[LLVMdev] RFC: MCJIT enhancements

[Jim Grosbach]

On Aug 17, 2012, at 2:50 AM, Paweł Bylica <pawel.bylica at ibs.org.pl> wrote:

> On Fri, Aug 17, 2012 at 12:16 AM, Kaylor, Andrew <andrew.kaylor at intel.com> wrote:
> Hi Paweł,
> 
>  
> 
> Thanks for continuing this discussion.
> 
>  
> 
> I like the simplicity of your suggestion.  My only concern involves the ambiguity of what is meant by “environment”.  Presently there are functions in the llvm::Triple class to access the environment as an enumeration of a fixed set of values.  It seems that some non-enumerated values are already in use, but introducing possible combinations of ABI and object format would seem to strain the API.
> 
>  
> 
> I couldn’t find an explanation anywhere of what is meant by environment in the context of the triple, and it isn’t clear from looking at the possible values whether object format properly belongs to the environment concept or whether it should be a new triple component.  Looking at the uses in the LLVM code base, it seems that the environment element has been used specifically to enable generation of MachO objects on non-Darwin OSes.  I see that clang  recognizes “iphoneos” as a value (though it’s not in the Triple::EnvironmentType enum), and it’s not clear to me what it uses it for.  Beyond that, the environment seems to be used to influence ABI selection, which would seem to have some degree of overlap with object format selection while being essentially different.
> 
>  
> 
> In other words, environment seems to be a bit of a catch-all dumping ground at the moment.  That’s not entirely bad, I suppose, but it does make the task of cleanly extending the target triple more complicated.
> 
>  
> 
> It would seem that at some point the target triple has evolved into a target quadruple with the addition of environment.  The question now, I think, is whether it makes sense to extend it further into a target quintuple, adding object format as a well-defined, but optional, component with full API support.  The difficulty, of course, is that with two optional extensions the expected canonical form becomes ambiguous, but I don’t think that’s much of a problem.  Given that object format is a relatively small enumeration, the triple parser should be able to handle it presence or absence while parsing or normalizing.
> 
>  
> 
> So here’s what I would suggest.
> 
>  
> 
> 1.       Define the target triple as follows (adapted from Triple.h):
> 
>  
> 
> /// Target triples are strings in the canonical form:
> 
> ///   ARCHITECTURE-VENDOR-OPERATING_SYSTEM
> 
> /// or
> 
> ///   ARCHITECTURE-VENDOR-OPERATING_SYSTEM-OBJECT_FORMAT
> 
> /// or
> 
> ///   ARCHITECTURE-VENDOR-OPERATING_SYSTEM-ENVIRONMENT
> 
> /// or
> 
> ///   ARCHITECTURE-VENDOR-OPERATING_SYSTEM-OBJECT_FORMAT-ENVIRONMENT
> 
>  
> 
> 2.       Add an llvm::Triple::ObjectFormatType enum with all llvm-supported object formats.
> 
> 3.       Remove “MachO” from the llvm::Triple::EnvironmentType enum.
> 
> 4.       Add the following methods to llvm::Triple:
> 
>  
> 
> Triple(const Twine &ArchStr, const Twine &VendorStr, const Twine &OSStr, const Twine &ObjFmtStr)
> 
> Triple(const Twine &ArchStr, const Twine &VendorStr, const Twine &OSStr, const Twine &ObjFmtStr, const Twine &EnvStr)
> 
> bool hasExplicitObjectFormat() const
> 
> ObjectFormatType getObjectFormat() const
> 
> StringRef getObjectFormatName() const
> 
> bool isOutputObjectFormatELF() const
> 
> bool isOutputObjectFormatCOFF() const
> 
> bool isOutputObjectFormatMachO() const
> 
> static const char * getObjectFormatName(ObjectFormatType Kind)
> 
>  
> 
> 5.       When an object format is specified, it will be used by MC and MCJIT if supported for the specified architecture
> 
> 6.       When an unsupport object format is specified, a fatal error will be thrown
> 
> 7.       When an object format and environment are both specified but are incompatible, a fatal error will be thrown
> 
> 8.       When an object format is not specified everything will behave as it currently does
> 
>  
> 
> How does that sound?
> 
>  
> 
> -Andy
> 
> 
> Hi Andy,
> 
> That sounds great. Do we need any approval for this change? I can do the coding I you don't mind.
> 

Changes to the Triple have pretty wide-reaching implications. Chris, what do you think?

-Jim

> I have one more question. Triples are specified in both modules and TargetMachine. In 3.1 triple from module was used by default. In trunk it has been changed, and LLVM_HOSTTRIPLE is used by default. Do we have any rules in case module triple conflicts with TargetMachine triple?
> 
> - Paweł
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