Portable alloca for Git

In the next patch we'll have to use alloca() for performance reasons,
but since alloca is non-standardized and is not portable, let's have a
trick with compatibility wrappers:

1. at configure time, determine, do we have working alloca() through
   alloca.h, and define

    #define HAVE_ALLOCA_H

   if yes.

2. in code

    #ifdef HAVE_ALLOCA_H
    # include <alloca.h>
    # define xalloca(size)      (alloca(size))
    # define xalloca_free(p)    do {} while(0)
    #else
    # define xalloca(size)      (xmalloc(size))
    # define xalloca_free(p)    (free(p))
    #endif

   and use it like

   func() {
       p = xalloca(size);
       ...

       xalloca_free(p);
   }

This way, for systems, where alloca is available, we'll have optimal
on-stack allocations with fast executions. On the other hand, on
systems, where alloca is not available, this gracefully fallbacks to
xmalloc/free.

Both autoconf and config.mak.uname configurations were updated. For
autoconf, we are not bothering considering cases, when no alloca.h is
available, but alloca() works some other way - its simply alloca.h is
available and works or not, everything else is deep legacy.

For config.mak.uname, I've tried to make my almost-sure guess for where
alloca() is available, but since I only have access to Linux it is the
only change I can be sure about myself, with relevant to other changed
systems people Cc'ed.

NOTE

SunOS and Windows had explicit -DHAVE_ALLOCA_H in their configurations.
I've changed that to now-common HAVE_ALLOCA_H=YesPlease which should be
correct.

Cc: Brandon Casey <drafnel@gmail.com>
Cc: Marius Storm-Olsen <mstormo@gmail.com>
Cc: Johannes Sixt <j6t@kdbg.org>
Cc: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Cc: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Cc: Gerrit Pape <pape@smarden.org>
Cc: Petr Salinger <Petr.Salinger@seznam.cz>
Cc: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Thomas Schwinge <thomas@codesourcery.com> (GNU Hurd changes)
Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/configure.ac b/configure.ac
index 2f433939dc..0eae70430c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -272,6 +272,14 @@ AS_HELP_STRING([],           [ARG can be also prefix for libpcre library and hea
 	GIT_CONF_SUBST([LIBPCREDIR])
     fi)
 #
+# Define HAVE_ALLOCA_H if you have working alloca(3) defined in that header.
+AC_FUNC_ALLOCA
+case $ac_cv_working_alloca_h in
+    yes)    HAVE_ALLOCA_H=YesPlease;;
+    *)      HAVE_ALLOCA_H='';;
+esac
+GIT_CONF_SUBST([HAVE_ALLOCA_H])
+#
 # Define NO_CURL if you do not have curl installed.  git-http-pull and
 # git-http-push are not built, and you cannot use http:// and https://
 # transports.