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From: "benjamin.meier70 at gmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug tree-optimization/106822] "Boolean-or" optimization opportunity not consistently used Date: Sat, 03 Sep 2022 23:55:41 +0000 [thread overview] Message-ID: <bug-106822-4-Z3gOe1M99Z@http.gcc.gnu.org/bugzilla/> (raw) In-Reply-To: <bug-106822-4@http.gcc.gnu.org/bugzilla/> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106822 --- Comment #3 from Benjamin B. Meier <benjamin.meier70 at gmail dot com> --- > The reason for the difference is because in the case where there is an int in the structure, the alignment is 4 bytes so a 2 or 4 byte read for a and b will not cause an fault while in the first case where there is no int, the alignment is 1 and might cause a fault. Gcc is conservative here on doing the optimization. Okay, got it. The alignment actually makes a difference here. Thanks, for the explanation. > Also || is a short cutting or in c so I don't think this optimization can be done for the original case in a well defined way. I see that as well, however reading the right hand side expression of the ||-operator has in this case no observable side-effect. Plus if we know that s->a can be read, we know as well that s cannot be a NULL pointer. Given that, I would assume the optimization should be valid (and applied). Another supporting argument for ignoring the short cutting behavior is the version of "MyStruct" with "int x;": In this case the compiler also always reads both operands (and therefore ignores the short cut). So if the compiler uses a nice optimization without jumps if it knows the alignment is at least 2 bytes, it probably could also use an optimization for the case that the alignment is only 1 byte. A possible optimization would then look like the code of f2. Finally, it's nothing really important, but I still do not yet completely see why optimizing the jump away in f is an issue:)
next prev parent reply other threads:[~2022-09-03 23:55 UTC|newest] Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top 2022-09-03 22:48 [Bug c/106822] New: " benjamin.meier70 at gmail dot com 2022-09-03 22:55 ` [Bug c/106822] " benjamin.meier70 at gmail dot com 2022-09-03 23:08 ` [Bug tree-optimization/106822] " pinskia at gcc dot gnu.org 2022-09-03 23:55 ` benjamin.meier70 at gmail dot com [this message] 2022-09-05 10:19 ` rguenth at gcc dot gnu.org 2022-09-05 14:27 ` benjamin.meier70 at gmail dot com
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