Write execution logs to a named log file (#409)

This patch adds a --log argument to spike. If not given, the behaviour
is unchanged: messages logging execution of instructions and (if
commit logging is enabled) commits go to stderr.

If --log=P is given, Spike now writes these messages to a log file at
the path P. This is nice, because they are no longer tangled up with
other errors and warnings.

The code is mostly plumbing: passing a FILE* object through to the
functions that were using stderr. I've written a simple "log_file_t"
class, which opens a log file if necessary and yields it or stderr.

1 files changed, 37 insertions, 0 deletions

diff --git a/riscv/log_file.h b/riscv/log_file.h
new file mode 100644
index 0000000..d039859
--- /dev/null
+++ b/riscv/log_file.h
@@ -0,0 +1,37 @@
+// See LICENSE for license details.
+#ifndef _RISCV_LOGFILE_H
+#define _RISCV_LOGFILE_H
+
+#include <stdio.h>
+#include <memory>
+#include <sstream>
+#include <stdexcept>
+
+// Header-only class wrapping a log file. When constructed with an
+// actual path, it opens the named file for writing. When constructed
+// with the null path, it wraps stderr.
+class log_file_t
+{
+public:
+  log_file_t(const char *path)
+    : wrapped_file (nullptr, &fclose)
+  {
+    if (!path)
+      return;
+
+    wrapped_file.reset(fopen(path, "w"));
+    if (! wrapped_file) {
+      std::ostringstream oss;
+      oss << "Failed to open log file at `" << path << "': "
+          << strerror (errno);
+      throw std::runtime_error(oss.str());
+    }
+  }
+
+  FILE *get() { return wrapped_file ? wrapped_file.get() : stderr; }
+
+private:
+  std::unique_ptr<FILE, decltype(&fclose)> wrapped_file;
+};
+
+#endif