[LLVMdev] RFC: MCJIT enhancements

[Kaylor, Andrew]

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


From: Paweł Bylica [mailto:pawel.bylica at ibs.org.pl]
Sent: Thursday, August 16, 2012 4:03 AM
To: Kaylor, Andrew
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] RFC: MCJIT enhancements


On Wed, Jul 25, 2012 at 12:24 AM, Kaylor, Andrew <andrew.kaylor at intel.com<mailto:andrew.kaylor at intel.com>> wrote:
-------------------------------
ELF Support on Windows
-------------------------------

There are various reasons that it would be nice to be able to support generation of ELF objects on Windows through the MCJIT interface, one of which is that it would allow users to debug JITed code with GDB (i.e. MinGW).  There was a proposal by Eli Bendersky to enable ELF generation on Windows by extending the target triple handling.  We have been using that approach, and it works.  However, at the time it was proposed there seemed to be a general feeling that it would be preferable to have a general way to specify a non-default object format for any platform rather than a Windows/ELF-specific extension.

The original discussion can be found here:

http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20120130/136053.html

Unfortunately, I don't believe a consensus was ever reached as to what the correct way to implement this would be.  I'd like to re-open that discussion now so that we can find a solution that would be generally accepted for inclusion in LLVM.

I'm using this patch mentioned and it works just fine.
However, I would like to see it implemented in more generic way:

1. When a container type is explicitly specified in triple it should be used by MCJIT.
2. When a container type is not specified it is deduced from OS.
3. Use ELF by default.

"Environment" part of the triple may be used to specify the container. Some options are already there (like MachO). But there may conflicts exist where both environment and container must have been specified together (e.g. "arm-apple-ios-eabi-macho").

- Paweł
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