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From: "roger at nextmovesoftware dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/114187] [14 regression] bizarre register dance on x86_64
 for pass-by-value struct since r14-2526
Date: Fri, 01 Mar 2024 16:08:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114187

Roger Sayle <roger at nextmovesoftware dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2024-03-01
             Status|UNCONFIRMED                 |NEW
     Ever confirmed|0                           |1

--- Comment #3 from Roger Sayle <roger at nextmovesoftware dot com> ---
There's a missing simplification in combine:

Trying 6 -> 11:
    6: r102:TI=3Dzero_extend(r109:DF#0)<<0x40|zero_extend(r108:DF#0)
      REG_DEAD r108:DF
      REG_DEAD r109:DF
   11: r105:DF=3Dr102:TI#0+r102:TI#8
      REG_DEAD r102:TI
Failed to match this instruction:
(set (reg:DF 105 [ _4 ])
    (plus:DF (subreg:DF (ior:TI (ashift:TI (zero_extend:TI (subreg:DI (reg:=
DF
109) 0))
                    (const_int 64 [0x40]))
                (zero_extend:TI (subreg:DI (reg:DF 108) 0))) 8)
        (reg:DF 108)))

where the lowpart is getting simplified to reg:DF 108, but the highpart isn=
't
getting simplified to reg:DF 109.  i.e.

(subreg:DF (ior:TI (ashift:TI (zero_extend:TI (subreg:DI (reg:DF 109) 0))
                              (const_int 64 [0x40]))
                              (zero_extend:TI (subreg:DI (reg:DF 108) 0))) =
8)
can be simplified to just (reg:DF 109).

I'm looking into why this isn't happening.=