[sancov] Ignore PC samples with value 0

[mcgrathr]

Summary

The sancov runtime for the (Fuchsia) Zircon kernel delivers results
in the standard format, but as the full array of possible samples
with 0 in uncovered slots. That runtime delivers "live" data and
has no final "export" pass to compactify out the uncovered slots,
and it seems silly to require another offline tool just for that.

[mcgrathr]

https://fuchsia-review.googlesource.com/c/fuchsia/+/295508 has the runtime implementation that motivated this.

[MaskRay]

llvm/tools/sancov/sancov.cpp

261–265

The erase(0) logic should probably be moved here.

[mcgrathr]

Move logic out of RawCoverage constructor.

[mcgrathr]

PTAL

[phosek]

LGTM

llvm/tools/sancov/sancov.cpp

RawCoverage::read(const std::string &FileName) {
   case Bitness32:
     readInts<uint32_t>(Buf->getBufferStart() + 8, Buf->getBufferEnd(),
                        Addrs.get());
     break;
   default:
     errs() << "Unsupported bitness: " << Header->Bitness << '\n';
     return make_error_code(errc::illegal_byte_sequence);
   }
 
+  // Ignore slots that are zero, so a runtime implementation is not required
+  // to compactify the data.
+  Addrs->erase(0);
+
   return std::unique_ptr<RawCoverage>(new RawCoverage(std::move(Addrs)));
 }
 
 // Print coverage addresses.
 raw_ostream &operator<<(raw_ostream &OS, const RawCoverage &CoverageData) {
   for (auto Addr : *CoverageData.Addrs) {
     OS << "0x";
     OS.write_hex(Addr);
int main(int Argc, char **Argv) {
   sys::PrintStackTraceOnErrorSignal(Argv[0]);
   PrettyStackTraceProgram X(Argc, Argv);
   llvm_shutdown_obj Y; // Call llvm_shutdown() on exit.
 
   llvm::InitializeAllTargetInfos();
   llvm::InitializeAllTargetMCs();
   llvm::InitializeAllDisassemblers();
 
-  cl::ParseCommandLineOptions(Argc, Argv, 
+  cl::ParseCommandLineOptions(Argc, Argv,
       "Sanitizer Coverage Processing Tool (sancov)\n\n"
       "  This tool can extract various coverage-related information from: \n"
       "  coverage-instrumented binary files, raw .sancov files and their "
       "symbolized .symcov version.\n"
       "  Depending on chosen action the tool expects different input files:\n"
       "    -print-coverage-pcs     - coverage-instrumented binary files\n"
       "    -print-coverage         - .sancov files\n"
       "    <other actions>         - .sancov files & corresponding binary "