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From: "linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/103353] Indefinite recursion when compiling -mmma
 requiring testcase w/ -maltivec
Date: Tue, 22 Feb 2022 08:49:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D103353

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |meissner at gcc dot gnu.or=
g,
                   |                            |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)=20

// back to the original, OOmode move from the hard register to another pseu=
do,
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 XOmo=
de
handling.

Any thoughts?=