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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/58416] Incorrect x87-based union copying code
Date: Mon, 16 Sep 2013 10:28:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58416

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

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Target|                            |i?86-*-*

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
It is SRA that transforms

  MCOp.DoubleVal = 0.0;
  MCOp.IntVal = Val_3(D);
  *dest_5(D) = MCOp;

into

  MCOp$DoubleVal_7 = 0.0;
  _8 = VIEW_CONVERT_EXPR<double>(Val_3(D));
  MCOp$DoubleVal_4 = _8;
  MEM[(union MyClass *)dest_5(D)] = MCOp$DoubleVal_4;

simplifying into

  _6 = VIEW_CONVERT_EXPR<double>(Val_3(D));
  MEM[(union MyClass *)dest_4(D)] = _6;

and

(insn 7 6 0 (set (mem:DF (reg/v/f:SI 60 [ dest ]) [0 MEM[(union MyClass
*)dest_4(D)]+0 S8 A32])
        (subreg:DF (reg/v:DI 61 [ Val ]) 0)) t.C:15 -1
     (nil))

causes a reload:

(insn 7 11 12 2 (set (reg:DF 8 st [62])
        (mem/c:DF (plus:SI (reg/f:SI 7 sp)
                (const_int 8 [0x8])) [0 Val+0 S8 A32])) t.C:15 134
{*movdf_internal}
     (nil))
(insn 12 7 0 2 (set (mem:DF (reg/v/f:SI 0 ax [orig:60 dest ] [60]) [0
MEM[(union MyClass *)dest_4(D)]+0 S8 A32])
        (reg:DF 8 st [62])) t.C:15 134 {*movdf_internal}
     (expr_list:REG_DEAD (reg:DF 8 st [62])
        (expr_list:REG_DEAD (reg/v/f:SI 0 ax [orig:60 dest ] [60])
            (nil))))

where *movdf_internal simply doesn't properly preserve sNaN.

It looks wrong for DFmode move instructions to not preserve a IEEE defined
flag.  I suppose it would be also wrong to not preserve the exact bit pattern
(think of canonicalizing NaNs).