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From: "marxin at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug lto/98912] valgrind error in default_elf_asm_output_ascii
Date: Tue, 02 Feb 2021 09:25:44 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D98912

--- Comment #4 from Martin Li=C5=A1ka <marxin at gcc dot gnu.org> ---
(In reply to Jakub Jelinek from comment #2)
> Well, lto_section is 8 byte long struct containing 2 ushort fields, 1 uch=
ar,
> one byte padding (on most hosts) and one ushort field.
> So bet valgrind is complaining about streaming the padding...

I bet it's complaining about the padding.
Please test the following debugging patch:

diff --git a/gcc/lto-streamer-out.c b/gcc/lto-streamer-out.c
index 405f3bfc56c..d96a7ee3ec8 100644
--- a/gcc/lto-streamer-out.c
+++ b/gcc/lto-streamer-out.c
@@ -2672,6 +2672,10 @@ produce_lto_section ()
   lto_section s
     =3D { LTO_major_version, LTO_minor_version, slim_object, 0 };
   s.set_compression (compression);
+  char *ptr =3D (char *)&s;
+  for (unsigned i =3D 0; i < sizeof s; i++)
+    fprintf (stderr, "v[%d]=3D%d\n", i, ptr[i]);
+  fprintf (stderr, "\n");
   lto_write_data (&s, sizeof s);
   lto_end_section ();
   destroy_output_block (ob);

with GCC 10 the object is constructed in the following way:

  MEM <char[4]> [(struct lto_section *)&s + 2B] =3D {};
  s.major_version =3D 9;
  s.slim_object =3D prephitmp_911;
  s.flags =3D 1;

and that covers all bytes in the struct.=