[PATCH] D61151: [SimpligyCFG] NFC, remove GCD that was only used for powers of two

[Shawn Landden via Phabricator via llvm-commits]

shawnl updated this revision to Diff 197005.

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D61151/new/

https://reviews.llvm.org/D61151

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp


Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -5543,25 +5543,24 @@
   // 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.
+  // 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);
+  // Cttz often has an edge condition on 0 which means that the bit-width
+  // is important, however here there is no such edge condition because if
+  // 0 is the only value then a shift does nothing, and LLVM requires
+  // well-formed IR to not have duplicate cases (so the minimum will not
+  // be BitWidth)
+  unsigned Shift = 64;
   for (auto &V : Values)
-    V = (int64_t)((uint64_t)V >> Shift);
+    Shift = std::min(Shift, countTrailingZeros((uint64_t)V);
+  if (Shift > 0)
+    for (auto &V : Values)
+      V = (int64_t)((uint64_t)V >> Shift);
 
   if (!isSwitchDense(Values))
     // Transform didn't create a dense switch.


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