[llvm] r181639 - Micro-optimization: don't shift an entire bitcode record over to get the code.

[Jordan Rose]

Author: jrose
Date: Fri May 10 17:17:10 2013
New Revision: 181639

URL: http://llvm.org/viewvc/llvm-project?rev=181639&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp

Modified: llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp?rev=181639&r1=181638&r2=181639&view=diff
==============================================================================
--- llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp (original)
+++ llvm/trunk/lib/Bitcode/Reader/BitstreamReader.cpp Fri May 10 17:17:10 2013
@@ -204,7 +204,16 @@ unsigned BitstreamCursor::readRecord(uns
 
   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(uns
     JumpToBit(NewEnd);
   }
 
-  unsigned Code = (unsigned)Vals[0];
-  Vals.erase(Vals.begin());
   return Code;
 }