From: Craig Topper <craig.topper@gmail.com>
Date: Sat, 18 Mar 2017 18:21:46 +0000 (+0000)
Subject: [ValueTracking] Remove deadish code from computeKnownBitsAddSub.
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=99e866c1f09f959043d4ea4180c4e74cf3fadf3e;p=llvm

[ValueTracking] Remove deadish code from computeKnownBitsAddSub.

The code assigned to KnownZero, but later code unconditionally assigned over it. I'm pretty sure the later code can handle the same cases and more equally well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@298190 91177308-0d34-0410-b5e6-96231b3b80d8
---

diff --git a/lib/Analysis/ValueTracking.cpp b/lib/Analysis/ValueTracking.cpp
index 38d9c290bfc..c401e08af92 100644
--- a/lib/Analysis/ValueTracking.cpp
+++ b/lib/Analysis/ValueTracking.cpp
@@ -255,30 +255,6 @@ static void computeKnownBitsAddSub(bool Add, const Value *Op0, const Value *Op1,
                                    APInt &KnownZero, APInt &KnownOne,
                                    APInt &KnownZero2, APInt &KnownOne2,
                                    unsigned Depth, const Query &Q) {
-  if (!Add) {
-    if (const ConstantInt *CLHS = dyn_cast<ConstantInt>(Op0)) {
-      // We know that the top bits of C-X are clear if X contains less bits
-      // than C (i.e. no wrap-around can happen).  For example, 20-X is
-      // positive if we can prove that X is >= 0 and < 16.
-      if (!CLHS->getValue().isNegative()) {
-        unsigned BitWidth = KnownZero.getBitWidth();
-        unsigned NLZ = (CLHS->getValue()+1).countLeadingZeros();
-        // NLZ can't be BitWidth with no sign bit
-        APInt MaskV = APInt::getHighBitsSet(BitWidth, NLZ+1);
-        computeKnownBits(Op1, KnownZero2, KnownOne2, Depth + 1, Q);
-
-        // If all of the MaskV bits are known to be zero, then we know the
-        // output top bits are zero, because we now know that the output is
-        // from [0-C].
-        if ((KnownZero2 & MaskV) == MaskV) {
-          unsigned NLZ2 = CLHS->getValue().countLeadingZeros();
-          // Top bits known zero.
-          KnownZero = APInt::getHighBitsSet(BitWidth, NLZ2);
-        }
-      }
-    }
-  }
-
   unsigned BitWidth = KnownZero.getBitWidth();
 
   // If an initial sequence of bits in the result is not needed, the