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

[James Molloy via llvm-dev]

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.

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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> >
>
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