[llvm-cov] Prevent llvm-cov from using too many threads

As reported here: https://reviews.llvm.org/D75153#1987272

Before, each instance of llvm-cov was creating one thread per hardware core, which wasn't needed probably because the number of inputs were small. This was probably causing a thread rlimit issue on large core count systems.

After this patch, the previous behavior is restored (to what was before rG8404aeb5):

If --num-threads is not specified, we create one thread per input, up to num.cores.
When specified, --num-threads indicates any number of threads, with no upper limit.

Differential Revision: https://reviews.llvm.org/D78408

1 files changed, 8 insertions, 5 deletions

diff --git a/llvm/tools/llvm-cov/CoverageReport.cpp b/llvm/tools/llvm-cov/CoverageReport.cpp
index 187e2dc..8509710 100644
--- a/llvm/tools/llvm-cov/CoverageReport.cpp
+++ b/llvm/tools/llvm-cov/CoverageReport.cpp
@@ -352,12 +352,15 @@ std::vector<FileCoverageSummary> CoverageReport::prepareFileReports(
     ArrayRef<std::string> Files, const CoverageViewOptions &Options,
     const CoverageFilter &Filters) {
   unsigned LCP = getRedundantPrefixLen(Files);
-  auto NumThreads = Options.NumThreads;
 
-  // If NumThreads is not specified, auto-detect a good default.
-  if (NumThreads == 0)
-    NumThreads = Files.size();
-  ThreadPool Pool(heavyweight_hardware_concurrency(NumThreads));
+  ThreadPoolStrategy S = hardware_concurrency(Options.NumThreads);
+  if (Options.NumThreads == 0) {
+    // If NumThreads is not specified, create one thread for each input, up to
+    // the number of hardware cores.
+    S = heavyweight_hardware_concurrency(Files.size());
+    S.Limit = true;
+  }
+  ThreadPool Pool(S);
 
   std::vector<FileCoverageSummary> FileReports;
   FileReports.reserve(Files.size());