[llvm-dev] Reasoning about results of min and max with a constant
Carlos Liam via llvm-dev
llvm-dev at lists.llvm.org
Thu Sep 1 10:06:00 PDT 2016
I'm wondering if anyone would be willing to help me implement this.
> On Aug 5, 2016, at 7:58 PM, Carlos Liam <carlos at aarzee.me> wrote:
> 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 <mailto:david.majnemer at gmail.com>> wrote:
>> Yes, I was proposing something like https://ghostbin.com/paste/r5uou <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 <mailto: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 <mailto: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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