Get subtractions the right way round.

mach/powerpc/mcg/table

@@ -609,7 +609,7 @@ PATTERNS
     ALUCC(ADD.I, "addi")
 
 	out:(int)reg = SUB.I(left:(int)reg, right:(int)reg)
-		emit "subf %out, %left, %right"
+		emit "subf %out, %right, %left"
 		cost 4;
 
 	out:(int)reg = SUB.I(left:(int)reg, right:CONST.I)

plat/qemuppc/tests/intsub.c

@@ -0,0 +1,31 @@
+#include "test.h"
+
+/* Constants in globals to defeat constant folding. */
+int two = 2;
+int one = 1;
+int zero = 0;
+int minusone = -1;
+
+/* Bypasses the CRT, so there's no stdio or BSS initialisation. */
+void _m_a_i_n(void)
+{
+    ASSERT((two - one) == 1);
+    ASSERT((one - two) == -1);
+
+    ASSERT((two - 1) == 1);
+    ASSERT((one - 2) == -1);
+
+    ASSERT((2   - one) == 1);
+    ASSERT((1   - two) == -1);
+
+    ASSERT(((unsigned int)two - (unsigned int)one) == 1);
+    ASSERT(((unsigned int)one - (unsigned int)two) == 0xffffffff);
+
+    ASSERT(((unsigned int)two - (unsigned int)1) == 1);
+    ASSERT(((unsigned int)one - (unsigned int)2) == 0xffffffff);
+
+    ASSERT(((unsigned int)2   - (unsigned int)one) == 1);
+    ASSERT(((unsigned int)1   - (unsigned int)two) == 0xffffffff);
+
+    finished();
+}