Bitcode: Fix major regression: large files w/ debug info

The metadata/value split introduced a major regression reading large
bitcode files that contain debug info (or other cyclic (non-self
reference) metadata graphs).  For the first time in a while, I dropped
from libLTO.dylib down to `llvm-lto` with a non-trivial bitcode file
(~350MB), and I hit this when reading the result of ld64's `-save-temps`
in `llvm-lto`.

Here's pseudo-code for what was going on:

    read-main-metadata-block:
      for each md:
        if has-fwd-ref: // Only true for cyclic graphs.
          any-fwd-refs <- true
      if any-fwd-refs:
        foreach md:
          resolve-cycles(md) // Handle cycles.

    foreach function:
      read-function-metadata-block: // Such as !alias, !loop
        if any-fwd-refs:
          foreach md: // (all metadata, not just this block)
            resolve-cycles(md) // A no-op, but the loop is expensive!!

This commit resets the `AnyFwdRefs` flag to `false`.  This on its own
was enough to change my Release+Asserts `llvm-lto` time for reading this
bitcode from over 20 minutes (I gave up on it) to 20 seconds.  I've gone
further by tracking the min/max metadata forward-references in a
metadata block.  This protects against a schema that has lots of
functions that each reference their own metadata cycle.

Unfortunately, this regression is in the 3.6 branch as well.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229421 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Bitcode/Reader/BitcodeReader.cpp

@@ -555,9 +555,17 @@ Metadata *BitcodeReaderMDValueList::getValueFwdRef(unsigned Idx) {
   if (Metadata *MD = MDValuePtrs[Idx])
     return MD;
 
-  // Create and return a placeholder, which will later be RAUW'd.
-  AnyFwdRefs = true;
+  // Track forward refs to be resolved later.
+  if (AnyFwdRefs) {
+    MinFwdRef = std::min(MinFwdRef, Idx);
+    MaxFwdRef = std::max(MaxFwdRef, Idx);
+  } else {
+    AnyFwdRefs = true;
+    MinFwdRef = MaxFwdRef = Idx;
+  }
   ++NumFwdRefs;
+
+  // Create and return a placeholder, which will later be RAUW'd.
   Metadata *MD = MDNode::getTemporary(Context, None).release();
   MDValuePtrs[Idx].reset(MD);
   return MD;
@@ -573,7 +581,8 @@ void BitcodeReaderMDValueList::tryToResolveCycles() {
     return;
 
   // Resolve any cycles.
-  for (auto &MD : MDValuePtrs) {
+  for (unsigned I = MinFwdRef, E = MaxFwdRef + 1; I != E; ++I) {
+    auto &MD = MDValuePtrs[I];
     auto *N = dyn_cast_or_null<MDNode>(MD);
     if (!N)
       continue;
@@ -581,6 +590,9 @@ void BitcodeReaderMDValueList::tryToResolveCycles() {
     assert(!N->isTemporary() && "Unexpected forward reference");
     N->resolveCycles();
   }
+
+  // Make sure we return early again until there's another forward ref.
+  AnyFwdRefs = false;
 }
 
 Type *BitcodeReader::getTypeByID(unsigned ID) {

lib/Bitcode/Reader/BitcodeReader.h

@@ -99,6 +99,8 @@ public:
 class BitcodeReaderMDValueList {
   unsigned NumFwdRefs;
   bool AnyFwdRefs;
+  unsigned MinFwdRef;
+  unsigned MaxFwdRef;
   std::vector<TrackingMDRef> MDValuePtrs;
 
   LLVMContext &Context;