[llvm] f82f39b - [SCEV] Add a comment about invariant in howManyLessThans

[Philip Reames via llvm-commits]

Author: Philip Reames
Date: 2021-07-26T16:39:26-07:00
New Revision: f82f39b9cfc9a807380ebb2348c1cf16610e1e87

URL: https://github.com/llvm/llvm-project/commit/f82f39b9cfc9a807380ebb2348c1cf16610e1e87
DIFF: https://github.com/llvm/llvm-project/commit/f82f39b9cfc9a807380ebb2348c1cf16610e1e87.diff

LOG: [SCEV] Add a comment about invariant in howManyLessThans

Added: 
    

Modified: 
    llvm/lib/Analysis/ScalarEvolution.cpp

Removed: 
    


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diff  --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index df38a850c287..f22d834b5e57 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -11728,6 +11728,15 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
       return getCouldNotCompute();
   }
 
+  // On all paths just preceeding, we established the following invariant:
+  //   IV can be assumed not to overflow up to and including the exiting
+  //   iteration.  We proved this in one of two ways:
+  //   1) We can show overflow doesn't occur before the exiting iteration
+  //      1a) canIVOverflowOnLT, and b) step of one
+  //   2) We can show that if overflow occurs, the loop must execute UB
+  //      before any possible exit.
+  // Note that we have not yet proved RHS invariant (in general).
+
   const SCEV *Start = IV->getStart();
 
   // Preserve pointer-typed Start/RHS to pass to isLoopEntryGuardedByCond.