GNU bug report logs - #11082
24.0.94; u.glyphless member in struct glyph does not fit in 32 bits

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: YAMAMOTO Mitsuharu <mituharu <at> math.s.chiba-u.ac.jp>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.0.94; u.glyphless member in struct glyph does not fit in 32 bits
Date: Sat, 24 Mar 2012 14:23:28 +0900

In dispextern.h:

   316	struct glyph
   317	{
(snip)
   418	  /* A union of sub-structures for different glyph types.  */
   419	  union
   420	  {
(snip)
   447	    /* Sub-stretch for type == GLYPHLESS_GLYPH.  */
   448	    struct
   449	    {
   450	      /* Value is an enum of the type glyphless_display_method.  */
   451	      unsigned method : 2;
   452	      /* 1 iff this glyph is for a character of no font. */
   453	      unsigned for_no_font : 1;
   454	      /* Length of acronym or hexadecimal code string (at most 8).  */
   455	      unsigned len : 4;
   456	      /* Character to display.  Actually we need only 22 bits.  */
   457	      unsigned ch : 26;
   458	    } glyphless;
   459	
   460	    /* Used to compare all bit-fields above in one step.  */
   461	    unsigned val;
   462	  } u;
   463	};

The member `u.glyphless' above requires at least 33 bits and does not
fit in the size (32 bits) of `u.val' on many environments.  As a
result, equality with respect to the `u.val' member (e.g., used in
GLYPH_EQUAL_P) does not necessarily mean the equality of glyphless
glyphs.

According to the comment above, it seems to be OK to shorten the
length of `u.glyphless.ch' member from 26 to 25.  Could someone
confirm this?

				     YAMAMOTO Mitsuharu
				mituharu <at> math.s.chiba-u.ac.jp

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