Re: [PATCH] Re: LLVM and PSEUDO_REG/PSEUDO_PHI

[Pekka Enberg <penberg@kernel.org>]

On 08/28/2011 02:23 PM, Linus Torvalds wrote:
>> On Sun, Aug 28, 2011 at 10:52 AM, Josh Triplett<josh@joshtriplett.org>
>>  wrote:
>>>>
>>>> You should consider all types to be just "blocks of memory", and
>>>> sparse has already calculated all offsets etc for you. As far as LLVM
>>>> is concerned, the memory has no structure, it's just a blob.
>>>
>>> I suspect LLVM's optimization passes won't particularly care for that
>>> approach.
>>
>> That's fine. We've already done the CSE and alias analysis on the
>> thing. And as mentioned, trying to turn overlapping (or partial)
>> accesses into some "named accesses" is just *wrong*. They weren't
>> named in the C code either. They are accesses through pointer
>> arithmetic. Trying to make them be somehow named would just be crazy.

On Sun, Aug 28, 2011 at 10:33 PM, Jeff Garzik <jeff@garzik.org> wrote:
> LLVM just cares about layout.
>
> It does not care about naming (or lack thereof).  Nobody/nothing is trying
> to make them named.

Yup. The LLVM backend pretty much expects to know about struct layout.
For example, this simple initialization:

  struct symbol {
    char *s;
  };

  extern struct symbol foo;

  struct symbol *foo_p = &foo;

Looks like this in LLVM bitcode:

  %struct.symbol = type { i8* }

  @foo = external global %struct.symbol
  @foo_p = global %struct.symbol* @foo, align 8

I'm sure we *could* turn treat 'foo' as simple block of memory and
turn 'foo_p' into an untyped pointer. However, it's likely to be more
difficult than to just let LLVM know about struct layouts.

                        Pekka
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