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.

llvm-svn: 229421

1 files changed, 2 insertions, 0 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitcodeReader.h b/llvm/lib/Bitcode/Reader/BitcodeReader.h
index 7f7eb70bce9..5090be48450 100644
--- a/llvm/lib/Bitcode/Reader/BitcodeReader.h
+++ b/llvm/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;