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From: "ianthompson at microsoft dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/113550] New: data512_t initializers dereference a
 clobbered register
Date: Mon, 22 Jan 2024 23:12:11 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D113550

            Bug ID: 113550
           Summary: data512_t initializers dereference a clobbered
                    register
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ianthompson at microsoft dot com
  Target Milestone: ---
            Target: aarch64

Created attachment 57189
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D57189&action=3Dedit
Additional non-minimal failing cases

When initializing or copying a data512_t, the compiler is generating code w=
hich
clobbers the register containing the source pointer of the copy. Initially
observed on Arm GNU Toolchain 12.2.Rel1, but this also reproduces on trunk.

Minimal reproduction, hits a segfault when compiled with "aarch64-none-elf-=
gcc
-march=3Darmv9-a+ls64":

#include <arm_acle.h>
void test_data512_init() {
    data512_t my_value =3D {};
}

This code generates this assembly snippet for initializing my_value:
        adrp    x0, .LC0
        add     x0, x0, :lo12:.LC0
        ldp     x0, x1, [x0]
        ldp     x2, x3, [x0, 16]
        ldp     x4, x5, [x0, 32]
        ldp     x6, x7, [x0, 48]

Notice that the first ldp clobbers x0, redirecting the remaining 3 loads to
whatever address happens to be in val[0] of the initializer.

Similar incorrect code is generated in many other situations that involve
copying a data512_t (passing a global variable to a function, dereferencing=
 a
data512_t*, etc). See the attached source file for the other failing cases =
I'm
seeing.=