Reuse debug info cache between HBCISel calls

Summary:
When the sourceMapURL is huge (due to being a literal base64-encoded
URL-embedded sourcemap), it actually makes sense to reuse the debug
cache between functions.

In one measured case, this reduced runtime from 55s to 3s

Reviewed By: avp

Differential Revision: D23123584

fbshipit-source-id: 86eb770c4ba41c71db165a9a7f8fce39b4e74315

include/hermes/BCGen/HBC/ISel.h

@@ -21,6 +21,15 @@
 namespace hermes {
 namespace hbc {
 
+// Looking up filename/sourcemap id for each instruction is pretty slow,
+// and it's almost always from the same bufId every time. Cache the previous
+// result in this struct.
+struct HBCISelDebugCache {
+  unsigned currentBufId = -1;
+  unsigned currentFilenameId;
+  unsigned currentSourceMappingUrlId;
+};
+
 class HBCISel {
   struct Relocation {
     enum RelocationType {
@@ -159,14 +168,7 @@ class HBCISel {
   uint8_t acquirePropertyReadCacheIndex(unsigned id);
   uint8_t acquirePropertyWriteCacheIndex(unsigned id);
 
-  // Looking up filename/sourcemap id for each instruction is pretty slow,
-  // and it's almost always from the same bufId every time. Cache the previous
-  // result here, to reuse it when possible.
-  struct {
-    unsigned currentBufId;
-    unsigned currentFilenameId;
-    unsigned currentSourceMappingUrlId;
-  } debugIdCache_;
+  HBCISelDebugCache debugIdCache_;
 
  public:
   /// C'tor.
@@ -201,6 +203,16 @@ class HBCISel {
 
   /// Generate the bytecode stream for the function.
   void generate(SourceMapGenerator *outSourceMap);
+
+  /// Get the current debug cache, to allow pre-populating another HBCISel in
+  /// the same module.
+  HBCISelDebugCache getDebugCache() {
+    return debugIdCache_;
+  }
+  /// Populate the debug cache with data from a previous function.
+  void populateDebugCache(HBCISelDebugCache cache) {
+    debugIdCache_ = cache;
+  }
 };
 
 } // namespace hbc

lib/BCGen/HBC/HBC.cpp

@@ -244,6 +244,10 @@ std::unique_ptr<BytecodeModule> hbc::generateBytecodeModule(
 
   // Construct the relative function scope depth map.
   FunctionScopeAnalysis scopeAnalysis{lexicalTopLevel};
+
+  // Allow reusing the debug cache between functions
+  HBCISelDebugCache debugCache;
+
   // Bytecode generation for each function.
   for (auto &F : *M) {
     if (!shouldGenerate(&F)) {
@@ -295,7 +299,9 @@ std::unique_ptr<BytecodeModule> hbc::generateBytecodeModule(
       funcGen =
           BytecodeFunctionGenerator::create(BMGen, RA.getMaxRegisterUsage());
       HBCISel hbciSel(&F, funcGen.get(), RA, scopeAnalysis);
+      hbciSel.populateDebugCache(debugCache);
       hbciSel.generate(sourceMapGen);
+      debugCache = hbciSel.getDebugCache();
     }
 
     BMGen.setFunctionGenerator(&F, std::move(funcGen));

lib/BCGen/HBC/ISel.cpp

@@ -247,7 +247,6 @@ void HBCISel::addDebugSourceLocationInfo(SourceMapGenerator *outSourceMap) {
   IRBuilder builder(F_);
 
   DebugSourceLocation info{};
-  debugIdCache_.currentBufId = (unsigned)-1;
 
   bool hasDebugInfo = false;
   for (auto &reloc : relocations_) {