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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/109609] [12/13/14 Regression] tail call for function
 even when passing a ptr which references a local array still
Date: Tue, 25 Apr 2023 07:15:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109609

--- Comment #11 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #10)
> On the first testcase reverting the offending rev. shows that it causes
>=20
>    <bb 2> [local count: 137085152]:
> -  MEM[(char *)&buf + 12B] =3D 0;
> -  _19 =3D *id_8(D);
> -  if (_19 !=3D 0)
> +  _18 =3D *id_7(D);
> +  if (_18 !=3D 0)
>=20
> thus we DSE the store to the end.
>=20
> The issue is that the fnspec we have for strncpy says the access size
> is specified by argument 3 but what it specified there is the _maximum_
> size read, not the actual size.  So instead of "1cO313" it should be
> "1cO31 " ('1' is somewhat odd then, it says we copy 'src' to 'dst'
> but we only say the 'dst' write covers arg 3 size - I guess that's OK
> for points-to analysis, the additional zeros written do not have pointers,
> but if we use it differently it might be a wrong spec?)
>=20
> I'm scanning other builtins for similar issues.

Note a different fix would be to re-interpret the case of reads and
say the size is _up to_ the specified length, thus use
ao_ref_init_from_ptr_and_range instead of _and_size.

strncat, stpncpy, strncmp, strnlen, strndup are affected similarly.

for functions like memchr I'm not sure if we can assume all 'n' bytes are
read (thus if that would cause known overread -> undefined behavior).

Honza?  Any opinion?


Fix for the testcase, but incomplete as noted above:

diff --git a/gcc/builtins.cc b/gcc/builtins.cc
index 0e06fa5b2e0..133707c1617 100644
--- a/gcc/builtins.cc
+++ b/gcc/builtins.cc
@@ -11562,11 +11562,12 @@ builtin_fnspec (tree callee)
       case BUILT_IN_STPCPY_CHK:
        return ".cO 1 ";
       case BUILT_IN_STRNCPY:
+      case BUILT_IN_STRNCPY_CHK:
+       return "1cO31 ";
       case BUILT_IN_MEMCPY:
       case BUILT_IN_MEMMOVE:
       case BUILT_IN_TM_MEMCPY:
       case BUILT_IN_TM_MEMMOVE:
-      case BUILT_IN_STRNCPY_CHK:
       case BUILT_IN_MEMCPY_CHK:
       case BUILT_IN_MEMMOVE_CHK:
        return "1cO313";=