llvm dev - Dec 2016 - analysis based on nonnull attribute

On Wed, Dec 14, 2016 at 2:51 PM, Hal Finkel <hfinkel at anl.gov> wrote:
>
>
> ------------------------------
>
> *From: *"Sanjay Patel via llvm-dev" <llvm-dev at
lists.llvm.org>
> *To: *"llvm-dev" <llvm-dev at lists.llvm.org>
> *Sent: *Wednesday, December 14, 2016 3:47:03 PM
> *Subject: *[llvm-dev] analysis based on nonnull attribute
>
> Does the nonnull parameter attribute give us information about subsequent
> uses of that value outside of the function that has the attribute?
>
> Yes. We're guaranteeing that we never pass a null value for the
argument,
> so that information can be used to optimize the caller as well.
>
Thanks! I don't know if that will actually solve our sub-optimal output for
dyn_cast (!), but it might help...
https://llvm.org/bugs/show_bug.cgi?id=28430


>
>
> Example:
>
> define i1 @bar(i32* nonnull %x) { ; %x must be non-null in this function
>   %y = load i32, i32* %x
>   %z = icmp ugt i32 %y, 23
>   ret i1 %z
> }
>
> define i1 @foo(i32* %x) {
>   %d = call i1 @bar(i32* %x)
>   %null_check = icmp eq i32* %x, null ; check if null after call that
> guarantees non-null?
>   br i1 %null_check, label %t, label %f
> t:
>   ret i1 1
> f:
>   ret i1 %d
> }
>
> $ opt  -inline  nonnull.ll -S
> ...
> define i1 @foo(i32* %x) {
>   %y.i = load i32, i32* %x   ; inlined and non-null knowledge is lost?
>
> It should be replaced by !nonnull metadata on the load. We might not be
> doing that today, however.
>
>
We can't tag this load with !nonnull though because this isn't a load of
the pointer?
"The existence of the !nonnull metadata on the instruction tells the
optimizer that the value loaded is known to never be null. This is
analogous to the nonnull attribute on parameters and return values. This
metadata can only be applied to loads of a pointer type."
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20161214/ca5ef458/attachment.html>

----- Original Message -----
> From: "Sanjay Patel" <spatel at rotateright.com>
> To: "Hal Finkel" <hfinkel at anl.gov>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Wednesday, December 14, 2016 4:03:40 PM
> Subject: Re: [llvm-dev] analysis based on nonnull attribute
> On Wed, Dec 14, 2016 at 2:51 PM, Hal Finkel < hfinkel at anl.gov >
> wrote:
> > > From: "Sanjay Patel via llvm-dev" < llvm-dev at
lists.llvm.org >
> > 
> 
> > > To: "llvm-dev" < llvm-dev at lists.llvm.org >
> > 
> 
> > > Sent: Wednesday, December 14, 2016 3:47:03 PM
> > 
> 
> > > Subject: [llvm-dev] analysis based on nonnull attribute
> > 
> 
> > > Does the nonnull parameter attribute give us information about
> > > subsequent uses of that value outside of the function that has
> > > the
> > > attribute?
> > 
> 
> > Yes. We're guaranteeing that we never pass a null value for the
> > argument, so that information can be used to optimize the caller as
> > well.
> 
> Thanks! I don't know if that will actually solve our sub-optimal
> output for dyn_cast (!), but it might help...
> https://llvm.org/bugs/show_bug.cgi?id=28430
> > > Example:
> > 
> 
> > > define i1 @bar(i32* nonnull %x) { ; %x must be non-null in this
> > > function
> > 
> 
> > > %y = load i32, i32* %x
> > 
> 
> > > %z = icmp ugt i32 %y, 23
> > 
> 
> > > ret i1 %z
> > 
> 
> > > }
> > 
> 
> > > define i1 @foo(i32* %x) {
> > 
> 
> > > %d = call i1 @bar(i32* %x)
> > 
> 
> > > %null_check = icmp eq i32* %x, null ; check if null after call
> > > that
> > > guarantees non-null?
> > 
> 
> > > br i1 %null_check, label %t, label %f
> > 
> 
> > > t:
> > 
> 
> > > ret i1 1
> > 
> 
> > > f:
> > 
> 
> > > ret i1 %d
> > 
> 
> > > }
> > 
> 
> > > $ opt -inline nonnull.ll -S
> > 
> 
> > > ...
> > 
> 
> > > define i1 @foo(i32* %x) {
> > 
> 
> > > %y.i = load i32, i32* %x ; inlined and non-null knowledge is
> > > lost?
> > 
> 
> > It should be replaced by !nonnull metadata on the load. We might
> > not
> > be doing that today, however.
> 
> We can't tag this load with !nonnull though because this isn't a
load
> of the pointer?
> "The existence of the !nonnull metadata on the instruction tells the
> optimizer that the value loaded is known to never be null. This is
> analogous to the nonnull attribute on parameters and return values.
> This metadata can only be applied to loads of a pointer type."
True, but we have range metadata for integers. 

-Hal 

-- 

Hal Finkel 
Lead, Compiler Technology and Programming Languages 
Leadership Computing Facility 
Argonne National Laboratory 
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20161214/a8be4fa4/attachment.html>

I think what Sanjay is getting at is that it's not an integer, it's
still a
pointer - but it's not clear where information about non-nullness of the
pointer should be propagated to.

In this particular case, since the def of %x in the caller is also an
argument, we could propagate it to the def directly, e.g.

define i1 @foo(i32* nonnull %x) {
  %y.i = load i32, i32* %x   ; inlined, still known to be nonnull

And if the def of %x was a load, we could use !nonnull. But I'm not sure
what we can do in the general case (say, %x = select...).
The best I can think of is generating an llvm.assume for the condition.

Michael

On 14 December 2016 at 14:05, Hal Finkel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> ------------------------------
>
> *From: *"Sanjay Patel" <spatel at rotateright.com>
> *To: *"Hal Finkel" <hfinkel at anl.gov>
> *Cc: *"llvm-dev" <llvm-dev at lists.llvm.org>
> *Sent: *Wednesday, December 14, 2016 4:03:40 PM
> *Subject: *Re: [llvm-dev] analysis based on nonnull attribute
>
>
>
>
> On Wed, Dec 14, 2016 at 2:51 PM, Hal Finkel <hfinkel at anl.gov>
wrote:
>
>>
>>
>> ------------------------------
>>
>> *From: *"Sanjay Patel via llvm-dev" <llvm-dev at
lists.llvm.org>
>> *To: *"llvm-dev" <llvm-dev at lists.llvm.org>
>> *Sent: *Wednesday, December 14, 2016 3:47:03 PM
>> *Subject: *[llvm-dev] analysis based on nonnull attribute
>>
>> Does the nonnull parameter attribute give us information about
subsequent
>> uses of that value outside of the function that has the attribute?
>>
>> Yes. We're guaranteeing that we never pass a null value for the
argument,
>> so that information can be used to optimize the caller as well.
>>
>
> Thanks! I don't know if that will actually solve our sub-optimal output
> for dyn_cast (!), but it might help...
> https://llvm.org/bugs/show_bug.cgi?id=28430
>
>
>
>>
>>
>> Example:
>>
>> define i1 @bar(i32* nonnull %x) { ; %x must be non-null in this
function
>>   %y = load i32, i32* %x
>>   %z = icmp ugt i32 %y, 23
>>   ret i1 %z
>> }
>>
>> define i1 @foo(i32* %x) {
>>   %d = call i1 @bar(i32* %x)
>>   %null_check = icmp eq i32* %x, null ; check if null after call that
>> guarantees non-null?
>>   br i1 %null_check, label %t, label %f
>> t:
>>   ret i1 1
>> f:
>>   ret i1 %d
>> }
>>
>> $ opt  -inline  nonnull.ll -S
>> ...
>> define i1 @foo(i32* %x) {
>>   %y.i = load i32, i32* %x   ; inlined and non-null knowledge is lost?
>>
>> It should be replaced by !nonnull metadata on the load. We might not be
>> doing that today, however.
>>
>>
> We can't tag this load with !nonnull though because this isn't a
load of
> the pointer?
> "The existence of the !nonnull metadata on the instruction tells the
> optimizer that the value loaded is known to never be null. This is
> analogous to the nonnull attribute on parameters and return values. This
> metadata can only be applied to loads of a pointer type."
>
> True, but we have range metadata for integers.
>
>  -Hal
>
>
>
>
>
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20161214/b23eef09/attachment.html>