[PATCH] D35990: [SCEV] Prohibit SCEV transformations for huge SCEVs

[Max Kazantsev via Phabricator via llvm-commits]

mkazantsev created this revision.

Currently SCEV attempts to limit transformations so that they do not work with
big SCEVs (that may take almost infinite compile time). But for this, it uses heuristics
such as recursion depth and number of operands, which do not give us a guarantee
that we don't actually have big SCEVs. This situation is still possible, though it is not
likely to happen. However, the bug PR33494 showed a bunch of simple corner case
tests where we still produce huge SCEVs, even not reaching big recursion depth etc.

This patch introduces a concept of 'huge' SCEVs. A SCEV is huge if its expression
size (intoduced in https://reviews.llvm.org/D35989) exceeds some threshold value. We prohibit optimizing
transformations if any of SCEVs we are dealing with is huge. This gives us a reliable
check that we don't spend too much time working with them.

As the next step, we can possibly get rid of old limiting mechanisms, such as recursion
depth thresholds.


https://reviews.llvm.org/D35990

Files:
  lib/Analysis/ScalarEvolution.cpp


Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -167,6 +167,11 @@
                   cl::desc("Max coefficients in AddRec during evolving"),
                   cl::init(16));
 
+static cl::opt<unsigned>
+    HugeExprThreshold("scalar-evolution-huge-expr-threshold", cl::Hidden,
+                  cl::desc("Size of the expression which is considered huge"),
+                  cl::init(4096));
+
 //===----------------------------------------------------------------------===//
 //                           SCEV class definitions
 //===----------------------------------------------------------------------===//
@@ -805,6 +810,14 @@
   return F.Size;
 }
 
+static bool isHugeExpression(const SCEV *S) {
+  return S->getExpressionSize() >= HugeExprThreshold;
+}
+
+static bool hasHugeExpression(SmallVectorImpl<const SCEV *> &Ops) {
+  return std::any_of(Ops.begin(), Ops.end(), isHugeExpression);
+}
+
 namespace {
 
 struct SCEVDivision : public SCEVVisitor<SCEVDivision, void> {
@@ -2278,7 +2291,7 @@
   }
 
   // Limit recursion calls depth.
-  if (Depth > MaxArithDepth)
+  if (Depth > MaxArithDepth || hasHugeExpression(Ops))
     return getOrCreateAddExpr(Ops, Flags);
 
   // Okay, check to see if the same value occurs in the operand list more than
@@ -2720,7 +2733,7 @@
   Flags = StrengthenNoWrapFlags(this, scMulExpr, Ops, Flags);
 
   // Limit recursion calls depth.
-  if (Depth > MaxArithDepth)
+  if (Depth > MaxArithDepth || hasHugeExpression(Ops))
     return getOrCreateMulExpr(Ops, Flags);
 
   // If there are any constants, fold them together.
@@ -2894,7 +2907,8 @@
       // Limit max number of arguments to avoid creation of unreasonably big
       // SCEVAddRecs with very complex operands.
       if (AddRec->getNumOperands() + OtherAddRec->getNumOperands() - 1 >
-          MaxAddRecSize)
+          MaxAddRecSize || isHugeExpression(AddRec) ||
+          isHugeExpression(OtherAddRec))
         continue;
 
       bool Overflow = false;


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