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

[Jeff Garzik <jeff@garzik.org>]

On 08/28/2011 01:04 PM, Linus Torvalds wrote:
> On Sun, Aug 28, 2011 at 3:04 AM, Jeff Garzik<jeff@garzik.org>  wrote:
>>
>> * need to start integrating struct support.  LLVM has a type system where
>> one defines data structures in the IR, then uses 'getelementptr' LLVM
>> instruction to build addresses for complex load/store operations. The Linux
>> kernel uses a lot of function calls (indirect branches) from values buried
>> deep within a struct.
>
> You really shouldn't need to do that.
>
> 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.

We _already_ do this in the backend, and LLVM supports 
LOAD-from-random-address just fine:
https://github.com/penberg/sparse-llvm/blob/master/sparse-llvm.c#L325

But that comment was thinking more medium term, supporting multiple 
platforms means properly handling struct layout in memory, which gets 
into the realm of platform-specific ABIs.

It didn't seem like we would want to encode a ton of ABI detail into the 
sparse C front-end.  But it sounds like there will be a lot of ABI 
detail found, if sparse is doing all the memory layout and such.


> Sometimes you cannot even get the "element" name. Look at
> ./test-linearize output of something like this:

Nobody cares about the element name, least of all LLVM.  LLVM provides a 
convenience naming feature, but you don't have to use that.

ABI-dictated memory layout is the main detail to get right, across 
platforms.

	Jeff