[PATCH] D40892: Add early out to O(n^2) switch analysis in switch-to-select conversion

[Andrew Scheidecker via Phabricator via llvm-commits]

AndrewScheidecker updated this revision to Diff 128411.
AndrewScheidecker added a comment.

I added parameters to the InitializeUniqueCases function to control the early-outs, so it could theoretically be called by another transform that can support more complex switch statements.


Repository:
  rL LLVM

https://reviews.llvm.org/D40892

Files:
  lib/Transforms/Utils/SimplifyCFG.cpp


Index: lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- lib/Transforms/Utils/SimplifyCFG.cpp
+++ lib/Transforms/Utils/SimplifyCFG.cpp
@@ -4670,30 +4670,31 @@
 }
 
 // Helper function used to add CaseVal to the list of cases that generate
-// Result.
-static void MapCaseToResult(ConstantInt *CaseVal,
-                            SwitchCaseResultVectorTy &UniqueResults,
-                            Constant *Result) {
+// Result. Returns the updated number of cases that generate this result.
+static uintptr_t MapCaseToResult(ConstantInt *CaseVal,
+                                 SwitchCaseResultVectorTy &UniqueResults,
+                                 Constant *Result) {
   for (auto &I : UniqueResults) {
     if (I.first == Result) {
       I.second.push_back(CaseVal);
-      return;
+      return I.second.size();
     }
   }
   UniqueResults.push_back(
       std::make_pair(Result, SmallVector<ConstantInt *, 4>(1, CaseVal)));
+  return 1;
 }
 
 // Helper function that initializes a map containing
 // results for the PHI node of the common destination block for a switch
 // instruction. Returns false if multiple PHI nodes have been found or if
 // there is not a common destination block for the switch.
-static bool InitializeUniqueCases(SwitchInst *SI, PHINode *&PHI,
-                                  BasicBlock *&CommonDest,
-                                  SwitchCaseResultVectorTy &UniqueResults,
-                                  Constant *&DefaultResult,
-                                  const DataLayout &DL,
-                                  const TargetTransformInfo &TTI) {
+static bool
+InitializeUniqueCases(SwitchInst *SI, PHINode *&PHI, BasicBlock *&CommonDest,
+                      SwitchCaseResultVectorTy &UniqueResults,
+                      Constant *&DefaultResult, const DataLayout &DL,
+                      const TargetTransformInfo &TTI,
+                      uintptr_t MaxUniqueResults, uintptr_t MaxCasesPerResult) {
   for (auto &I : SI->cases()) {
     ConstantInt *CaseVal = I.getCaseValue();
 
@@ -4706,7 +4707,18 @@
     // Only one value per case is permitted
     if (Results.size() > 1)
       return false;
-    MapCaseToResult(CaseVal, UniqueResults, Results.begin()->second);
+
+    // Add the case->result mapping to UniqueResults.
+    const uintptr_t NumCasesForResult =
+        MapCaseToResult(CaseVal, UniqueResults, Results.begin()->second);
+
+    // Early out if there are too many cases for this result.
+    if (NumCasesForResult > MaxCasesPerResult)
+      return false;
+
+    // Early out if there are too many unique results.
+    if (UniqueResults.size() > MaxUniqueResults)
+      return false;
 
     // Check the PHI consistency.
     if (!PHI)
@@ -4806,7 +4818,7 @@
   SwitchCaseResultVectorTy UniqueResults;
   // Collect all the cases that will deliver the same value from the switch.
   if (!InitializeUniqueCases(SI, PHI, CommonDest, UniqueResults, DefaultResult,
-                             DL, TTI))
+                             DL, TTI, 2, 1))
     return false;
   // Selects choose between maximum two values.
   if (UniqueResults.size() != 2)


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