[Bug target/94173] [RISCV] Superfluous stackpointer manipulation

["wilson at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94173

Jim Wilson <wilson at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |wilson at gcc dot gnu.org

--- Comment #1 from Jim Wilson <wilson at gcc dot gnu.org> ---
struct Pair has size 8 and align 4, and we have no unaligned load/store
support, so we are not able to allocate the temporary local variable to a
register.  It must be allocated a stack slot.  The RTL optimizer is able to
figure out that the stack stores and loads don't alias anything and hence are
not necessary and optimizes them away.  However, we don't have any support to
unallocate a stack slot after it has already been allocated, so we end up with
the unnecessary stack pointer increment and decrement.  In a degenerate case
like this, where there are no longer any stack loads/stores, we may be able to
notice that and get rid of the stack pointer manipulation.  But in a more
complicated case where there are multiple stack slots, and references to all
but one is optimized away, then we would still need the stack pointer change,
though we would just be wasting stack space in this case with larger
decrements/increments than needed.

If you change the type to
truct Pair {
    char *s;
    char *t;
} __attribute__ ((aligned(8)));
then you get the result you want.  That isn't a practical solution, but it
demonstrates that this is a size/alignment/strict-alignment problem.

This is more of a middle end problem than a RISC-V backend problem.  It should
be possible to reproduce on any target with similar strict alignment
constraints, and similar calling conventions that allow returning the structure
in registers, though I don't know if there are any offhand.