[llvm] 4c27ca2 - [NFC][SimplifyCFG] SinkCommonCodeFromPredecessors(): reword comment about PR30244

[Roman Lebedev via llvm-commits]

Author: Roman Lebedev
Date: 2021-04-29T01:01:00+03:00
New Revision: 4c27ca21d9e5a99d49b7791795ff9dd7d312f760

URL: https://github.com/llvm/llvm-project/commit/4c27ca21d9e5a99d49b7791795ff9dd7d312f760
DIFF: https://github.com/llvm/llvm-project/commit/4c27ca21d9e5a99d49b7791795ff9dd7d312f760.diff

LOG: [NFC][SimplifyCFG] SinkCommonCodeFromPredecessors(): reword comment about PR30244

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp

Removed: 
    


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diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 3bf5463efe16..6f19453aba03 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2041,11 +2041,11 @@ static bool SinkCommonCodeFromPredecessors(BasicBlock *BB,
   };
 
   if (Cond) {
-    // Check if we would actually sink anything first! This mutates the CFG and
-    // adds an extra block. The goal in doing this is to allow instructions that
-    // couldn't be sunk before to be sunk - obviously, speculatable instructions
-    // (such as trunc, add) can be sunk and predicated already. So we check that
-    // we're going to sink at least one non-speculatable instruction.
+    // It is always legal to sink common instructions from unconditional
+    // predecessors. However, if not all predecessors are unconditional,
+    // this transformation might be pessimizing. So as a rule of thumb,
+    // don't do it unless we'd sink at least one non-speculatable instruction.
+    // See https://bugs.llvm.org/show_bug.cgi?id=30244
     LRI.reset();
     unsigned Idx = 0;
     bool Profitable = false;