Re: [RFH] Fixing -fsection-anchors on powerpc-darwin

[Richard Sandiford <richard at codesourcery dot com>]

Andrew Pinski <pinskia@physics.uc.edu> writes:
> Since -fsection-anchors is very useful for PPC-Darwin, I decided to
> see what I needed to do to support them.
> I started by just doing a bootstrap with them enabled by default
> and ran into the first issue of SET_ASM_OP not being set.  Next
> I ran into the issue of quoting ". + 0" which caused the assembler
> to think it is a symbol instead of an addition of . and 0.

Have you thought about overriding the output_anchor hook for Darwin?
It sounds simpler than what you did in the patch, and the hook was
there specifically for cases where the ASM_OUTPUT_DEF thing wouldn't
work.

> The next issue I could not figure out how to fix, it is wrong code
> caused by merging constant strings together.
> Here is the reduced testcase (reduced from genmodes.c):
> int modes;
> emit_mode_class (void)
> {
>    int t;
>    for(t = 0;t<modes;t++)
>        __builtin_puts ("");
> }
> int main(void)
> {
>    char a[3] = "-h";
>    if (!strcmp (a, "-h")) ; else  __builtin_abort ();
>    emit_mode_class ();
> }
>
> If someone could look into this, it would be nice, I think the use
> of .cstring and anchors are not supported together as .cstring section
> can be merged.

Right.  This is handled for ELF by:

  /* Don't use anchors for mergeable sections.  The linker might move
     the objects around.  */
  sect = SYMBOL_REF_BLOCK (symbol)->sect;
  if (sect->common.flags & SECTION_MERGE)
    return false;

in default_use_anchor_for_symbol_p.  If .cstring contains mergeable
data, it sounds like you should either (a) set SECTION_MERGE for it
or (b) override use_anchor_for_symbol_p for Darwin and check whether
SYMBOL_REF_BLOCK (symbol)->sect == cstring_section.

Richard