[LLVMdev] Moving towards a singular pointer type

[Colin LeMahieu]

This idea seems to have good merits to me.  It seems like any functionality related to pointer types could determined at load/store time without the overhead of specifying pointer types in IR only to cast it away right before usage.

 

I often found myself doing the i8* trick any time what I was pointing to didn’t easy fit in to the type system since, as others pointed out, the frontend should know what the actual types are anyway.

 

One migration issue I could see is any time a frontend depends on the LLVM type system for tracking types rather than doing it itself.

 

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On Behalf Of David Blaikie
Sent: Tuesday, February 17, 2015 11:34 AM
To: Chris Lattner
Cc: LLVM Developers Mailing List
Subject: Re: [LLVMdev] Moving towards a singular pointer type

 

 

 

On Mon, Feb 16, 2015 at 3:53 PM, Chris Lattner <clattner at apple.com <mailto:clattner at apple.com> > wrote:

 

On Feb 6, 2015, at 3:38 PM, David Blaikie <dblaikie at gmail.com <mailto: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 <http://llvm.org/viewvc/llvm-project?rev=226781&view=rev> &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?

 

Hi David,

 

You can emulate this by having clang always generate "i8*” as the general type, then bitcast to "%Foo*” before doing a GEP.  Have you prototyped this change (with this approach, or any other one) to see what it does to the optimizers?


No, I haven't tried prototyping it. It's a fair point, though I'm not sure how much of that work would be thrown out (some of it would be reusable - since it'll force the front end to track type information wehre it might've previously been relying on the IR types). Reckon it's worthwhile? (I can certainly see that side of it - throwing all this churn in when we don't quite know how it'll play out in the end seem risky, but it seemed like those who're invested in this stuff felt pretty certain it was the right way to go)

- David
 

 

-Chris

 

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