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

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "James Molloy via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Philip Reames" <listmail at philipreames.com>,
> reviews+D14227+public+6a20206dc9791b02 at reviews.llvm.org, "James
> Molloy" <james.molloy at arm.com>, "manman ren" <manman.ren at gmail.com>,
> dexonsmith at apple.com, "mehdi amini" <mehdi.amini at apple.com>, "LLVM
> Dev" <llvm-dev at lists.llvm.org>
> Cc: "LLVM Commits" <llvm-commits at lists.llvm.org>
> Sent: Thursday, November 5, 2015 9:44:45 AM
> Subject: Re: [llvm-dev] [PATCH] D14227: Add a new attribute:
> norecurse

> [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).
+1 

This is a very generic property, and I'm happy with the conciseness provided by an attribute. 

-Hal 

> 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.

> 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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> > > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
> > 
> 
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> 
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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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