Why was handling of program_prefix changed?

[Ian Lance Taylor]

(I'm no longer on the address@hidden list; please CC me on any
replies.)

In autoconf 2.13, I see this code:

test "$host_alias" != "$target_alias" &&
  test "$program_prefix$program_suffix$program_transform_name" = \
    NONENONEs,x,x, &&
  program_prefix=${target_alias}-

In the current autoconf CVS sources, I see this:

test -n "$target_alias" &&
  test "$program_prefix$program_suffix$program_transform_name" = \
    NONENONEs,x,x, &&
  program_prefix=${target_alias}-[]dnl

Note that this is significantly different.  In the old code,
program_prefix was set by default if host_alias != target_alias.  In
the new code, program_prefix is set by default is target_alias is set
at all.

I believe the original version is correct.  Suppose I am using an
automatic configurator which works over a whole tree, like the Cygnus
configure script.  It is natural to always pass --host and --target
options.  When those options are the same, it indicates a native
build, implying that the default value of program_prefix should be
empty.

The original logic was that program_prefix should be set by default
for a non-native build.  That is what the original code tested for.
That code is based on the same code in the Cygnus configure script.
Why is it correct to set program_prefix merely because a --target
option was used?

Moreover, the code does not match the documentation.  The
documentation describes the original behaviour, not the new behaviour:

    If any of the options described below are given to @command{configure},
    program names are transformed accordingly.  Otherwise, if
    @code{AC_CANONICAL_TARGET} has been called and a @option{--target} value
    is given that differs from the host type (specified with @option{--host}),
    the target type followed by a dash is used as a prefix.  Otherwise, no
    program name transformation is done.

The change was made with the following ChangeLog entry, which does not
mention this behaviour change:

2000-05-24  Akim Demaille  <address@hidden>

        The options --build etc. used to set `$build'.  *If*
        AC_CANONICAL_SYSTEM was run, then the value of `$build' given by
        the user was saved into `$build_alias', and `$build' was
        normalized using `config.sub'.

        Now, let `--build' set `$build_alias' so that scripts with or
        without `AC_CANONICAL_BUILD' have the same semantics.  This allows
        to use `AC_CHECK_TOOL' without requiring `config.guess' and
        `config.sub' (which was bizarre anyway).

        * acgeneral.m4 (_AC_INIT_PARSE_ARGS): --build, --host and --target
        set `build_alias' etc.
        After the option handling, set host to $host_alias etc. for
        bugward compatibility.
        (AC_CANONICALIZE): Mutate into...
        (_AC_CANONICAL_SPLIT): this, which does not canonicalize, it just
        computes `$foo_os' etc.
        (_AC_CANONICAL_THING): Remove, too confusing.
        (AC_CANONICAL_BUILD): Adjust.
        Do not assign any value to `build_alias', just compute `build', and
        `build_{cpy, vendor, os}'.
        (AC_CANONICAL_HOST, AC_CANONICAL_TARGET): Likewise.
        (AC_CHECK_TOOL_PREFIX): Fire your rule when `$host_alias' is
        given, not when `$host != $build'.

Here is a patch to restore the original behaviour:

2001-11-19  Maciej W. Rozycki  <address@hidden>

        * lib/autoconf/general.m4 (AC_CANONICAL_HOST):  Don't set
        program_prefix if target_alias equals host_alias.

autoconf-2.52-target_prefix.patch
diff -up --recursive --new-file autoconf-2.52.macro/acgeneral.m4 
autoconf-2.52/acgeneral.m4
--- autoconf-2.52.macro/acgeneral.m4    Wed Jul  4 15:05:43 2001
+++ autoconf-2.52/acgeneral.m4  Sun Nov 18 15:27:47 2001
@@ -1774,7 +1774,7 @@ _AC_CANONICAL_SPLIT([target])
 
 # The aliases save the names the user supplied, while $host etc.
 # will get canonicalized.
-test -n "$target_alias" &&
+test -n "$target_alias" && test "x$target_alias" != "x$host_alias" &&
   test "$program_prefix$program_suffix$program_transform_name" = \
     NONENONEs,x,x, &&
   program_prefix=${target_alias}-[]dnl

Ian