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

[Andrew Lenharth]

On Tue, Jun 8, 2010 at 4:15 PM, Eugene Toder <eltoder at gmail.com> wrote:
> Let me point out that projects using standard toolchain (e.g.
> binutils) can already reorder code and data pretty much arbitrary
> using sections and linker scripts.
> I think it's still attractive to have reordering in LLVM to be
> independent from external toolchain. This will allow reordering in JIT
> and other interesting things.
>
> I agree with John that special global with ordered list looks like the
> clean approach. However, for the specific need in GHC I'm not sure
> that this is enough.
> What it's trying to do is placing a global variable (a struct) and a
> function next to each other, so that it can use pointer arithmetic to
> go from one address to another.
> So, say, we take the address of the struct, add it's size and call
> into resulting address. Sufficiently smart optimizer can spot that we
> dereference a pointer pointing outside of the object. This probably
> has undefined semantics and can be replaced with unreachable?
> It's also a missed opportunity for optimizations -- if optimizer knew
> where outside-of-the-struct pointer is really going it could make
> direct call instead of indirect -- however, I don't know if this is a
> big deal.

If I understand, you want more than just ordering within a section,
you want to override the section of a global to be in the text (or
equivalent) section.  Is the global structure constant?  It seems a
way to splat constants directly before/after/in a function and refer
to them as a global would be useful for all manner of side-tables.
Being able to just order output of globals and functions make picking
sections for them fairly hackish.

Andrew


> Eugene
>
> On Tue, Jun 8, 2010 at 3:50 PM, Sebastian Redl
> <sebastian.redl at getdesigned.at> wrote:
>>
>> On Tue, 8 Jun 2010 11:42:41 +0100, David Terei <davidterei at gmail.com>
>> wrote:
>>> Hi All,
>>>
>>> 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'.
>>>
>>
>> Whichever way is chosen, the ability to reorder and intermingle functions
>> and data arbitrarily is interesting to more than just GHC. In particular, I
>> would like to point out the efforts by Mozilla to make Firefox startup
>> faster, which essentially came down to reordering stuff in the executables
>> so that everything is ordered by the sequence of accesses during program
>> startup. This means that programs can be read sequentially from the front
>> to the end, thus reducing I/O latency.
>>
>> Tools for automating this process would probably benefit from being able
>> to specify the layout this way.
>>
>> Sebastian
>>
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