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From: "avieira at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109543] Avoid using BLKmode for unions with a
 non-BLKmode member when possible
Date: Mon, 24 Apr 2023 10:10:49 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109543

--- Comment #2 from avieira at gcc dot gnu.org ---
Sorry for the delay. Here's the typedefs with GNU vectors.=20=20

typedef struct=20
{
    float __attribute__ ((vector_size(16))) v[2];
} STRUCT;

#ifdef GOOD
typedef STRUCT TYPE;
#else
typedef union
{
    STRUCT s;
    double d[2];
} TYPE;
#endif

To be fair I suspect you could see similar behaviour with just 16-byte vect=
ors,
but with aarch64 the backend will know to use 64-bit scalar moves for 128-b=
it
BLKmodes, though even then, picking the vector mode would result in more
optimal (single vector move) code.=