llvm dev - Jun 2016 - Early CSE clobbering llvm.assume

Yeah, that change is completely unrelated, that is about correctness, this
is about optimization.
I'm working on a proposal to just fix assume at some point to deal with the
former issue.

The problem with this testcase is that all the ways assume is propagate
expect the variable in the assume to later be used.


<This is the main way assume constants are propagated>
bool GVN::processAssumeIntrinsic(IntrinsicInst *Inst) {
...

   // We can replace assume value with true, which covers cases like this:
   // call void @llvm.assume(i1 %cmp)
   // br i1 %cmp, label %bb1, label %bb2 ; will change %cmp to true
   ReplaceWithConstMap[V] = True;
...
}

...
bool GVN::processBlock(BasicBlock *BB) {
...
   // Clearing map before every BB because it can be used only for single
BB.
   ReplaceWithConstMap.clear();
....

}


So it's going to go through the rest of the bb, see nothing with %2, do
nothing, and then next iteration, clear the constant map.  It's not valid
to avoid clearing the constant map, and in fact,  what is happening here is
a pretty complicated to help with.
There is no easy way to see that the load  at %3 is affected at all by the
assume.

It's possible to make this work using the predication/value inference
algorithms in the paper newgvn is based on, but it's not even implemented
there.

Short answer, without special casing this in magic ways, i wouldn't expect
this to get fixed anytime soon.

If we fixed assume in one of the ways i thought about, like bodik's
extended ssa:
http://homepages.dcc.ufmg.br/~fernando/classes/dcc888/ementa/slides/RangeAnalysis.pdf

You would at least see that the load result is used by an assume, and could
go look at that assume and so something with it.  Currently, it's a few
steps away.

In the current scheme, assumes just float in air, and so it can be hard to
see what their effects touch
:)


On Fri, Jun 10, 2016 at 2:00 PM, Josh Klontz via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Thanks for the lead Kevin. Unfortunately when I updated to ToT the problem
> persists. Will put together a minimum reproducing example.
>
> On Fri, Jun 10, 2016 at 12:26 PM, Smith, Kevin B <kevin.b.smith at
intel.com>
> wrote:
>
>> You might look at this commit to fix the problem: r270823
>> "MemorySSA: Revert r269678 and r268068; replace with special
casing in
>> MemorySSA."
>>
>> I think that might fix the issue for you.
>>
>> Kevin Smith
>>
>>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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Thanks Daniel, with that knowledge I think I can at least work around the
issue in my frontend.

Ignoring GVN for a second though, and just looking at Early CSE, it seems
to me that at least in this pass that there is the potential for an easy
fix. Here the issue appears to be that we hit

  if (Value *V = SimplifyInstruction(Inst, DL, &TLI, &DT, &AC))

immediately replacing %1 with 3 before we even reach %3. If we were to
record this replacement in EarlyCSE::AvailableValues, wouldn't that address
the issue? I'll try this out and see.

v/r,
Josh

On Fri, Jun 10, 2016 at 4:23 PM, Daniel Berlin <dberlin at dberlin.org>
wrote:
> Yeah, that change is completely unrelated, that is about correctness, this
> is about optimization.
> I'm working on a proposal to just fix assume at some point to deal with
> the former issue.
>
> The problem with this testcase is that all the ways assume is propagate
> expect the variable in the assume to later be used.
>
>
> <This is the main way assume constants are propagated>
> bool GVN::processAssumeIntrinsic(IntrinsicInst *Inst) {
> ...
>
>    // We can replace assume value with true, which covers cases like this:
>    // call void @llvm.assume(i1 %cmp)
>    // br i1 %cmp, label %bb1, label %bb2 ; will change %cmp to true
>    ReplaceWithConstMap[V] = True;
> ...
> }
>
> ...
> bool GVN::processBlock(BasicBlock *BB) {
> ...
>    // Clearing map before every BB because it can be used only for single
> BB.
>    ReplaceWithConstMap.clear();
> ....
>
> }
>
>
> So it's going to go through the rest of the bb, see nothing with %2, do
> nothing, and then next iteration, clear the constant map.  It's not
valid
> to avoid clearing the constant map, and in fact,  what is happening here is
> a pretty complicated to help with.
> There is no easy way to see that the load  at %3 is affected at all by the
> assume.
>
> It's possible to make this work using the predication/value inference
> algorithms in the paper newgvn is based on, but it's not even
implemented
> there.
>
> Short answer, without special casing this in magic ways, i wouldn't
expect
> this to get fixed anytime soon.
>
> If we fixed assume in one of the ways i thought about, like bodik's
> extended ssa:
>
>
http://homepages.dcc.ufmg.br/~fernando/classes/dcc888/ementa/slides/RangeAnalysis.pdf
>
> You would at least see that the load result is used by an assume, and
> could go look at that assume and so something with it.  Currently, it's
a
> few steps away.
>
> In the current scheme, assumes just float in air, and so it can be hard to
> see what their effects touch
> :)
>
>
> On Fri, Jun 10, 2016 at 2:00 PM, Josh Klontz via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Thanks for the lead Kevin. Unfortunately when I updated to ToT the
>> problem persists. Will put together a minimum reproducing example.
>>
>> On Fri, Jun 10, 2016 at 12:26 PM, Smith, Kevin B <kevin.b.smith at
intel.com
>> > wrote:
>>
>>> You might look at this commit to fix the problem: r270823
>>> "MemorySSA: Revert r269678 and r268068; replace with special
casing in
>>> MemorySSA."
>>>
>>> I think that might fix the issue for you.
>>>
>>> Kevin Smith
>>>
>>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
>
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Maybe.  It may not fix it directly because you never use %1 or %2 again.
I haven't looked to see how good the lookup is.

On Fri, Jun 10, 2016, 3:45 PM Josh Klontz <josh.klontz at gmail.com>
wrote:
> Thanks Daniel, with that knowledge I think I can at least work around the
> issue in my frontend.
>
> Ignoring GVN for a second though, and just looking at Early CSE, it seems
> to me that at least in this pass that there is the potential for an easy
> fix. Here the issue appears to be that we hit
>
>   if (Value *V = SimplifyInstruction(Inst, DL, &TLI, &DT, &AC))
>
> immediately replacing %1 with 3 before we even reach %3. If we were to
> record this replacement in EarlyCSE::AvailableValues, wouldn't that
address
> the issue? I'll try this out and see.
>
> v/r,
> Josh
>
> On Fri, Jun 10, 2016 at 4:23 PM, Daniel Berlin <dberlin at
dberlin.org>
> wrote:
>
>> Yeah, that change is completely unrelated, that is about correctness,
>> this is about optimization.
>> I'm working on a proposal to just fix assume at some point to deal
with
>> the former issue.
>>
>> The problem with this testcase is that all the ways assume is propagate
>> expect the variable in the assume to later be used.
>>
>>
>> <This is the main way assume constants are propagated>
>> bool GVN::processAssumeIntrinsic(IntrinsicInst *Inst) {
>> ...
>>
>>    // We can replace assume value with true, which covers cases like
this:
>>    // call void @llvm.assume(i1 %cmp)
>>    // br i1 %cmp, label %bb1, label %bb2 ; will change %cmp to true
>>    ReplaceWithConstMap[V] = True;
>> ...
>> }
>>
>> ...
>> bool GVN::processBlock(BasicBlock *BB) {
>> ...
>>    // Clearing map before every BB because it can be used only for
single
>> BB.
>>    ReplaceWithConstMap.clear();
>> ....
>>
>> }
>>
>>
>> So it's going to go through the rest of the bb, see nothing with
%2, do
>> nothing, and then next iteration, clear the constant map.  It's not
valid
>> to avoid clearing the constant map, and in fact,  what is happening
here is
>> a pretty complicated to help with.
>> There is no easy way to see that the load  at %3 is affected at all by
>> the assume.
>>
>> It's possible to make this work using the predication/value
inference
>> algorithms in the paper newgvn is based on, but it's not even
implemented
>> there.
>>
>> Short answer, without special casing this in magic ways, i wouldn't
>> expect this to get fixed anytime soon.
>>
>> If we fixed assume in one of the ways i thought about, like bodik's
>> extended ssa:
>>
>>
http://homepages.dcc.ufmg.br/~fernando/classes/dcc888/ementa/slides/RangeAnalysis.pdf
>>
>> You would at least see that the load result is used by an assume, and
>> could go look at that assume and so something with it.  Currently,
it's a
>> few steps away.
>>
>> In the current scheme, assumes just float in air, and so it can be hard
>> to see what their effects touch
>> :)
>>
>>
>> On Fri, Jun 10, 2016 at 2:00 PM, Josh Klontz via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Thanks for the lead Kevin. Unfortunately when I updated to ToT the
>>> problem persists. Will put together a minimum reproducing example.
>>>
>>> On Fri, Jun 10, 2016 at 12:26 PM, Smith, Kevin B <
>>> kevin.b.smith at intel.com> wrote:
>>>
>>>> You might look at this commit to fix the problem: r270823
>>>> "MemorySSA: Revert r269678 and r268068; replace with
special casing in
>>>> MemorySSA."
>>>>
>>>> I think that might fix the issue for you.
>>>>
>>>> Kevin Smith
>>>>
>>>>
>>>
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>>
>>
>
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