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

[Aaron Ballman via llvm-dev]

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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