[Bug target/103353] Indefinite recursion when compiling -mmma requiring testcase w/ -maltivec

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103353

Kewen Lin <linkw at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |meissner at gcc dot gnu.org,
                   |                            |segher at gcc dot gnu.org,
                   |                            |wschmidt at gcc dot gnu.org

--- Comment #3 from Kewen Lin <linkw at gcc dot gnu.org> ---
And I am also curious about what will we miss if just changing it back to
return const0_rtx?

Back to the failure itself, without TARGET_MMA set we don't have the optab
movoo support. When it is expanding the bif, it tries to emit_move_insn
(target, valreg)

(gdb) pr target
(reg:OO 117 [ _1 ])

(gdb) pr valreg
(reg:OO 66 2)

For the target, it's pseudo and able to get subreg:SI; while for the valreg,
it's hard reg, fails to gen subreg, but it will call operand_subword_force (y,
i, mode) to get subword further, it goes with:

   copy_to_reg (op);

     further call: emit_move_insn (temp, x) 

// back to the original, OOmode move from the hard register to another pseudo,
and again and again... until memory run out for allocation.

If the answer to the question above is it's still meaningful to expand this
call without an expected context, I think we have to extend OOmode and XOmode
handling.

Any thoughts?