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

[Carlos Liam via llvm-dev]

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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>