[llvm] r300309 - [ValueTracking] Calculate the KnownZeros for Intrinsic::ctpop without using a temporary APInt to count leading zeros on.

[Craig Topper via llvm-commits]

Author: ctopper
Date: Fri Apr 14 01:43:34 2017
New Revision: 300309

URL: http://llvm.org/viewvc/llvm-project?rev=300309&view=rev
Log:
[ValueTracking] Calculate the KnownZeros for Intrinsic::ctpop without using a temporary APInt to count leading zeros on.

The APInt was created from an 'unsigned' and we just wanted to know how many bits the value needed to represent it. We can just use Log2_32 from MathExtras.h to get the info.

Modified:
    llvm/trunk/lib/Analysis/ValueTracking.cpp

Modified: llvm/trunk/lib/Analysis/ValueTracking.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ValueTracking.cpp?rev=300309&r1=300308&r2=300309&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ValueTracking.cpp (original)
+++ llvm/trunk/lib/Analysis/ValueTracking.cpp Fri Apr 14 01:43:34 2017
@@ -1428,11 +1428,8 @@ static void computeKnownBitsFromOperator
         // We can bound the space the count needs.  Also, bits known to be zero
         // can't contribute to the population.
         unsigned BitsPossiblySet = BitWidth - KnownZero2.countPopulation();
-        unsigned LeadingZeros =
-          APInt(BitWidth, BitsPossiblySet).countLeadingZeros();
-        assert(LeadingZeros <= BitWidth);
-        KnownZero.setHighBits(LeadingZeros);
-        KnownOne &= ~KnownZero;
+        unsigned LowBits = Log2_32(BitsPossiblySet)+1;
+        KnownZero.setBitsFrom(LowBits);
         // TODO: we could bound KnownOne using the lower bound on the number
         // of bits which might be set provided by popcnt KnownOne2.
         break;