[Bug rtl-optimization/87678] Redundant vmovss with -fPIC

["ubizjak at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87678

--- Comment #8 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Segher Boessenkool from comment #7)

> It's not clear to me what you would have liked it to do instead?

The loads from constant memory pools always have REG_EQUAL of a relevant
constant attached to the instruction. As seen from Comment #3, combine first
tries to combine memory reference to a constant pool with the plus operation,
but fails the combination, since subst simplifies the load from constant pool
to an immediate constant. Later, combine retries the combination with the
REG_EQUAL from the load, which again results in the same immediate, and again
fails.

I would like to propose that combine handles constant pool memory references in
a different way. In the second try, it shouldn't just substitute REG_EQUAL
(since avoid_costant_pool_reference will return the same immediate constant
anyway), but should try to combine memory reference itself. This way, combine
will return combined instruction with a memory reference to a constant pool,
which is valid instruction in the x86 case.

So, tl;dr version reduces to - how to convice combine to return combined
instruction that refers to a constant pool in the second try instead of the
same invalid combined instruction with an immediate constant.