[LLVMdev] Adding support to LLVM for data & code layout (needed by GHC)

[John McCall]

On Jun 8, 2010, at 3:42 AM, David Terei wrote:
> The GHC developers would like to add support to llvm to enable the
> order that code and data are laid out in, in the resulting assembly
> code produced by llvm to be defined by the user. The reason we would
> like to have this feature is explained in the blog post on GHC's use
> of llvm here: http://blog.llvm.org/2010/05/glasgow-haskell-compiler-and-llvm.html,
> specifically under the title, 'Problems with backend'.
> 
> Basically we want to be able to produce code using llvm that looks like this:
> 
> .text
>    .align 4,0x90
>    .long  _ZCMain_main_srt-(_ZCMain_main_info)+0
>    .long  0
>    .long  196630
> .globl _ZCMain_main_info
> _ZCMain_main_info:
> .Lcg6:
>    leal -12(%ebp),%eax
>    cmpl 84(%ebx),%eax
>    [...]
> 
> So in the above code we can access the code for the function
> '_ZCMain_main_info' and the metadata for it need by the runtime with
> just the one label. At the moment llvm just outputs all global
> variables at the end.
> 
> It seems to me that there are three slightly different ways to support
> this in llvm:
> 
> 1) Have llvm preserve order of data and code from input file when in
> the same section

I dislike this approach;  implicit requirements are bad.  Many clients don't care
about the order in which variables are emitted, and indeed GHC doesn't care
either outside of a very narrow range of constraints.

It seems to me that a module property (or special global value) holding a list of
ordering lists would be reasonably appropriate.  Constraints:  values can only
appear in a single list, values in a list must be definitions, and (for now) values
in a list should not have merging linkage.

John.