Micro-optimization: don't shift an entire bitcode record over to get the code.

Previously, BitstreamCursor read an abbreviated record by splatting the
whole thing into a data vector, then extracting and removing the /first/
element. Now, it reads the first element--the record code--separately from
the actual field values.

No (intended) functionality change.

llvm-svn: 181639

1 files changed, 10 insertions, 3 deletions

diff --git a/llvm/lib/Bitcode/Reader/BitstreamReader.cpp b/llvm/lib/Bitcode/Reader/BitstreamReader.cpp
index 9dafe2a0367..1fd9abd8b18 100644
--- a/llvm/lib/Bitcode/Reader/BitstreamReader.cpp
+++ b/llvm/lib/Bitcode/Reader/BitstreamReader.cpp
@@ -204,7 +204,16 @@ unsigned BitstreamCursor::readRecord(unsigned AbbrevID,
 
   const BitCodeAbbrev *Abbv = getAbbrev(AbbrevID);
 
-  for (unsigned i = 0, e = Abbv->getNumOperandInfos(); i != e; ++i) {
+  // Read the record code first.
+  assert(Abbv->getNumOperandInfos() != 0 && "no record code in abbreviation?");
+  const BitCodeAbbrevOp &CodeOp = Abbv->getOperandInfo(0);
+  if (CodeOp.isLiteral())
+    readAbbreviatedLiteral(CodeOp, Vals);
+  else
+    readAbbreviatedField(CodeOp, Vals);
+  unsigned Code = (unsigned)Vals.pop_back_val();
+
+  for (unsigned i = 1, e = Abbv->getNumOperandInfos(); i != e; ++i) {
     const BitCodeAbbrevOp &Op = Abbv->getOperandInfo(i);
     if (Op.isLiteral()) {
       readAbbreviatedLiteral(Op, Vals);
@@ -264,8 +273,6 @@ unsigned BitstreamCursor::readRecord(unsigned AbbrevID,
     JumpToBit(NewEnd);
   }
 
-  unsigned Code = (unsigned)Vals[0];
-  Vals.erase(Vals.begin());
   return Code;
 }