Date: Fri, 30 Apr 2021 00:13:52 +0200 From: Dimitry Andric <dim@FreeBSD.org> To: Hans Petter Selasky <hps@selasky.org> Cc: FreeBSD Current <freebsd-current@freebsd.org>, Konstantin Belousov <kib@FreeBSD.org> Subject: Re: Undefined compiler behaviour or a compiler bug? Message-ID: <84FC71CC-FE92-4327-8536-0A04416432C2@FreeBSD.org> In-Reply-To: <16b60c22-be04-cbbf-8fc4-a90c4031c529@selasky.org> References: <59f81323-74c5-e6a8-a62d-6abdf78902e8@selasky.org> <16b60c22-be04-cbbf-8fc4-a90c4031c529@selasky.org>
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--Apple-Mail=_C2BF98D0-CEEF-4A70-9DA6-304A70CFB90F Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset=us-ascii On 29 Apr 2021, at 23:11, Hans Petter Selasky <hps@selasky.org> wrote: >=20 > On 4/29/21 11:06 PM, Hans Petter Selasky wrote: >> Hi, >> Can someone please explain what C-compiler flag I'm missing, to make = this simple C-program terminate? >> I have a function _abs() which at some point is equal to one, but the = compiler thinks it is not required to emit the test for the while() at = all, looking at the assembly code! A printf() clearly verifies that the = _abs() function has returned one. >=20 >> clang11 -O2 test.c && ./a.out >=20 > Looks like -fwrapv fixes the issue. Indeed, in this case UBSan can help you diagnose the issue. E.g.: % clang -fsanitize=3Dundefined test.c -o test % ./test Y:0x00000001 ABS:0x00000011 Y:0x00000007 ABS:0x00000041 Y:0x00000019 ABS:0x00000281 Y:0x00000027 ABS:0x00000601 Y:0x000000d9 ABS:0x0000b801 Y:0x00000327 ABS:0x0009f001 Y:0x000004d9 ABS:0x00178001 Y:0x00003b27 ABS:0x0dab0001 Y:0x000044d9 ABS:0x12840001 test.c:20:26: runtime error: signed integer overflow: 46341 * 46341 = cannot be represented in type 'int' SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior test.c:20:26 in Y:0x0001bb27 ABS:0xff200001 Y:0x000e44d9 ABS:0x9a400001 Y:0x0011bb27 ABS:0x64000001 Y:0x01ee44d9 ABS:0xc8000001 ^C -Dimitry --Apple-Mail=_C2BF98D0-CEEF-4A70-9DA6-304A70CFB90F Content-Transfer-Encoding: 7bit Content-Disposition: attachment; filename=signature.asc Content-Type: application/pgp-signature; name=signature.asc Content-Description: Message signed with OpenPGP -----BEGIN PGP SIGNATURE----- Version: GnuPG/MacGPG2 v2.2 iF0EARECAB0WIQR6tGLSzjX8bUI5T82wXqMKLiCWowUCYIsvoAAKCRCwXqMKLiCW o2ZSAJ98xP+shoXJopMKNkFS3Ydycpz5YgCgm0j8he49bPR6lDyYXM7oGHks4HY= =lDtn -----END PGP SIGNATURE----- --Apple-Mail=_C2BF98D0-CEEF-4A70-9DA6-304A70CFB90F--
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