SimplifyCFG: disable some expensive optimizations for huge functions.

Disable constant folding and jump threading for functions with > 10000 blocks.
Those optimizations are not strictly linear with the number of blocks and cause compile time issues if the function is really huge.

SR-10209
rdar://problem/49522869

Author: eeckstein

lib/SILOptimizer/Transforms/SimplifyCFG.cpp

@@ -81,6 +81,10 @@ namespace {
 
     ConstantFolder ConstFolder;
 
+    // True if the function has a large amount of blocks. In this case we turn off some expensive
+    // optimizations.
+    bool isVeryLargeFunction = false;
+
     void constFoldingCallback(SILInstruction *I) {
       // If a terminal instruction gets constant folded (like cond_br), it
       // enables further simplify-CFG optimizations.
@@ -1226,11 +1230,13 @@ bool SimplifyCFG::simplifyBranchBlock(BranchInst *BI) {
       if (DestBB->getArgument(i) != BI->getArg(i)) {
         SILValue Val = BI->getArg(i);
         DestBB->getArgument(i)->replaceAllUsesWith(Val);
-        if (auto *I = dyn_cast<SingleValueInstruction>(Val)) {
-          // Replacing operands may trigger constant folding which then could
-          // trigger other simplify-CFG optimizations.
-          ConstFolder.addToWorklist(I);
-          ConstFolder.processWorkList();
+        if (!isVeryLargeFunction) {
+          if (auto *I = dyn_cast<SingleValueInstruction>(Val)) {
+            // Replacing operands may trigger constant folding which then could
+            // trigger other simplify-CFG optimizations.
+            ConstFolder.addToWorklist(I);
+            ConstFolder.processWorkList();
+          }
         }
       } else {
         // We must be processing an unreachable part of the cfg with a cycle.
@@ -1290,7 +1296,7 @@ bool SimplifyCFG::simplifyBranchBlock(BranchInst *BI) {
   // If this unconditional branch has BBArgs, check to see if duplicating the
   // destination would allow it to be simplified.  This is a simple form of jump
   // threading.
-  if (!BI->getArgs().empty() &&
+  if (!isVeryLargeFunction && !BI->getArgs().empty() &&
       tryJumpThreading(BI))
     return true;
 
@@ -3067,6 +3073,9 @@ bool SimplifyCFG::run() {
 
   LLVM_DEBUG(llvm::dbgs() << "### Run SimplifyCFG on " << Fn.getName() << '\n');
 
+  // Disable some expensive optimizations if the function is huge.
+  isVeryLargeFunction = (Fn.size() > 10000);
+
   // First remove any block not reachable from the entry.
   bool Changed = removeUnreachableBlocks(Fn);