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From: "jakub at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/109954] x86-64's -m32 does not conform to documentation
Date: Wed, 24 May 2023 10:05:00 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109954

--- Comment #5 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
If -march=3D isn't specified, then the default configured value (explicitly=
 or
implicitly) is used for it.  That is the case on lots of architectures, not
just for -m32.
We say in the documentation:
@item --with-cpu=3D@var{cpu}
@itemx --with-cpu-32=3D@var{cpu}
@itemx --with-cpu-64=3D@var{cpu}
Specify which cpu variant the compiler should generate code for by default.
@var{cpu} will be used as the default value of the @option{-mcpu=3D} switch.
This option is only supported on some targets, including ARC, ARM, i386, M6=
8k,
PowerPC, and SPARC@.  It is mandatory for ARC@.  The @option{--with-cpu-32}=
 and
@option{--with-cpu-64} options specify separate default CPUs for
32-bit and 64-bit modes; these options are only supported for aarch64, i386,
x86-64, PowerPC, and SPARC@.

@item --with-schedule=3D@var{cpu}
@itemx --with-arch=3D@var{cpu}
@itemx --with-arch-32=3D@var{cpu}
@itemx --with-arch-64=3D@var{cpu}
@itemx --with-tune=3D@var{cpu}
@itemx --with-tune-32=3D@var{cpu}
@itemx --with-tune-64=3D@var{cpu}
@itemx --with-abi=3D@var{abi}
@itemx --with-fpu=3D@var{type}
@itemx --with-float=3D@var{type}
These configure options provide default values for the @option{-mschedule=
=3D},
@option{-march=3D}, @option{-mtune=3D}, @option{-mabi=3D}, and @option{-mfp=
u=3D}
options and for @option{-mhard-float} or @option{-msoft-float}.  As with
@option{--with-cpu}, which switches will be accepted and acceptable values
of the arguments depend on the target.

So, if you configure gcc --with-arch-32=3Di386 vs. --with-arch-32=3Dx86-64,=
 it will
behave differently.
And if you don't use any of those --with-arch*=3D configure options, a defa=
ult is
picked.=