Fix reading gcov data that does not have function names

[arseny.kapoulkine]

Summary

In order for recent gcov versions to read the coverage data, you have to use UseCfgChecksum=true and FunctionNamesInData=false options for coverage profiling pass. This is because gcov is expecting the function section in .gcda to be exactly 3 words in size, containing ident and two checksums.

While llvm-cov is compatible with UseCfgChecksum=true, it always expects a function name in .gcda function sections (it's not compatible with FunctionNamesInData=false). Thus it's currently impossible to generate one set of coverage files that works with both gcov and llvm-cov.

This change fixes the reading of coverage information to only read the function name if it's present.

[arseny.kapoulkine]

Updated the diff to use "have we read the entire chunk" as a condition so that we don't have to track the required length specifically. Also added a test (compiled with clang with options that make the output gcov-compatible).

[arseny.kapoulkine]

Updated the diff to use "have we read the entire chunk" as a condition so that we don't have to track the required length specifically. Also added a test (compiled with clang with options that make the output gcov-compatible).

[arseny.kapoulkine]

Ping.

lib/IR/GCOV.cpp

 /// readGCDA - Read a function from the GCDA buffer. Return false if an error
 /// occurs.
 bool GCOVFunction::readGCDA(GCOVBuffer &Buff, GCOV::GCOVVersion Version) {
-  uint32_t Dummy;
-  if (!Buff.readInt(Dummy))
+  uint32_t HeaderLength;
+  if (!Buff.readInt(HeaderLength))
     return false; // Function header length
 
+  uint32_t MinHeaderLength = 2;
+
+  if (Version != GCOV::V402) {
+    MinHeaderLength++; // CfgChecksum
+  }
+
+  if (HeaderLength < MinHeaderLength) {
+    errs() << "Function header is invalid: expected " << MinHeaderLength
+           << " words, got " << HeaderLength << " (in " << Name << ").\n";
+    return false;
+  }
+
   uint32_t GCDAIdent;
   if (!Buff.readInt(GCDAIdent))
     return false;

     }
   }
 
-  StringRef GCDAName;
-  if (!Buff.readString(GCDAName))
-    return false;
-  if (Name != GCDAName) {
-    errs() << "Function names do not match: " << Name << " != " << GCDAName
-           << ".\n";
-    return false;
+  if (MinHeaderLength < HeaderLength) {
+    StringRef GCDAName;
+    if (!Buff.readString(GCDAName))
+      return false;
+    if (Name != GCDAName) {
+      errs() << "Function names do not match: " << Name << " != " << GCDAName
+             << ".\n";
+      return false;
+    }
   }
 
   if (!Buff.readArcTag()) {