[LLVMdev] GVN incorrectly handling readnone parameter attribute?

[Robert Lougher]

Hi Philip,

Thanks for the reply.

On 22 May 2014 01:31, Philip Reames <listmail at philipreames.com> wrote:
>
> On 05/21/2014 02:52 PM, Robert Lougher wrote:
>>
>> On 21 May 2014 21:40, Robert Lougher <rob.lougher at gmail.com> wrote:
>>>
>>> define i32* @get_pntr(i32* readnone %p) {
>>> entry:
>>>    ret i32* %p
>>> }
>>>
>>> define void @store(i32* nocapture readnone %p) {
>>> entry:
>>>    store i32 10, i32* %p, align 4, !tbaa !1
>>>    ret void
>>> }
>>>
>> Further to my first post, based on the definition of readnone on an
>> argument, this is also incorrect.  After get_pntr() has been inlined
>> into store(), we are dereferencing %p, but it is still marked
>> readnone.
>>
>> So we seem to have a couple of issues.  First GVN seems to be making
>> incorrect assumptions based on argument attributes, and secondly
>> inlining can invalidate existing attributes?
>
> I'm not clear on the GVN issue, but looking at your example IR, the readnone
> attribute is definitely incorrect after inlining.
>

Yes.  I haven't investigated it any further, but the parameter was
already readnone before inlining, so it looks like the attributes
aren't revisited afterwards.  But that is just a guess.

> A question worth asking here: does this definition of readnone make sense?
> I can see where it came from, but it seems to give very unintuitive
> reasoning here.
>

Exactly.  That was my point in asking here before I went any further.
The definition in the LangRef seems odd.  If it could still be
accessed by another pointer, I can't see where the information is
useful.

> p.s. Is this with TOT or an earlier version?

Yes, this was with TOT as of yesterday (r209294).

Thanks,
Rob.

>
> Philip
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev