[LLVMdev] Moving towards a singular pointer type

[Sean Silva]

On Fri, Feb 6, 2015 at 3:38 PM, 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?
>

I'm wondering how geps will work when the pointer type doesn't encode the
stride that each index must take. Maybe it would have to be (strawman
syntax) gep<%T> off of a typeless base pointer?

-- Sean Silva


>
>
> - David
>
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