SILOptimizer: add complexity limit in ARCCodeMotion and DeadStoreElimination

Add an emergency exit to avoid bad compile time problems in rare corner cases.
The introduced limit is more than enough for "real world" code. Even large functions have < 100 locations.
But in some corner cases - especially in generated code - we can run into quadratic complexity for large functions without that limit.

Fixes a compile time problem.
Unfortunately I don't have isolated test cases for these problems.

rdar://106516360

Author: eeckstein

lib/SILOptimizer/Transforms/ARCCodeMotion.cpp

@@ -254,6 +254,17 @@ bool CodeMotionContext::run() {
   // Initialize the data flow.
   initializeCodeMotionDataFlow();
 
+  if (RCRootVault.size() > 500) {
+    // Emergency exit to avoid bad compile time problems in rare corner cases.
+    // This limit is more than enough for "real world" code.
+    // Even large functions have < 100 locations.
+    // But in some corner cases - especially in generated code - we can run
+    // into quadratic complexity for large functions.
+    // TODO: eventually the ARCCodeMotion passes will be replaced by OSSA
+    //       optimizations which shouldn't have this problem.
+    return false;
+  }
+
   // Converge the BBSetOut with iterative data flow.
   if (MultiIteration) {
     initializeCodeMotionBBMaxSet();

lib/SILOptimizer/Transforms/DeadStoreElimination.cpp

@@ -1204,6 +1204,17 @@ bool DSEContext::run() {
   //
   // Initialize the BBToLocState mapping.
   unsigned LocationNum = this->getLocationVault().size();
+
+  if (LocationNum > 500) {
+    // Emergency exit to avoid bad compile time problems in rare corner cases.
+    // This limit is more than enough for "real world" code.
+    // Even large functions have < 100 locations.
+    // But in some corner cases - especially in generated code - we can run
+    // into quadratic complexity for large functions.
+    // TODO: implement DSE with a better (non-quadratic) algorithm
+    return false;
+  }
+
   for (auto bs : BBToLocState) {
     bs.data.init(&bs.block, LocationNum, Optimistic);
     bs.data.initStoreSetAtEndOfBlock(*this);