[llvm-dev] [LLVMdev] Moving towards a singular pointer type

[David Blaikie via llvm-dev]

On Sun, Apr 16, 2017 at 2:34 AM James Courtier-Dutton via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi,
>
> Did this work ever get done? There was a long thread about it back in 2015.
>
> I wish to use IRBuilder.
> Is there any documentation?
> How do I use the singular pointer type in GEP, LOAD, STORE instructions?
>

Sorry, no, the work is not complete - for now you'll need to pass the
pointer and pointee type to a few of these functions and some will do it
for you (Getting the pointee type from the pointer type, etc). I'll
hopefully be coming back around to this in 6-9 months to continue on it
(finishing off one or two IR constructs, then making sure all the
optimizations can cope with this)

- Dave


>
> Kind Regards
>
> James
>
> On 6 February 2015 at 23:38, David Blaikie <dblaikie at gmail.com> wrote:
>
>> It's an idea been thrown around in a few different threads (including
>> Rafael's recent
>> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20141201/247285.html
>> and Chandler's http://llvm.org/viewvc/llvm-project?rev=226781&view=rev )
>> so I'm putting up my hand to volunteer to do the work & interested in
>> getting a bit more feedback, thoughts on best approaches, timing, etc.
>>
>> For some more detail: pointer types (i32*, %foo*, etc) complicate IR
>> canonicalization. store + load should be the same instructions given the
>> same number of bytes stored to memory, but instead we can have store float,
>> store int, etc, etc. Another point Chandler made was that the bitcasts
>> involved when a pointer isn't of the right type results in extra IR
>> instructions we don't really need.
>>
>> So the general idea is that all pointers would just be called "ptr"
>> (pointer? void*?).
>>
>> Is this something everyone feels is the right direction? Any reason why
>> it wouldn't be?
>>
>> Beyond that, I'm trying to think about how to do this & I haven't hit on
>> a terribly convincing way to do this incrementally. I could introduce the
>> alternative form of "store" that provides the magic pointer type, then set
>> about adding overloads (is that possible?) of any instruction consuming a
>> pointer type, writing the usual LLVM regression tests as I go. Eventually,
>> once this looks like it's functioning, I could start porting IRbuilder and
>> Clang over to the new store operations & other sources of pointers. Then
>> remove the old stuff.
>>
>> Are IR instructions overloadable like this? If not, would it be
>> worthwhile to introduce separate names for the typeless-pointer forms
>> (gep_ptr, store_ptr, etc) as a temporary means to have both sets of
>> semantics then rename them all back once the old ones are removed?
>>
>> Other ideas/thoughts?
>>
>> - David
>>
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