[LLVMdev] GVN incorrectly handling readnone parameter attribute?

Fri May 23 12:41:59 PDT 2014

Would it make sense to count the return value as an escape point?  ie it
seems (and hopefully is programmed to be) incorrect to infer a readnone
attribute for a pointer that gets passed to a non-readnone function, or
stored to a global variable, etc.  In that sense the pointer seems to also
escape if returned from the function, because the function can't guarantee
any longer that no reads or writes happen to that value.  So my take would
have been that get_pntr() shouldn't get annotated with readnone...

On Fri, May 23, 2014 at 11:56 AM, Nick Lewycky <nlewycky at google.com> wrote:

> On 23 May 2014 09:42, Robert Lougher <rob.lougher at gmail.com> wrote:
>
>> Hi Nick,
>>
>> Thanks for replying.  Bug filed:
>> http://llvm.org/bugs/show_bug.cgi?id=19842
>
>
> Thank you!
>
> Strangely enough, my first conclusion was that %p was being marked
>> readnone incorrectly as it wasn't handling the copy via @get_addr.
>>
>
> Sorry -- saying %p alone is ambiguous because there's the %p parameter of @get_pntr
> and the %p parameter of @store. It is correct to mark %p readnone in
> @get_pntr. From function entrance to exit, it does not write to the pointer
> passed in. It is incorrect to mark %p readnone in @store, as it receives a
> pointer then writes to it. Supposing the @get_pntr loaded a pointer from
> some global variable then wrote through it, then @get_pntr could still mark
> its %p as being readnone, since the write was not through %p particularly,
> it just happens to be through a pointer that aliased it.
>
> Somebody (not here) then pointed out the readnone definition in the
>> LangRef (specifically the "even though it may read or write
>> the memory that the pointer points to if accessed through other
>> pointers").  I decided the copy fell within this definition and
>> changed my mind and decided GVN was at fault.  How should the
>> definition be interpreted?
>>
>
> Consider:
>
> define void @example(i32* %p, i32* %q) {
>   store i32 10, i32* %q
>   %A = icmp ult i32* %p, %q
>    call void @use(i1 %A) readnone
>   ret void
> }
>
> @example does not modify the memory pointed to by %p, unless it does it
> through a different pointer, hence it may mark that parameter readnone. A
> caller may choose to pass in %p == %q in which case the memory pointed-to
> may change, but @example still lives up to its contract.
>
> The real-world implications of this are that callers may use this
> information to optimize themselves, combined with other information such as
> knowing that %p != %q, or better, that %p is a local alloca or freshly
> malloc'd memory and the pointer %p has not yet escaped (aka. been captured).
>
> If we read the language as permitting the function to make copies of the
> pointers the this code would be valid:
>
> define void @writes_over_everything(i32* readnone %p, i32* readnone %q) {
>   %P = bitcast i32* %p to i32*
>   %Q = bitcast i32* %p to i32*
>   store i32 10, i32* %P
>   store i32 10, i32* %Q
>   ret void
> }
>
> and that's not useful for optimization.
>
> Nick
>
>
>>  On 23 May 2014 04:22, Nick Lewycky <nlewycky at google.com> wrote:
>> > Confirmed, this is a bug. This
>> >
>> > define i32* @get_pntr(i32* %p) nounwind uwtable {
>> > entry:
>> >   ret i32* %p
>> > }
>> > define void @store(i32* %p) noinline nounwind uwtable {
>> > entry:
>> >   %call = call i32* @get_pntr(i32* %p)
>> >   store i32 10, i32* %call, align 4
>> >   ret void
>> > }
>> >
>> > run through opt -functionattrs gets a 'readnone' on @store's %p. That's
>> > wrong, it clearly stores to it. The bug is due to incorrectly handling
>> the
>> > copy of the pointer made by @get_pntr, because @get_pntr itself is
>> marked
>> > 'readnone'.
>> >
>> > Please file a bug.
>> >
>> > Nick
>> >
>> >
>> >
>> > On 22 May 2014 08:15, Robert Lougher <rob.lougher at gmail.com> wrote:
>> >>
>> >> 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
>> >> >
>> >> > _______________________________________________
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>> >
>>
>
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