Does Clang have an equivalent to GCC's -malign-double flag?

Question

It seems like the -malign-double compiler option has been removed from Clang. Example code:

#include <stddef.h>
#include <stdio.h>

typedef struct X { char a; long long b; } X;

int main(void)
{
  printf("%zd\n", offsetof(X, b));
  return 0;
}

When compiled with GCC in 32-bit mode (-m32), this can be made to output either 8 or 4 depending on if -malign-double is enabled or not respectively. But Clang does not appear to support this option:

$ clang test.c -m32 -malign-double
clang: warning: argument unused during compilation: '-malign-double'
$ ./a.out
4

Clang version:

Apple clang version 3.1 (tags/Apple/clang-318.0.58) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin11.3.0
Thread model: posix

I can't seem to find any official documentation on the full list of compiler flags supported by Clang, they just seem to defer to GCC's documentation for the most part.

Is there any current equivalent to -malign-double in Clang? Or am I stuck having to use a different compiler for now? I need it to compile some code that links against a binary-only third-party library that uses that flag.

Ok, look like it's still not possible, as somebody told me on irc: " there is no option for that, if possible (e.g. not constrained by the ABI), LLVM will use natural alignment | if it is generated code, you can always force the alignment using attributes" — tito, Jul 22 '12 at 21:28

score 1 · Accepted Answer · edited Dec 28 '21 at 09:33

This seems to be fixed now:

commit 3205dbb3f1f96e77e106f964dcf1b5f69fba4ecc
Author: Craig Topper <craig.topper@intel.com>
Date:   Thu Mar 21 20:07:24 2019 +0000

    [Driver] Pass -malign-double from the driver to the cc1 command line

    -malign-double is currently only implemented in the -cc1 interface.
    But its declared in Options.td so it is a driver option too.
    But you try to use it with the driver you'll get a message about the
    option being unused.

    This patch teaches the driver to pass the option through to cc1 so it won't
    be unused. The Options.td says the option is x86 only but I didn't see any
    x86 specific code in its implementation in cc1 so not sure if the
    documentation is wrong or if I should only pass this option through the
    driver on x86 targets.

    Differential Revision: https://reviews.llvm.org/D59624

    llvm-svn: 356706

Does Clang have an equivalent to GCC's -malign-double flag?

1 Answers1