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From: "bernd.edlinger at hotmail dot de" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug middle-end/58570] [4.9 Regression] wrong code for bitfields at -O2 and above Date: Tue, 08 Oct 2013 17:43:00 -0000 [thread overview] Message-ID: <bug-58570-4-dH2VrO5LZd@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-58570-4@http.gcc.gnu.org/bugzilla/> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58570 --- Comment #9 from Bernd Edlinger <bernd.edlinger at hotmail dot de> --- Eric, there is one more thing to consider for your proposed patch, that is the damned -fstrict-volatile-bitfields: if strict_volatile_bitfields>0 and the BIT_FIELD access is _volatile_ it does not respect the BIT_FIELD_REPRESENTATIVE at all. that means for instance: struct { int a:24; char b; } s; s.a=1; s.b=2; we have a INT32 read-modify-write over b. even worse: struct { int a:24; } __attribute__((packed)) s; s.a=1; uses an (unaligned) INT32 read-modify-write and may overwrite one member of an adjacent structure. Note that at least the second example should no longer write beyond the structure if this patch is applied: http://gcc.gnu.org/ml/gcc-patches/2013-09/msg02058.html So you should expect an alias if either field is a VOLATILE bit-field access and flag_strict_volatile_bitfields > 0. Bernd.
next prev parent reply other threads:[~2013-10-08 17:43 UTC|newest] Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top 2013-09-28 20:53 [Bug tree-optimization/58570] New: wrong code at -Os and above on x86_64-linux-gnu (both 32-bit and 64-bit modes) su at cs dot ucdavis.edu 2013-09-30 10:15 ` [Bug tree-optimization/58570] [4.9 Regression] " rguenth at gcc dot gnu.org 2013-10-06 7:07 ` bernd.edlinger at hotmail dot de 2013-10-06 10:48 ` bernd.edlinger at hotmail dot de 2013-10-06 15:08 ` mikpelinux at gmail dot com 2013-10-07 21:07 ` ebotcazou at gcc dot gnu.org 2013-10-07 21:13 ` [Bug tree-optimization/58570] [4.9 Regression] wrong code for bitfields at -O2 and above ebotcazou at gcc dot gnu.org 2013-10-08 6:47 ` bernd.edlinger at hotmail dot de 2013-10-08 8:07 ` ebotcazou at gcc dot gnu.org 2013-10-08 8:09 ` [Bug middle-end/58570] " ebotcazou at gcc dot gnu.org 2013-10-08 17:43 ` bernd.edlinger at hotmail dot de [this message] 2013-10-08 17:56 ` ebotcazou at gcc dot gnu.org 2013-10-08 18:21 ` bernd.edlinger at hotmail dot de 2013-10-09 12:59 ` ebotcazou at gcc dot gnu.org 2013-10-09 13:10 ` ebotcazou at gcc dot gnu.org 2013-10-10 12:02 ` rguenth at gcc dot gnu.org
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