Update CFGWalker to generate consolidated exit blocks (#6079)

Previously CFGWalker designated a particular block as the "exit" block, but it
was just the block that happened to appear at the end of the function that
returned values by implicitly flowing them out. That exit block was not tied in
any way to other blocks that might end in returns, so analyses that needed to
perform some action at the end of the function would have had to perform that
action at the end of the designated exit block but also separately at any return
instruction.

Update CFGWalker to make the exit block a synthetic empty block that is a
successor of all other blocks tthat implicitly or explicitly return from the
function in case there are multiple such blocks, or to make the exit block the
single returning block if there is only one. This means that analyses will only
perform their end-of-function actions at the end of the exit block rather than
additionally at every return instruction.

2 files changed, 19 insertions, 0 deletions

diff --git a/src/analysis/cfg.cpp b/src/analysis/cfg.cpp
index ba638ec5a..5d939f7a6 100644
--- a/src/analysis/cfg.cpp
+++ b/src/analysis/cfg.cpp
@@ -62,6 +62,12 @@ CFG CFG::fromFunction(Function* func) {
     }
   }
 
+  assert(!cfg.blocks.empty());
+  cfg.blocks[0].entry = true;
+  if (builder.exit) {
+    oldToNewBlocks.at(builder.exit)->exit = true;
+  }
+
   // Move-construct a new CFG to get mandatory copy elision, preserving basic
   // block addresses through the return.
   return CFG(std::move(cfg));
@@ -96,6 +102,14 @@ void BasicBlock::print(std::ostream& os, Module* wasm, size_t start) const {
   }
   os << "]\n";
 
+  if (isEntry()) {
+    os << ";; entry\n";
+  }
+
+  if (isExit()) {
+    os << ";; exit\n";
+  }
+
   os << index << ":\n";
   size_t instIndex = start;
   for (auto* inst : *this) {
diff --git a/src/analysis/cfg.h b/src/analysis/cfg.h
index baabe9591..b05f3c47c 100644
--- a/src/analysis/cfg.h
+++ b/src/analysis/cfg.h
@@ -51,8 +51,13 @@ struct BasicBlock {
 
   Index getIndex() const { return index; }
 
+  bool isEntry() const { return entry; }
+  bool isExit() const { return exit; }
+
 private:
   Index index;
+  bool entry = false;
+  bool exit = false;
   std::vector<Expression*> insts;
   std::vector<const BasicBlock*> predecessors;
   std::vector<const BasicBlock*> successors;