|Version 4 (modified by dterei, 6 years ago) (diff)|
LlVM Back-end Issues
This page list some of the issues and problems with the current implementation of the LLVM back-end. Hopefully they will slowly be resolved.
GHC for heap objects places the info table (meta data) and the code adjacent to each other. That is, in memory, the object firstly has a head structure, which consists of a pointer to an info table and a payload structure. The pointer points to the bottom of the info table and the closures code is placed to be straight after the info table, so to jump to the code we can just jump one past the info table pointer. The other way to do this would be to have the info table contain a pointer to the closure code. However this would then require two jumps to get to the code instead of just one jump in the optimised layout. Achieving this layout can create some difficulty, the current back-ends handle it as follows:
- The NCG can create this layout itself
- The C code generator can't. So the Evil Mangler rearranges the GCC assembly code to achieve the layout.
There is a build option in GHC to use the unoptimised layout and instead use a pointer to the code in the info table. This layout can be enabled/disabled by using the compiler #def TABLES_NEXT_TO_CODE. As LLVM has no means to achieve the optimised layout and we don't wish to write an LLVM sister for the Evil Mangler, the LLVM back-end currently uses the unoptimised layout. This apparently incurs a performance penalty of 5% (source, Making a Fast Curry: Push/Enter vs. Eval/Apply for Higher-order Languages, Simon Marlow and Simon Peyton Jones, 2004).
LLVM IR Representation
The LLVM IR is modeled in GHC using an algebraic data type to represent the first order abstract syntax of the LLVM assembly code. The LLVM representation lives in the 'Llvm' subdirectory and also contains code for pretty printing. This is the same approach taken by EHC's LLVM Back-end, and we adapted the module developed by them for this purpose.
It is an open question as to if this binding should be split out into its own cabal package. Please contact the GHC mailing list if you think you might be a user of such a package.