Re: master 37889523278: Add new `swap` macro and use it

[Stefan Kangas <stefankangas@gmail.com>]

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Po Lu <luangruo@yahoo.com> writes:

> Stefan Kangas <stefankangas@gmail.com> writes:
>
>> branch: master
>> commit 37889523278fe65733938fb11c3701898309961c
>> Author: Stefan Kangas <stefankangas@gmail.com>
>> Commit: Stefan Kangas <stefankangas@gmail.com>
>>
>>     Add new `swap` macro and use it
>>
>>     A `swap` macro prevents programming errors and is more concise.
>>     It is a natural addition to our existing `min` and `max` macros.
>
> It's well known that a generic "swap" macro is impossible to express in
> Standard C for lack of a standard "typeof"-like expression.  To
> compensate for this limitation, the macro must accept an argument
> identifying the type of the field where the temporary value is saved,
> which is more trouble than it's worth and reintroduces the type
> conversion problems such macros are intended to prevent.
>
> Neither typeof nor __typeof__ are portable by our standards, the latter
> being an extension compatible with the Standard enabled when GCC is
> configured to obey the standard to the letter, while the former is a
> keyword enabled in other situations.

Ouch, right.  I was inspired by some other large project, let's leave it
unnamed, but their C chops were big enough that I let my guard down.
Unfortunately for me, they happen to support far fewer compilers than we
do.  Sorry about that, and thanks for being on your toes.

I had a look at what Gnulib has, and came up with the attached patch.
It still lets us avoid the type conversion problems on GCC, clang and
reasonably recent versions of IBM C and sunc.  I'm not sure it's worth
it though, so perhaps my original patch should just be reverted.

I copied in Paul just in case, and I'm hoping Eli will chime in too.

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diff --git a/lwlib/xlwmenu.c b/lwlib/xlwmenu.c
index a3d9474bed0..d6fa23841a3 100644
--- a/lwlib/xlwmenu.c
+++ b/lwlib/xlwmenu.c
@@ -671,7 +671,7 @@ draw_shadow_rectangle (XlwMenuWidget mw, Window window, int x, int y,
     }
 
   if (!erase_p && down_p)
-    swap (top_gc, bottom_gc);
+    swap (GC, top_gc, bottom_gc);
 
   /* Do draw (or erase) shadows */
   points [0].x = x;
@@ -752,7 +752,7 @@ draw_shadow_rhombus (XlwMenuWidget mw, Window window, int x, int y,
     }
 
   if (!erase_p && down_p)
-    swap (top_gc, bottom_gc);
+    swap (GC, top_gc, bottom_gc);
 
   points [0].x = x;
   points [0].y = y + height / 2;
diff --git a/src/androidterm.c b/src/androidterm.c
index 34734e63c37..851a066316a 100644
--- a/src/androidterm.c
+++ b/src/androidterm.c
@@ -5869,7 +5869,7 @@ android_get_surrounding_text (void *data)
      bad input methods.  */
 
   if (request->end < request->start)
-    swap (request->start, request->end);
+    swap (ptrdiff_t, request->start, request->end);
 
   /* Retrieve the conversion region.  */
 
diff --git a/src/buffer.c b/src/buffer.c
index 14c67224551..c03170e8c2d 100644
--- a/src/buffer.c
+++ b/src/buffer.c
@@ -3591,7 +3591,7 @@ DEFUN ("make-overlay", Fmake_overlay, Smake_overlay, 2, 5, 0,
   CHECK_FIXNUM_COERCE_MARKER (end);
 
   if (XFIXNUM (beg) > XFIXNUM (end))
-    swap (beg, end);
+    swap (Lisp_Object, beg, end);
 
   ptrdiff_t obeg = clip_to_bounds (BUF_BEG (b), XFIXNUM (beg), BUF_Z (b));
   ptrdiff_t oend = clip_to_bounds (obeg, XFIXNUM (end), BUF_Z (b));
@@ -3611,7 +3611,7 @@ DEFUN ("make-overlay", Fmake_overlay, Smake_overlay, 2, 5, 0,
 modify_overlay (struct buffer *buf, ptrdiff_t start, ptrdiff_t end)
 {
   if (start > end)
-    swap (start, end);
+    swap (ptrdiff_t, start, end);
 
   BUF_COMPUTE_UNCHANGED (buf, start, end);
 
@@ -3651,7 +3651,7 @@ DEFUN ("move-overlay", Fmove_overlay, Smove_overlay, 3, 4, 0,
   CHECK_FIXNUM_COERCE_MARKER (end);
 
   if (XFIXNUM (beg) > XFIXNUM (end))
-    swap (beg, end);
+    swap (Lisp_Object, beg, end);
 
   specbind (Qinhibit_quit, Qt); /* FIXME: Why?  */
 
diff --git a/src/dispnew.c b/src/dispnew.c
index 78ec3537a35..a3e6ca416db 100644
--- a/src/dispnew.c
+++ b/src/dispnew.c
@@ -649,7 +649,7 @@ reverse_rows (struct glyph_matrix *matrix, int start, int end)
   int i, j;
 
   for (i = start, j = end - 1; i < j; ++i, --j)
-    swap (matrix->rows[i], matrix->rows[j]);
+    swap (struct glyph_row, matrix->rows[i], matrix->rows[j]);
 }
 
 
@@ -975,7 +975,7 @@ swap_glyphs_in_rows (struct glyph_row *a, struct glyph_row *b)
 
       while (glyph_a < glyph_a_end)
 	{
-	  swap (*glyph_a, *glyph_b);
+	  swap (struct glyph, *glyph_a, *glyph_b);
 	  ++glyph_a;
 	  ++glyph_b;
 	}
diff --git a/src/editfns.c b/src/editfns.c
index 2e455a2efed..f31d8f5aef3 100644
--- a/src/editfns.c
+++ b/src/editfns.c
@@ -1753,7 +1753,7 @@ DEFUN ("insert-buffer-substring", Finsert_buffer_substring, Sinsert_buffer_subst
   b = !NILP (start) ? fix_position (start) : BUF_BEGV (bp);
   e = !NILP (end) ? fix_position (end) : BUF_ZV (bp);
   if (b > e)
-    swap (b, e);
+    swap (EMACS_INT, b, e);
 
   if (!(BUF_BEGV (bp) <= b && e <= BUF_ZV (bp)))
     args_out_of_range (start, end);
@@ -1808,7 +1808,7 @@ DEFUN ("compare-buffer-substrings", Fcompare_buffer_substrings, Scompare_buffer_
   begp1 = !NILP (start1) ? fix_position (start1) : BUF_BEGV (bp1);
   endp1 = !NILP (end1) ? fix_position (end1) : BUF_ZV (bp1);
   if (begp1 > endp1)
-    swap (begp1, endp1);
+    swap (EMACS_INT, begp1, endp1);
 
   if (!(BUF_BEGV (bp1) <= begp1
 	&& begp1 <= endp1
@@ -1833,7 +1833,7 @@ DEFUN ("compare-buffer-substrings", Fcompare_buffer_substrings, Scompare_buffer_
   begp2 = !NILP (start2) ? fix_position (start2) : BUF_BEGV (bp2);
   endp2 = !NILP (end2) ? fix_position (end2) : BUF_ZV (bp2);
   if (begp2 > endp2)
-    swap (begp2, endp2);
+    swap (EMACS_INT, begp2, endp2);
 
   if (!(BUF_BEGV (bp2) <= begp2
 	&& begp2 <= endp2
diff --git a/src/eval.c b/src/eval.c
index 6a866d6cc32..7d89c0b621e 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2794,9 +2794,9 @@ DEFUN ("run-hook-with-args-until-failure", Frun_hook_with_args_until_failure,
 static Lisp_Object
 run_hook_wrapped_funcall (ptrdiff_t nargs, Lisp_Object *args)
 {
-  swap (args[0], args[1]);
+  swap (Lisp_Object, args[0], args[1]);
   Lisp_Object ret = Ffuncall (nargs, args);
-  swap (args[1], args[0]);
+  swap (Lisp_Object, args[1], args[0]);
   return ret;
 }
 
diff --git a/src/fns.c b/src/fns.c
index c8adc5cb891..9e7f43b58e1 100644
--- a/src/fns.c
+++ b/src/fns.c
@@ -5674,7 +5674,7 @@ extract_data_from_object (Lisp_Object spec,
       b = !NILP (start) ? fix_position (start) : BEGV;
       e = !NILP (end) ? fix_position (end) : ZV;
       if (b > e)
-	swap (b, e);
+	swap (EMACS_INT, b, e);
 
       if (!(BEGV <= b && e <= ZV))
 	args_out_of_range (start, end);
diff --git a/src/lisp.h b/src/lisp.h
index f96932ab0c1..a9782868157 100644
--- a/src/lisp.h
+++ b/src/lisp.h
@@ -68,8 +68,13 @@ #define max(a, b) ((a) > (b) ? (a) : (b))
 #define min(a, b) ((a) < (b) ? (a) : (b))
 
 /* Swap values of a and b.  */
-#define swap(a, b) \
-  do { typeof (a) __tmp; __tmp = (a); (a) = (b); (b) = __tmp; } while (0);
+#if _GL_HAVE___TYPEOF__
+#define swap(type, a, b)							\
+  do { __typeof__ (a) __tmp; __tmp = (a); (a) = (b); (b) = __tmp; } while (0);
+#else
+#define swap(type, a, b)							\
+  do { type __tmp; __tmp = (a); (a) = (b); (b) = __tmp; } while (0);
+#endif
 
 /* Number of elements in an array.  */
 #define ARRAYELTS(arr) (sizeof (arr) / sizeof (arr)[0])
diff --git a/src/regex-emacs.c b/src/regex-emacs.c
index fdc2cc63445..afaf0d06bd1 100644
--- a/src/regex-emacs.c
+++ b/src/regex-emacs.c
@@ -2930,7 +2930,7 @@ forall_firstchar_1 (re_char *p, re_char *pend,
 	    /* We have to check that both destinations are safe.
 	       Arrange for `newp1` to be the smaller of the two.  */
 	    if (newp1 > newp2)
-	      swap (newp1, newp2);
+	      swap (re_char *, newp1, newp2);
 
 	    if (newp2 <= p_orig) /* Both destinations go backward!  */
 	      {
diff --git a/src/textconv.c b/src/textconv.c
index e0707522d7e..c8599f78a54 100644
--- a/src/textconv.c
+++ b/src/textconv.c
@@ -383,8 +383,8 @@ textconv_query (struct frame *f, struct textconv_callback_struct *query,
   if (end < pos)
     {
       eassert (end_byte < pos_byte);
-      swap (pos_byte, end_byte);
-      swap (pos, end);
+      swap (ptrdiff_t, pos_byte, end_byte);
+      swap (ptrdiff_t, pos, end);
     }
 
   /* Return the string first.  */
@@ -1903,7 +1903,7 @@ get_extracted_text (struct frame *f, ptrdiff_t n,
 
 	  /* Sort start and end.  */
 	  if (start > end)
-	    swap (start, end);
+	    swap (ptrdiff_t, start, end);
 	}
       else
 	goto finish;
@@ -2002,7 +2002,7 @@ get_surrounding_text (struct frame *f, ptrdiff_t left,
   /* Now sort start and end.  */
 
   if (end < start)
-    swap (start, end)
+    swap (ptrdiff_t, start, end)
 
   /* And subtract left and right.  */
 
diff --git a/src/textprop.c b/src/textprop.c
index ec9435219ea..118a5831c10 100644
--- a/src/textprop.c
+++ b/src/textprop.c
@@ -142,7 +142,7 @@ validate_interval_range (Lisp_Object object, Lisp_Object *begin,
     return NULL;
 
   if (XFIXNUM (*begin) > XFIXNUM (*end))
-    swap (*begin, *end);
+    swap (Lisp_Object, *begin, *end);
 
   if (BUFFERP (object))
     {
@@ -2196,7 +2196,7 @@ verify_interval_modification (struct buffer *buf,
     return;
 
   if (start > end)
-    swap (start, end);
+    swap (ptrdiff_t, start, end);
 
   /* For an insert operation, check the two chars around the position.  */
   if (start == end)
diff --git a/src/w32uniscribe.c b/src/w32uniscribe.c
index c417159cf9e..798a90bf594 100644
--- a/src/w32uniscribe.c
+++ b/src/w32uniscribe.c
@@ -768,7 +768,7 @@ #define OTF_INT16_VAL(TABLE, OFFSET, PTR)		     \
     BYTE data[2];					     \
     if (GetFontData (context, TABLE, OFFSET, data, 2) != 2)  \
       goto font_table_error;				     \
-    swap (data[0], data[1]);				     \
+    swap (BYTE, data[0], data[1]);			     \
     memcpy (PTR, data, 2);				     \
   } while (0)
 
diff --git a/src/xfaces.c b/src/xfaces.c
index f79eb022e15..95ae3b4b1bc 100644
--- a/src/xfaces.c
+++ b/src/xfaces.c
@@ -1357,7 +1357,7 @@ load_face_colors (struct frame *f, struct face *face,
 
   /* Swap colors if face is inverse-video.  */
   if (EQ (attrs[LFACE_INVERSE_INDEX], Qt))
-    swap (fg, bg);
+    swap (Lisp_Object, fg, bg);
 
   /* Check for support for foreground, not for background because
      face_color_supported_p is smart enough to know that grays are
diff --git a/src/xterm.c b/src/xterm.c
index 0b83b0554b3..92e51577cec 100644
--- a/src/xterm.c
+++ b/src/xterm.c
@@ -1571,8 +1571,8 @@ #define SWAPCARD32(l)				\
     struct { unsigned t : 32; } bit32;		\
     char *tp = (char *) &bit32;			\
     bit32.t = l;				\
-    swap (tp[0], tp[3]);			\
-    swap (tp[1], tp[2]);			\
+    swap (char, tp[0], tp[3]);			\
+    swap (char, tp[1], tp[2]);			\
     l = bit32.t;				\
   }
 
@@ -1581,7 +1581,7 @@ #define SWAPCARD16(s)				\
     struct { unsigned t : 16; } bit16;		\
     char *tp = (char *) &bit16;			\
     bit16.t = s;				\
-    swap (tp[0], tp[1]);			\
+    swap (char, tp[0], tp[1]);			\
     s = bit16.t;				\
   }