[llvm-dev] Reasoning about results of min and max with a constant

[David Majnemer via 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!
> >> _______________________________________________
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> >
> >
>
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