[llvm-dev] [RFC] : LLVM IR should allow bitcast between address spaces with the same size

[Sankisa, Krishna (Chaitanya) via llvm-dev]

[AMD Official Use Only]

Re-iterating the proposed change and clarifying the concerns raised :

Proposed change:
- Allow no-op bitcast between pointers of same size but from different addrspaces.

Clarifying the concerns:
- No-op reinterprent of bits between addrspaces is already done in the IR like introducing ptrtoint/inttoptr(ex. GVN).
  Main motivation for this change is to give the IR a way to represent a cast we know is a no-op without resorting to ptrtoint/inttoptr.

- With this change we do not want to introduce new illegal address space conversions that were not already present in the original program.
  The original code was not performing an address space cast, it was doing type punning and reinterpreting some bits as a different address space.
  It was broken to begin with if the target didn't allow no-op conversions between given pair of address spaces. We are not intending to change that.
  Since the IR transform like in GVN case, did a no-op reinterpretation of bits between addrspaces, We are just trying to optimise it by using a no-op bitcast.

- Since the original IR already did this reinterpret of bits without querying the target, the change we propose will also not query the target.
  This pointer-reinterpret-to-cast transformation is and should be done unconditionally. Querying TTI to check for no-op would introduce target dependence on Core IR instruction semantics.
  We used DataLayout to match the pointer sizes. This check is sufficient to allow the no-op bitcast.

- Also, there is no target specific pattern to search for here. Irrespective of the target, we need to introduce new IR that represents the same no-op reinterpret of bits between addrspaces.
  All the targets are intended to benefit from this change.

Regards,
Chaitanya
________________________________
From: Matt Arsenault <whatmannerofburgeristhis at gmail.com> on behalf of Matt Arsenault <arsenm2 at gmail.com>
Sent: 01 December 2021 05:57
To: Renato Golin <rengolin at gmail.com>
Cc: Sankisa, Krishna (Chaitanya) <Krishna.Sankisa at amd.com>; llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [RFC] : LLVM IR should allow bitcast between address spaces with the same size

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On Nov 30, 2021, at 04:51, Renato Golin <rengolin at gmail.com<mailto:rengolin at gmail.com>> wrote:

On Mon, 29 Nov 2021 at 18:15, Matt Arsenault <arsenm2 at gmail.com<mailto:arsenm2 at gmail.com>> wrote:
This is true, but this isn’t directly related to this change. If the target does not have no-op conversions between a given pair of address spaces, the original code was broken to begin with. This change does not allow introducing new illegal address space conversions that were not already present in the original program. It’s garbage in, garbage out. If the target can’t really reinterpret these pointers, it would be UB on access.

Ah, I see.

So IIUC, adding the casts in the first place would have to know about target address spaces, but this specific change only allows eliding the inttoptr cast in the specific case where the address spaces have the same data layout?

Assuming the initial cast was legal to begin with, this looks sensible to me.
The target’s notion of address space casts doesn’t apply here. The user decided to reinterpret the pointer in a different address space, which may or may not be valid to use on the target.


I was worried this would allow (encourage?) front-ends and other passes to add an address space cast where none (could have) existed in the first place.

No, we’re converting from an in-memory representation of reinterpreting bits to one in values. Frontends still need to have some contextual knowledge of what the language and target expects out of address spaces.

-Matt
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