Fix comparing comparisons

Sometimes the result of a comparison is not directly used in a jump,
but in arithmetic or further comparisons.  If those further things
do a vswap() with the VT_CMP as current top, and then generate
instructions for the new top, this most probably destroys the flags
(e.g. if it's a bitfield load like in the example).

vswap() must do the same like vsetc() and not allow VT_CMP vtops
to be moved down.

tccgen.c

@@ -452,6 +452,14 @@ ST_FUNC void vswap(void)
 {
     SValue tmp;
 
+    /* cannot let cpu flags if other instruction are generated. Also
+       avoid leaving VT_JMP anywhere except on the top of the stack
+       because it would complicate the code generator. */
+    if (vtop >= vstack) {
+        int v = vtop->r & VT_VALMASK;
+        if (v == VT_CMP || (v & ~1) == VT_JMP)
+            gv(RC_INT);
+    }
     tmp = vtop[0];
     vtop[0] = vtop[-1];
     vtop[-1] = tmp;

tests/tcctest.c

@@ -86,6 +86,7 @@ void asm_test(void);
 void builtin_test(void);
 void weak_test(void);
 void global_data_test(void);
+void cmp_comparison_test(void);
 
 int fib(int n);
 void num(int n);
@@ -592,6 +593,7 @@ int main(int argc, char **argv)
     builtin_test();
     weak_test();
     global_data_test();
+    cmp_comparison_test();
     return 0; 
 }
 
@@ -2562,3 +2564,31 @@ void global_data_test (void)
   global_data_callit (0);
   printf ("%d\n", global_data.a[0]);
 }
+
+struct cmpcmpS
+{
+  unsigned char fill : 3;
+  unsigned char b1 : 1;
+  unsigned char b2 : 1;
+  unsigned char fill2 : 3;
+};
+
+int glob1, glob2, glob3;
+
+void compare_comparisons (struct cmpcmpS *s)
+{
+  if (s->b1 != (glob1 == glob2)
+      || (s->b2 != (glob1 == glob3)))
+    printf ("comparing comparisons broken\n");
+}
+
+void cmp_comparison_test(void)
+{
+  struct cmpcmpS s;
+  s.b1 = 1;
+  glob1 = 42; glob2 = 42;
+  s.b2 = 0;
+  glob3 = 43;
+  compare_comparisons (&s);
+  return 0;
+}