Re: obstack module has alignment issues on sparc? (Re: set_labels_identifiers_target -fsanitize=undefined error)

[Bruno Haible]

Hi Jeffrey,

> On SPARC, 64-bit words can be loaded and saved through one of two
> instructions. The first version is optimized, the second version is
> not. The optimized version is faster, but the 64-bit words have to be
> aligned to 8-byte boundaries. I.e., naturally aligned.
> 
> If you are performing unaligned loads of 64-bit words, then you have
> to specify the compiler option -xmemalign=4i. -xmemalign=4i will
> generate the inefficient load, but it will avoid the SIGBUS.
> 
> When using the default toolchain settings, -xmemalign=8s is used,
> which causes the toolchain to use the optimized loads. I think that is
> what is generating the UBsan finding "runtime error: member access
> within misaligned address ... which requires 8 byte alignment."
> 
> Also see "3.4.151 –xmemalign[=<a><b>]",
> <https://docs.oracle.com/cd/E37069_01/html/E37076/aevkc.html>, in the
> Solaris manual.

I know that you wanted to be helpful, but these details about the Solaris
Studio compiler option miss the point.

  * We are discussing a problem on Linux/glibc/sparc64 [1][2], not on Solaris.

  * For gnulib, we generally assume the default compiler settings,
    except for a few essentially mandatory options on some platforms.[3]
    It is futile to even read about '-xmemalign' and similar options
    because
      - Gnulib cannot make use of them without excessive Makefile.am hacking.
      - Gnulib cannot support people who use such arbitrary compiler options,
        as it would vastly increase the number of build configurations that
        we would need to support.

    We can use compiler-specific built-ins, intrinsics, asms, etc. because
    that is limited to the .h / .c file source code. But not compiler-specific
    options.

Bruno

[1] https://lists.gnu.org/archive/html/bug-texinfo/2023-11/msg00000.html
[2] https://lists.gnu.org/archive/html/bug-texinfo/2023-11/msg00076.html
[3] https://gitlab.com/ghwiki/gnow-how/-/wikis/Platforms/Configuration