[SCEV] Limit AddRec "simplifications" to avoid combinatorial explosions

SCEV's transform that turns `{A1,+,A2,+,...,+,An}<L> * {B1,+,B2,+,...,+,Bn}<L>` into
a single AddRec of size `2n+1` with complex combinatorial coefficients can easily
trigger exponential growth of the SCEV (in case if nothing gets folded and simplified).
We tried to restrain this transform using the option `scalar-evolution-max-add-rec-size`,
but its default value seems to be insufficiently small: the test attached to this patch
with default value of this option `16` has a SCEV of >3M symbols (when printed out).

This patch reduces the simplification limit. It is not a cure to combinatorial
explosions, but at least it reduces this corner case to something more or less
reasonable.

Differential Revision: https://reviews.llvm.org/D53282
Reviewed By: sanjoy

llvm-svn: 344584

1 files changed, 1 insertions, 1 deletions

diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 4a30447..60cd1cb 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -204,7 +204,7 @@ static cl::opt<unsigned>
 static cl::opt<unsigned>
     MaxAddRecSize("scalar-evolution-max-add-rec-size", cl::Hidden,
                   cl::desc("Max coefficients in AddRec during evolving"),
-                  cl::init(16));
+                  cl::init(8));
 
 //===----------------------------------------------------------------------===//
 //                           SCEV class definitions