> Sorry, I was unclear. I meant to Clang users as a language-level
> attribute (e.g., [[clang::norecurse]]), not LLVM users as an IR-level
> attribute.
>
> ~Aaron
>
>
> >
> > James
> >
> > On Thu, 5 Nov 2015 at 16:03 Aaron Ballman <aaron at aaronballman.com>
> wrote:
> >>
> >> On Thu, Nov 5, 2015 at 10:44 AM, James Molloy via llvm-dev
> >> <llvm-dev at lists.llvm.org> wrote:
> >> > [Adding llvm-dev and re-stating the situation for llvm-dev's benefit]
> >> >
> >> > RFC: A new attribute, "norecurse".
> >> >
> >> > In some cases, it is possible to demote global variables to local
> >> > variables.
> >> > This is possible when the global is only used in one function, and
> that
> >> > function is known not to recurse (because if the function recurses, a
> >> > local
> >> > variable cannot be equivalent to a global as the global would hold
> state
> >> > over the recursive call).
> >> >
> >> > GlobalOpt has this ability already, however it is currently (a) weak
> and
> >> > (b)
> >> > broken.
> >> >
> >> > GlobalOpt has a special case for the function "main". It assumes that
> >> > any
> >> > function called "main" with external linkage is by definition
> >> > non-recursive.
> >> > It does this because in C++, main is defined by the standard not to
> >> > recurse.
> >> > However, this is not the case in C, so GlobalOpt will happily break C
> >> > code.
> >> >
> >> > GlobalOpt also currently won't even attempt to discover functions that
> >> > don't
> >> > recurse - it only checks "main".
> >> >
> >> > What I'd like to do is improve GlobalOpt to be more aggressive with
> >> > demoting
> >> > globals to locals, and to do that I need to teach it which functions
> are
> >> > known to be non-recursive. At the same time I'd really like to fix the
> >> > "main" kludge.
> >> >
> >> > There are three options I see:
> >> >   1. Just use an analysis pass to infer non-recursiveness as a
> property.
> >> > This can be a simple SCC pass which is queried by GlobalOpt.
> >> >   2. Add an attribute, "norecurse". This would be inferred just like
> >> > above
> >> > by FunctionAttrs in the general case, but it also allows frontends to
> >> > add
> >> > the attribute if they have specific knowledge - for example "main" in
> >> > C++
> >> > mode.
> >> >   3. (1), but use metadata to add the non-recursiveness information in
> >> > the
> >> > frontend, to solve the "main" problem.
> >> >
> >> > What I'm suggesting here is (2). I have no real problem implementing
> (3)
> >> > instead, and I think it's something that's worthwhile discussing.
> Adding
> >> > new
> >> > metadata is very cheap and easy and doesn't seem to have many
> downsides
> >> > as
> >> > an option.
> >> >
> >> > Hopefully I've described the problem and potential solutions well
> enough
> >> > -
> >> > what are peoples' thoughts? I quite like the attribute solution best,
> >> > but
> >> > I'd be happy with anything that moves us forward.
> >>
> >> I think (2) is a good solution, but am wondering whether you intend to
> >> expose that attribute to users, or whether it's one that can only be
> >> created implicitly by the compiler?
> >>
> >> ~Aaron
> >>
> >> >
> >> > Cheers,
> >> >
> >> > James
> >> >
> >> > On Wed, 4 Nov 2015 at 17:46 Philip Reames via llvm-commits
> >> > <llvm-commits at lists.llvm.org> wrote:
> >> >>
> >> >> This should probably be raised on llvm-dev for boarder visibility.
> >> >>
> >> >> Side question: What optimizations or codegen opportunities does this
> >> >> enable?  i.e. what's the actual use case?
> >> >>
> >> >>
> >> >> On 11/02/2015 05:03 AM, James Molloy via llvm-commits wrote:
> >> >>
> >> >> jmolloy created this revision.
> >> >> jmolloy added reviewers: manmanren, dexonsmith, joker.eph.
> >> >> jmolloy added a subscriber: llvm-commits.
> >> >> jmolloy set the repository for this revision to rL LLVM.
> >> >>
> >> >> This attribute allows the compiler to assume that the function never
> >> >> recurses into itself, either directly or indirectly (transitively).
> >> >> This can
> >> >> be used among other things to demote global variables to locals.
> >> >>
> >> >> The norecurse attribute indicates that the function does not call
> >> >> itself
> >> >> either directly or indirectly down any possible call path.
> >> >>
> >> >> Repository:
> >> >>   rL LLVM
> >> >>
> >> >> http://reviews.llvm.org/D14227
> >> >>
> >> >> Files:
> >> >>   docs/LangRef.rst
> >> >>   include/llvm/Bitcode/LLVMBitCodes.h
> >> >>   include/llvm/IR/Attributes.h
> >> >>   include/llvm/IR/Function.h
> >> >>   lib/AsmParser/LLLexer.cpp
> >> >>   lib/AsmParser/LLParser.cpp
> >> >>   lib/AsmParser/LLToken.h
> >> >>   lib/Bitcode/Reader/BitcodeReader.cpp
> >> >>   lib/Bitcode/Writer/BitcodeWriter.cpp
> >> >>   lib/IR/Attributes.cpp
> >> >>   lib/IR/Verifier.cpp
> >> >>   test/Bindings/llvm-c/invalid-bitcode.test
> >> >>   test/Bitcode/attributes.ll
> >> >>   test/Bitcode/compatibility.ll
> >> >>   test/Bitcode/invalid.ll
> >> >>   test/LTO/X86/invalid.ll
> >> >>
> >> >>
> >> >>
> >> >> _______________________________________________
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> >> >> llvm-commits at lists.llvm.org
> >> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
> >> >>
> >> >>
> >> >> _______________________________________________
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> >> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
> >> >
> >> >
> >> > _______________________________________________
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> >> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >> >
>
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