xz: Add support --threads=+N so that -T+1 gives threaded mode.

src/xz/args.c

@@ -258,11 +258,23 @@ parse_real(args_info *args, int argc, char **argv)
 			suffix_set(optarg);
 			break;
 
-		case 'T':
+		case 'T': {
+			// Since xz 5.4.0: Ignore leading '+' first.
+			const char *s = optarg;
+			if (optarg[0] == '+')
+				++s;
+
 			// The max is from src/liblzma/common/common.h.
-			hardware_threads_set(str_to_uint64("threads",
+			uint32_t t = str_to_uint64("threads", s, 0, 16384);
-					optarg, 0, 16384));
+
+			// If leading '+' was used then use multi-threaded
+			// mode even if exactly one thread was specified.
+			if (t == 1 && optarg[0] == '+')
+				t = UINT32_MAX;
+
+			hardware_threads_set(t);
 			break;
+		}
 
 		// --version
 		case 'V':

src/xz/hardware.c

@@ -21,6 +21,10 @@ static uint32_t threads_max = 1;
 /// on the available hardware threads.
 static bool threads_are_automatic = false;
 
+/// If true, then try to use multi-threaded mode (if memlimit allows)
+/// even if only one thread was requested explicitly (-T+1).
+static bool use_mt_mode_with_one_thread = false;
+
 /// Memory usage limit for compression
 static uint64_t memlimit_compress = 0;
 
@@ -57,9 +61,16 @@ static uint64_t total_ram;
 extern void
 hardware_threads_set(uint32_t n)
 {
+	// Reset these to false first and set them to true when appropriate.
+	threads_are_automatic = false;
+	use_mt_mode_with_one_thread = false;
+
 	if (n == 0) {
 		// Automatic number of threads was requested.
+		// If there is only one hardware thread, multi-threaded
+		// mode will still be used if memory limit allows.
 		threads_are_automatic = true;
+		use_mt_mode_with_one_thread = true;
 
 		// If threading support was enabled at build time,
 		// use the number of available CPU cores. Otherwise
@@ -72,9 +83,11 @@ hardware_threads_set(uint32_t n)
 #else
 		threads_max = 1;
 #endif
+	} else if (n == UINT32_MAX) {
+		use_mt_mode_with_one_thread = true;
+		threads_max = 1;
 	} else {
 		threads_max = n;
-		threads_are_automatic = false;
 	}
 
 	return;
@@ -92,7 +105,7 @@ extern bool
 hardware_threads_is_mt(void)
 {
 #ifdef MYTHREAD_ENABLED
-	return threads_max > 1 || threads_are_automatic;
+	return threads_max > 1 || use_mt_mode_with_one_thread;
 #else
 	return false;
 #endif

src/xz/hardware.h

@@ -16,6 +16,7 @@ extern void hardware_init(void);
 
 
 /// Set the maximum number of worker threads.
+/// A special value of UINT32_MAX sets one thread in multi-threaded mode.
 extern void hardware_threads_set(uint32_t threadlimit);
 
 /// Get the maximum number of worker threads.

src/xz/xz.1

@@ -5,7 +5,7 @@
 .\" This file has been put into the public domain.
 .\" You can do whatever you want with this file.
 .\"
-.TH XZ 1 "2022-11-09" "Tukaani" "XZ Utils"
+.TH XZ 1 "2022-11-19" "Tukaani" "XZ Utils"
 .
 .SH NAME
 xz, unxz, xzcat, lzma, unlzma, lzcat \- Compress or decompress .xz and .lzma files
@@ -1214,6 +1214,25 @@ even if the system supports only one hardware thread.
 5.2.x
 used single-threaded mode in this situation.)
 .IP ""
+To use multi-threaded mode with only one thread, set
+.I threads
+to
+.BR +1 .
+The
+.B +
+prefix has no effect with values other than
+.BR 1 .
+A memory usage limit can still make
+.B xz
+switch to single-threaded mode unless
+.B \-\-no\-adjust
+is used.
+Support for the
+.B +
+prefix was added in
+.B xz
+5.4.0.
+.IP ""
 If an automatic number of threads has been requested and
 no memory usage limit has been specified,
 then a system-specific default soft limit will be used to possibly