[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



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363422 91177308-0d34-0410-b5e6-96231b3b80d8

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.