Add support for writing gzip-compressed files

Signed-off-by: David Shah <dave@ds0.me>

CHANGELOG

@@ -16,6 +16,7 @@ Yosys 0.9 .. Yosys 0.9-dev
     - "synth_xilinx" to now infer wide multiplexers (-widemux <min> to enable)
     - Added automatic gzip decompression for frontends
     - Added $_NMUX_ cell type
+    - Added automatic gzip compression (based on filename extension) for backends
 
 Yosys 0.8 .. Yosys 0.8-dev
 --------------------------

kernel/register.cc

@@ -41,6 +41,45 @@ void decompress_gzip(const std::string &filename, std::stringstream &out)
 	}
 	gzclose(gzf);
 }
+
+/*
+An output stream that uses a stringbuf to buffer data internally,
+using zlib to write gzip-compressed data every time the stream is flushed.
+*/
+class gzip_ostream : public std::ostream  {
+public:
+	gzip_ostream()
+	{
+		rdbuf(&outbuf);
+	}
+	bool open(const std::string &filename)
+	{
+		return outbuf.open(filename);
+	}
+private:
+	class gzip_streambuf : public std::stringbuf {
+	public:
+		gzip_streambuf() { };
+		bool open(const std::string &filename)
+		{
+			gzf = gzopen(filename.c_str(), "wb");
+			return gzf != nullptr;
+		}
+		virtual int sync() override
+		{
+			gzwrite(gzf, reinterpret_cast<const void *>(str().c_str()), unsigned(str().size()));
+			str("");
+			return 0;
+		}
+		~gzip_streambuf()
+		{
+			sync();
+			gzclose(gzf);
+		}
+	private:
+		gzFile gzf = nullptr;
+	} outbuf;
+};
 PRIVATE_NAMESPACE_END
 
 #endif
@@ -588,14 +627,28 @@ void Backend::extra_args(std::ostream *&f, std::string &filename, std::vector<st
 
 		filename = arg;
 		rewrite_filename(filename);
-		std::ofstream *ff = new std::ofstream;
-		ff->open(filename.c_str(), std::ofstream::trunc);
-		yosys_output_files.insert(filename);
-		if (ff->fail()) {
-			delete ff;
-			log_cmd_error("Can't open output file `%s' for writing: %s\n", filename.c_str(), strerror(errno));
+		if (filename.size() > 3 && filename.substr(filename.size()-3) == ".gz") {
+#ifdef YOSYS_ENABLE_ZLIB
+			gzip_ostream *gf = new gzip_ostream;
+			if (!gf->open(filename)) {
+				delete gf;
+				log_cmd_error("Can't open output file `%s' for writing: %s\n", filename.c_str(), strerror(errno));
+			}
+			yosys_output_files.insert(filename);
+			f = gf;
+#else
+			log_cmd_error("Yosys is compiled without zlib support, unable to write gzip output.\n");
+#endif
+		} else {
+			std::ofstream *ff = new std::ofstream;
+			ff->open(filename.c_str(), std::ofstream::trunc);
+			yosys_output_files.insert(filename);
+			if (ff->fail()) {
+				delete ff;
+				log_cmd_error("Can't open output file `%s' for writing: %s\n", filename.c_str(), strerror(errno));
+			}
+			f = ff;
 		}
-		f = ff;
 	}
 
 	if (called_with_fp)