Fix for bug #43634: keep sign of rA for ADD/SUB yielding 0

4 files changed, 27 insertions, 15 deletions

diff --git a/mixlib/mix_types.c b/mixlib/mix_types.c
index 4dac3ba..a29f381 100644
--- a/mixlib/mix_types.c
+++ b/mixlib/mix_types.c
@@ -1,7 +1,7 @@
 /* -*-c-*- ------------------ mix_types.c :
  *  Implementation file for mix_types.h declarations.
  * ------------------------------------------------------------------
- * Copyright (C) 2000, 2001, 2002, 2004, 2006, 2007 Free Software Foundation, Inc.
+ * Copyright (C) 2000, 2001, 2002, 2004, 2006, 2007, 2019 Free Software Foundation, Inc.
  *
  * This program is free software; you can redistribute it and/or modify
  * it under the terms of the GNU General Public License as published by
@@ -202,18 +202,17 @@ mix_word_add (mix_word_t x, mix_word_t y)
 	/* for instance MIX_WORD_MAX + 1 = - MIX_WORD_MAX */
 	result = MIX_WORD_MAX_SIM - result;
 	result |= mix_word_sign (mix_word_negative (x));
-      } else if ( result != 0 )
+      } else {
 	result |= mix_word_sign (x);
-      /* keep positive sign for 0 so that w - w == -w + w */
+      }
     }
   else
     {
       result = mix_word_magnitude (x) -  mix_word_magnitude (y);
       if (result < 0)
 	result = -result|mix_word_sign (y);
-      else if (result > 0)
+      else
 	result = result|mix_word_sign (x);
-      /* keep positive sign for 0 so that w - w == -w + w */
     }
 
   g_assert ( result >= 0 );
@@ -266,10 +265,8 @@ mix_word_add_and_carry (mix_word_t x, mix_word_t y,
 	  gint32 dif = mix_word_magnitude (x) -  mix_word_magnitude (y);
 	  if ( dif < 0)
 	    *l = (-dif)|mix_word_sign (y);
-	  else if ( dif > 0)
+	  else
 	    *l = dif|mix_word_sign (x);
-	  else /* keep positive sign for 0 so that w - w == -w + w */
-	    *l = MIX_WORD_ZERO;
 	}
     }
 
@@ -595,7 +592,3 @@ mix_short_print_to_buffer (mix_short_t s, gchar *buf)
 	   mix_byte_new (s>>6), mix_byte_new (s));
   /* g_print ("(%04d)", mix_short_magnitude (s));*/
 }
-
-
-
-
diff --git a/mixlib/testsuite/mix_vm_ins_t.c b/mixlib/testsuite/mix_vm_ins_t.c
index 1d368a7..239b737 100644
--- a/mixlib/testsuite/mix_vm_ins_t.c
+++ b/mixlib/testsuite/mix_vm_ins_t.c
@@ -135,6 +135,18 @@ test_arithmetics_(mix_vm_t *vm, mix_dump_context_t *dc)
   test.rA_a = mix_word_new_b(20,54,6,3,8);
   run_test_(&test, vm, dc);
 
+  mix_vm_set_rA(vm, mix_word_new_bn(0,0,0,0,1));
+  mix_vm_set_addr_contents(vm, 1000, mix_word_new_b(0,0,0,0,1));
+  fill_test_desc_(&test,vm,&ins);
+  test.rA_a = MIX_WORD_MINUS_ZERO;
+  run_test_(&test, vm, dc);
+
+  mix_vm_set_rA(vm, mix_word_new_b(0,0,0,0,1));
+  mix_vm_set_addr_contents(vm, 1000, mix_word_new_bn(0,0,0,0,1));
+  fill_test_desc_(&test,vm,&ins);
+  test.rA_a = MIX_WORD_ZERO;
+  run_test_(&test, vm, dc);
+
   mix_ins_fill_from_id(ins,mix_SUB);
   mix_vm_set_rA(vm,mix_word_new_bn(19,18,0,0,9));
   mix_vm_set_addr_contents(vm,1000,mix_word_new_bn(31,16,2,22,0));
@@ -519,4 +531,3 @@ main(int argc, const char **argv)
 
   return EXIT_SUCCESS;
 }
-
diff --git a/samples/Makefile.am b/samples/Makefile.am
index 357b5f3..c1dcfa6 100644
--- a/samples/Makefile.am
+++ b/samples/Makefile.am
@@ -1,6 +1,6 @@
 ## Process this file with automake to produce Makefile.in
 
-# Copyright (C) 2000, 2001, 2004, 2006, 2013 Free Software Foundation, Inc.
+# Copyright (C) 2000, 2001, 2004, 2006, 2013, 2019 Free Software Foundation, Inc.
 #
 # This file is free software; as a special exception the author gives
 # unlimited permission to copy and/or distribute it, with or without
@@ -14,4 +14,4 @@ SUBDIRS = tests
 
 EXTRA_DIST = primes.result hello.mixal primes.mixal echo.mixal \
              permutations.mixal permutations.cardrd isains.mixal \
-             elevator.mixal mystery.mixal
+             elevator.mixal mystery.mixal minus_zero.mixal
diff --git a/samples/minus_zero.mixal b/samples/minus_zero.mixal
new file mode 100644
index 0000000..9adb914
--- /dev/null
+++ b/samples/minus_zero.mixal
@@ -0,0 +1,8 @@
+                ORIG    3000
+START           ENTA    1
+                STA     1000
+                LDA     0
+                DECA    1
+                ADD     1000
+                HLT
+                END 3000