[SimpligyCFG] NFC intended, remove GCD that was only used for powers …

…of two

and replace with an equilivent countTrailingZeros.

GCD is much more expensive than this, with repeated division.

This depends on D60823

Differential Revision: https://reviews.llvm.org/D61151

llvm-svn: 363422

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

@@ -5559,25 +5559,23 @@ static bool ReduceSwitchRange(SwitchInst *SI, IRBuilder<> &Builder,
   // Now we have signed numbers that have been shifted so that, given enough
   // precision, there are no negative values. Since the rest of the transform
   // is bitwise only, we switch now to an unsigned representation.
-  uint64_t GCD = 0;
-  for (auto &V : Values)
-    GCD = GreatestCommonDivisor64(GCD, (uint64_t)V);
 
-  // This transform can be done speculatively because it is so cheap - it results
-  // in a single rotate operation being inserted. This can only happen if the
-  // factor extracted is a power of 2.
-  // FIXME: If the GCD is an odd number we can multiply by the multiplicative
-  // inverse of GCD and then perform this transform.
+  // This transform can be done speculatively because it is so cheap - it
+  // results in a single rotate operation being inserted.
   // FIXME: It's possible that optimizing a switch on powers of two might also
   // be beneficial - flag values are often powers of two and we could use a CLZ
   // as the key function.
-  if (GCD <= 1 || !isPowerOf2_64(GCD))
-    // No common divisor found or too expensive to compute key function.
-    return false;
 
-  unsigned Shift = Log2_64(GCD);
+  // countTrailingZeros(0) returns 64. As Values is guaranteed to have more than
+  // one element and LLVM disallows duplicate cases, Shift is guaranteed to be
+  // less than 64.
+  unsigned Shift = 64;
   for (auto &V : Values)
-    V = (int64_t)((uint64_t)V >> Shift);
+    Shift = std::min(Shift, countTrailingZeros((uint64_t)V));
+  assert(Shift < 64);
+  if (Shift > 0)
+    for (auto &V : Values)
+      V = (int64_t)((uint64_t)V >> Shift);
 
   if (!isSwitchDense(Values))
     // Transform didn't create a dense switch.