[llvm-dev] Potential ambiguity in the grammar of LLVM IR assembly

[Mehdi Amini via llvm-dev]

> On May 25, 2016, at 6:59 PM, Robin Eklind via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello Tim,
> 
> Thank you for getting back to me.
> 
> The language grammar as defined by the LLVM Language Reference Manual [1] does not include the details of the LLVM IR parser reference implementation.
> 
> The following extract from "lib/AsmParser/LLParser.cpp" illustrates that unnamed globals are allowed [2].
> 
> > /// ParseUnnamedGlobal:
> > ///   OptionalVisibility (ALIAS | IFUNC) ...
> > ///   OptionalLinkage OptionalVisibility OptionalDLLStorageClass
> > ///                                                     ...   -> global variable
> > ///   GlobalID '=' OptionalVisibility (ALIAS | IFUNC) ...
> > ///   GlobalID '=' OptionalLinkage OptionalVisibility OptionalDLLStorageClass
> > ///                                                     ...   -> global variable
> 
> Also, using lli to interpret the following example program [3] produces the status code 42.

I believe you're not on ToT, I get:

lli: b.ll:1:1: error: expected top-level entity
global i32 42

-- 
Mehdi



> 
> global i32 42
> define i32 @main() {
> 	%foo = load i32, i32* @0
> 	ret i32 %foo
> }
> 
> As neither the LLVM Language Reference Manual, nor the comments of the LLVM IR reference implementation, include a full EBNF of the language grammar, one has to make educated guesses and cross-reference information from LangRef.html, comments in LLParser.cpp and the code in LLParser.cpp.
> 
> I'd love to see an EBNF grammar for LLVM IR at some point in the future, as this would open up for very interesting possibilities.
> 
> Given that global variables may be unnamed, does the unnamed_addr introduce an ambiguity in the LLVM IR grammar?
> 
> Cheers
> /u
> 
> [1]: http://llvm.org/docs/LangRef.html
> [2]: https://github.com/llvm-mirror/llvm/blob/master/lib/AsmParser/LLParser.cpp#L432
> [3]: https://github.com/mewspring/poc/blob/master/ll/b.ll
> 
> On 05/26/2016 02:42 AM, Tim Northover wrote:
>> On 25 May 2016 at 16:10, Robin Eklind via llvm-dev
>> <llvm-dev at lists.llvm.org> wrote:
>>> declare void @foo() unnamed_addr
>>> global i32 42
>> 
>> Doesn't a global have to be named? The syntax in the IR reference
>> doesn't make it optional:
>> 
>>    @<GlobalVarName> = [Linkage] [Visibility] [DLLStorageClass]
>> [ThreadLocal] ...
>> 
>> Cheers.
>> 
>> Tim.
>> 
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