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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/97071] Fails to CSE / inherit constant pool load
Date: Wed, 16 Sep 2020 13:49:24 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97071

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |powerpc64le
           Keywords|                            |missed-optimization

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Right before combine we see the following, still nicely with REG_EQUAL notes

(insn 7 17 9 2 (set (reg:DF 119)
        (mem/u/c:DF (unspec:DI [
                    (symbol_ref/u:DI ("*.LC0") [flags 0x82])
                    (reg:DI 2 2)
                ] UNSPEC_TOCREL) [0  S8 A64])) "t.c":3:20 533
{*movdf_hardfloat64}=20=20=20
     (expr_list:REG_EQUAL (const_double:DF -3.0e+0 [-0x0.cp+2])
        (nil)))
(insn 9 7 14 2 (set (reg:DF 121)
        (mem/u/c:DF (unspec:DI [
                    (symbol_ref/u:DI ("*.LC2") [flags 0x82])
                    (reg:DI 2 2)
                ] UNSPEC_TOCREL) [0  S8 A64])) "t.c":3:20 533
{*movdf_hardfloat64}=20=20=20
     (expr_list:REG_EQUAL (const_double:DF 3.0e+0 [0x0.cp+2])
        (nil)))
(insn 14 9 15 2 (set (reg/i:DF 33 1)
        (fma:DF (reg:DF 124)
            (reg:DF 119)
            (reg:DF 121))) "t.c":4:1 894 {*fmadf4_fpr}
     (expr_list:REG_DEAD (reg:DF 124)
        (expr_list:REG_DEAD (reg:DF 121)
            (expr_list:REG_DEAD (reg:DF 119)
                (nil)))))

eventually the easiest pass to teach this to is fwprop though as it already
works DF DEF -> USE.  Alternatively PRE could make the subtract and/or the
negated value anticipated.=