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

Thu Nov 5 08:08:16 PST 2015

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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>> >>
>> >>
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>> >
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