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From: "pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/113280] Strange error for empty inline
 assembly with +X constraint
Date: Mon, 08 Jan 2024 19:12:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113280

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> So the problem seems like combine decides somehow to=20
> combine:
> ```
> (insn 7 4 9 2 (set (reg/v:SF 100 [ xD.4459 ])
>         (plus:SF (reg/v:SF 102 [ aD.4455 ])
>             (reg/v:SF 103 [ bD.4456 ]))) "/app/example.cpp":6:7 1043 {add=
sf3}
>      (expr_list:REG_DEAD (reg/v:SF 102 [ aD.4455 ])
>         (nil)))
> (insn 9 7 10 2 (set (reg:SF 104 [ xD.4459 ])
>         (asm_operands:SF ("") ("=3DX") 0 [
>                 (reg/v:SF 100 [ xD.4459 ])
>             ]
>              [
>                 (asm_input:SF ("0") /app/example.cpp:8)
>             ]
>              [] /app/example.cpp:8)) "/app/example.cpp":8:5 -1
>      (expr_list:REG_DEAD (reg/v:SF 100 [ xD.4459 ])
>         (nil)))
> into:
> ```
> (insn 9 7 10 2 (set (reg:SF 104 [ xD.4459 ])
>         (asm_operands:SF ("") ("=3DX") 0 [
>                 (plus:SF (reg:SF 106)
>                     (reg/v:SF 103 [ bD.4456 ]))
>             ]
>              [
>                 (asm_input:SF ("0") /app/example.cpp:8)
>             ]
>              [] /app/example.cpp:8)) "/app/example.cpp":8:5 -1
>      (expr_list:REG_DEAD (reg:SF 106)
>         (nil)))
> ```
>=20
> But the dump from combine does not make sense:
> ```
> Trying 7 -> 9:
>     7: r100:SF=3Dr106:SF+r103:SF
>       REG_DEAD r106:SF
>     9: r104:SF=3Dasm_operands
>       REG_DEAD r100:SF
> Failed to match this instruction:
> (set (reg:SF 104 [ xD.4459 ])
>     (asm_operands:SF ("") ("=3DX") 0 [
>             (plus:SF (reg:SF 106)
>                 (reg/v:SF 103 [ bD.4456 ]))
>         ]
>          [
>             (asm_input:SF ("0") /app/example.cpp:8)
>         ]
>          [] /app/example.cpp:8))
> allowing combination of insns 7 and 9
> original costs 8 + 4 =3D 12
> replacement cost 4
> deferring deletion of insn with uid =3D 7.
> modifying insn i3     9: r104:SF=3Dasm_operands
>       REG_DEAD r106:SF
> deferring rescan insn with uid =3D 9.
> ```

So basically check_asm_operands accepts the insn because the plus matches t=
he
operand but this is an output operand which does not work as it is not an
lvalue ...=