Subject: bug#39857: 27.0.90; Compilation warning about alloca in 32-bit MinGW build

From: Eli Zaretskii <[email-masked]>
Date: Sun, 01 Mar 2020 19:20:31 +0200

Building a 32-bit Emacs 27.0.90 with MinGW GCC 8.2 and -Og
optimization produces the following compilation warning:

    CC       edittfns.o
  In file included from lisp.h:24,
		   from editfns.c:36:
  editfns.c: In function 'styled_format':
  ../lib/alloca.h:46:18: warning: argument 1 range [2147483648, 4294961362] exceeds maximum object size 2147483647 [-Walloc-size-larger-than=]
   #  define alloca __builtin_alloca
  lisp.h:4810:49: note: in expansion of macro 'alloca'
   #define AVAIL_ALLOCA(size) (sa_avail -= (size), alloca (size))
						   ^~~~~~
  lisp.h:4815:9: note: in expansion of macro 'AVAIL_ALLOCA'
	 ? AVAIL_ALLOCA (size)   \
	   ^~~~~~~~~~~~
  editfns.c:3167:10: note: in expansion of macro 'SAFE_ALLOCA'
     info = SAFE_ALLOCA (alloca_size);
	    ^~~~~~~~~~~
  ../lib/alloca.h:46:18: note: in a call to built-in allocation function '__builtin_alloca'
   #  define alloca __builtin_alloca
  lisp.h:4810:49: note: in expansion of macro 'alloca'
   #define AVAIL_ALLOCA(size) (sa_avail -= (size), alloca (size))
						   ^~~~~~
  lisp.h:4815:9: note: in expansion of macro 'AVAIL_ALLOCA'
	 ? AVAIL_ALLOCA (size)   \
	   ^~~~~~~~~~~~
  editfns.c:3167:10: note: in expansion of macro 'SAFE_ALLOCA'
     info = SAFE_ALLOCA (alloca_size);
	    ^~~~~~~~~~~

The GCC manual says about -Walloc-size-larger-than=N:

  '-Walloc-size-larger-than=N'
       Warn about calls to functions decorated with attribute 'alloc_size'
       that attempt to allocate objects larger than the specified number
       of bytes, or where the result of the size computation in an integer
       type with infinite precision would exceed 'SIZE_MAX / 2'.  The
       option argument N may end in one of the standard suffixes
       designating a multiple of bytes such as 'kB' and 'KiB' for kilobyte
       and kibibyte, respectively, 'MB' and 'MiB' for megabyte and
       mebibyte, and so on.  '-Walloc-size-larger-than='PTRDIFF_MAX is
       enabled by default.  Warnings controlled by the option can be
       disabled by specifying N of SIZE_MAX or more.  *Note Function
       Attributes::.

I guess __builtin_alloca is "decorated with alloc_size" attribute, and
thus the warning.  Of course large memory allocations never end up in
alloca, we call malloc instead.  Which probably means GCC is not smart
enough to understand SAFE_ALLOCA?

WIBNI this annoying warning could be shut up?

TIA.

In GNU Emacs 27.0.90 (build 1, i686-pc-mingw32)
 of 2020-03-01 built on HOME-C4E4A596F7
Windowing system distributor 'Microsoft Corp.', version 5.1.2600
System Description: Microsoft Windows XP Service Pack 3 (v5.1.0.2600)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure -C --prefix=/d/usr --with-wide-int --with-modules
 'CFLAGS=-Og -gdwarf-4 -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL GNUTLS LIBXML2
HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1255

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win w32-vars
term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer cl-generic cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms
cp51932 hebrew greek romanian slovak czech european ethiopic indian
cyrillic chinese composite charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote threads w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 51245 8998)
 (symbols 48 7171 1)
 (strings 16 18830 2275)
 (string-bytes 1 532711)
 (vectors 16 10213)
 (vector-slots 8 131934 8682)
 (floats 8 21 109)
 (intervals 40 252 92)
 (buffers 888 11))