[llvm-cov] Add a cl::opt to control the number of threads

When an output directory is specified, llvm-cov spawns some threads to
speed up the process of writing out file reports. Add an option which
allows users to control how many threads llvm-cov uses.

A CommandGuide.rst update + test is included.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307609 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/llvm-cov.rst

@@ -262,6 +262,12 @@ OPTIONS
  The demangler is expected to read a newline-separated list of symbols from
  stdin and write a newline-separated list of the same length to stdout.
 
+.. option:: -num-threads=N, -j=N
+
+ Use N threads to write file reports (only applicable when -output-dir is
+ specified). When N=0, llvm-cov auto-detects an appropriate number of threads to
+ use. This is the default.
+
 .. option:: -line-coverage-gt=<N>
 
  Show code coverage only for functions with line coverage greater than the

test/tools/llvm-cov/threads.c

@@ -0,0 +1,11 @@
+// Coverage/profile data recycled from the showLineExecutionCounts.cpp test.
+//
+// RUN: llvm-profdata merge %S/Inputs/lineExecutionCounts.proftext -o %t.profdata
+// RUN: llvm-cov show %S/Inputs/lineExecutionCounts.covmapping -j 1 -o %t1.dir -instr-profile %t.profdata -filename-equivalence %S/showLineExecutionCounts.cpp
+// RUN: llvm-cov show %S/Inputs/lineExecutionCounts.covmapping -num-threads 2 -o %t2.dir -instr-profile %t.profdata -filename-equivalence %S/showLineExecutionCounts.cpp
+// RUN: llvm-cov show %S/Inputs/lineExecutionCounts.covmapping -o %t3.dir -instr-profile %t.profdata -filename-equivalence %S/showLineExecutionCounts.cpp
+//
+// RUN: diff %t1.dir/index.txt %t2.dir/index.txt
+// RUN: diff %t1.dir/coverage/tmp/showLineExecutionCounts.cpp.txt %t2.dir/coverage/tmp/showLineExecutionCounts.cpp.txt
+// RUN: diff %t1.dir/index.txt %t3.dir/index.txt
+// RUN: diff %t1.dir/coverage/tmp/showLineExecutionCounts.cpp.txt %t3.dir/coverage/tmp/showLineExecutionCounts.cpp.txt

tools/llvm-cov/CodeCoverage.cpp

@@ -32,6 +32,7 @@
 #include "llvm/Support/Process.h"
 #include "llvm/Support/Program.h"
 #include "llvm/Support/ScopedPrinter.h"
+#include "llvm/Support/Threading.h"
 #include "llvm/Support/ThreadPool.h"
 #include "llvm/Support/ToolOutputFile.h"
 #include <functional>
@@ -705,6 +706,12 @@ int CodeCoverageTool::show(int argc, const char **argv,
       "project-title", cl::Optional,
       cl::desc("Set project title for the coverage report"));
 
+  cl::opt<unsigned> NumThreads(
+      "num-threads", cl::init(0),
+      cl::desc("Number of merge threads to use (default: autodetect)"));
+  cl::alias NumThreadsA("j", cl::desc("Alias for --num-threads"),
+                        cl::aliasopt(NumThreads));
+
   auto Err = commandLineParser(argc, argv);
   if (Err)
     return Err;
@@ -790,15 +797,19 @@ int CodeCoverageTool::show(int argc, const char **argv,
     }
   }
 
-  // FIXME: Sink the hardware_concurrency() == 1 check into ThreadPool.
-  if (!ViewOpts.hasOutputDirectory() ||
-      std::thread::hardware_concurrency() == 1) {
+  // If NumThreads is not specified, auto-detect a good default.
+  if (NumThreads == 0)
+    NumThreads =
+        std::max(1U, std::min(llvm::heavyweight_hardware_concurrency(),
+                              unsigned(SourceFiles.size())));
+
+  if (!ViewOpts.hasOutputDirectory() || NumThreads == 1) {
     for (const std::string &SourceFile : SourceFiles)
       writeSourceFileView(SourceFile, Coverage.get(), Printer.get(),
                           ShowFilenames);
   } else {
     // In -output-dir mode, it's safe to use multiple threads to print files.
-    ThreadPool Pool;
+    ThreadPool Pool(NumThreads);
     for (const std::string &SourceFile : SourceFiles)
       Pool.async(&CodeCoverageTool::writeSourceFileView, this, SourceFile,
                  Coverage.get(), Printer.get(), ShowFilenames);