llvm dev - Aug 2016 - Reasoning about results of min and max with a constant

Looks like LVI actually does have cases for max and min; would it be better to
allow ValueTracking to use range analysis instead?

 - CL
> On Aug 1, 2016, at 8:44 PM, Philip Reames <listmail at
philipreames.com> wrote:
> 
>> On 07/29/2016 07:08 PM, Carlos Liam via llvm-dev wrote:
>> Hi all,
>> 
>> Say we have this IR:
>> 
>> %1 = icmp slt i16 %x, 0
>> %.x = select i1 %1, i16 0, i16 %x
>> 
>> This is the canonical form of what is effectively max(x, 0).
>> 
>> From what I can tell LLVM has no facilities to determine from this code
that %.x >= 0, so (for example) an SExt on %.x will not be converted to a
ZExt.
>> 
>> I'm interested in seeing what sorts of changes would be needed to
recognize this pattern in existing facilities (computeKnownBits, etc.) in order
to more broadly apply optimizations that use those facilities.
> LVI has a couple of special cases around select idioms.  This would be
another reasonable one to add.  This would give range analysis (used in
JumpThreading and CVP) for this idiom.  I thought we already had this one
actually.
>> 
>> Any insight would be appreciated. Thanks!
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
>

Yes, I was proposing something like https://ghostbin.com/paste/r5uou

On Fri, Aug 5, 2016 at 2:05 PM, Carlos Liam via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Looks like LVI actually does have cases for max and min; would it be
> better to allow ValueTracking to use range analysis instead?
>
>  - CL
>
> > On Aug 1, 2016, at 8:44 PM, Philip Reames <listmail at
philipreames.com>
> wrote:
> >
> >> On 07/29/2016 07:08 PM, Carlos Liam via llvm-dev wrote:
> >> Hi all,
> >>
> >> Say we have this IR:
> >>
> >> %1 = icmp slt i16 %x, 0
> >> %.x = select i1 %1, i16 0, i16 %x
> >>
> >> This is the canonical form of what is effectively max(x, 0).
> >>
> >> From what I can tell LLVM has no facilities to determine from this
code
> that %.x >= 0, so (for example) an SExt on %.x will not be converted to
a
> ZExt.
> >>
> >> I'm interested in seeing what sorts of changes would be needed
to
> recognize this pattern in existing facilities (computeKnownBits, etc.) in
> order to more broadly apply optimizations that use those facilities.
> > LVI has a couple of special cases around select idioms.  This would be
> another reasonable one to add.  This would give range analysis (used in
> JumpThreading and CVP) for this idiom.  I thought we already had this one
> actually.
> >>
> >> Any insight would be appreciated. Thanks!
> >> _______________________________________________
> >> LLVM Developers mailing list
> >> llvm-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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I actually meant using the range analysis provided by LVI in computeKnownBits,
instead of matching select patterns again in computeKnownBits. For example,
consider this code:

// num is unsigned
if (num < 4)
  num = num & 4; // this can be proven to be 0

If we allowed computeKnownBits to work in conjunction with range analysis, we
could optimize this case.

 - CL
> On Aug 5, 2016, at 5:29 PM, David Majnemer <david.majnemer at
gmail.com> wrote:
> 
> Yes, I was proposing something like https://ghostbin.com/paste/r5uou
> 
>> On Fri, Aug 5, 2016 at 2:05 PM, Carlos Liam via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
>> Looks like LVI actually does have cases for max and min; would it be
better to allow ValueTracking to use range analysis instead?
>> 
>>  - CL
>> 
>> > On Aug 1, 2016, at 8:44 PM, Philip Reames <listmail at
philipreames.com> wrote:
>> >
>> >> On 07/29/2016 07:08 PM, Carlos Liam via llvm-dev wrote:
>> >> Hi all,
>> >>
>> >> Say we have this IR:
>> >>
>> >> %1 = icmp slt i16 %x, 0
>> >> %.x = select i1 %1, i16 0, i16 %x
>> >>
>> >> This is the canonical form of what is effectively max(x, 0).
>> >>
>> >> From what I can tell LLVM has no facilities to determine from
this code that %.x >= 0, so (for example) an SExt on %.x will not be
converted to a ZExt.
>> >>
>> >> I'm interested in seeing what sorts of changes would be
needed to recognize this pattern in existing facilities (computeKnownBits, etc.)
in order to more broadly apply optimizations that use those facilities.
>> > LVI has a couple of special cases around select idioms.  This
would be another reasonable one to add.  This would give range analysis (used in
JumpThreading and CVP) for this idiom.  I thought we already had this one
actually.
>> >>
>> >> Any insight would be appreciated. Thanks!
>> >> _______________________________________________
>> >> LLVM Developers mailing list
>> >> llvm-dev at lists.llvm.org
>> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>> >
>> 
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
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