[PATCH] D47087: [ScopHelper] Cache ScopExpander results.

[Eli Friedman via Phabricator via llvm-commits]

efriedma created this revision.
efriedma added reviewers: grosser, Meinersbur.
Herald added a subscriber: javed.absar.

A naive SCEV visitor not linear in the number of SCEV expressions. For an expression like x*x, "x" will be visited twice.  If x is itself an expression like x*x, that will be visited twice, etc, and the overall runtime is O(2^N) in the number of SCEV expressions.

To prevent this from happening, add a cache, so we only visit each SCEV expression once.

Not sure this is the best solution. Maybe we can instead check whether the SCEV is scop-invariant (in which case we never need to map the value). But we don't have a utility for that at the moment.


Repository:
  rPLO Polly

https://reviews.llvm.org/D47087

Files:
  lib/Support/ScopHelper.cpp


Index: lib/Support/ScopHelper.cpp
===================================================================
--- lib/Support/ScopHelper.cpp
+++ lib/Support/ScopHelper.cpp
@@ -249,13 +249,25 @@
     return Expander.expandCodeFor(E, Ty, I);
   }
 
+  const SCEV *visit(const SCEV *E) {
+    // Cache the expansion results for intermediate SCEV expressions. A SCEV
+    // expression can refer to an operand multiple times (e.g. "x*x), so
+    // a naive visitor takes exponential time.
+    if (SCEVCache.count(E))
+      return SCEVCache[E];
+    const SCEV *Result = SCEVVisitor::visit(E);
+    SCEVCache[E] = Result;
+    return Result;
+  }
+
 private:
   SCEVExpander Expander;
   ScalarEvolution &SE;
   const char *Name;
   const Region &R;
   ValueMapT *VMap;
   BasicBlock *RTCBB;
+  DenseMap<const SCEV *, const SCEV *> SCEVCache;
 
   const SCEV *visitGenericInst(const SCEVUnknown *E, Instruction *Inst,
                                Instruction *IP) {


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D47087.147585.patch
Type: text/x-patch
Size: 958 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20180518/1e82f4e2/attachment.bin>