Hi, it's a bit complicated: the _existing_ warnings warn about things _using the C semantics_, even going so far as to say what shifted values are (eg, 1<<37 gives 0x2000000000 which is beyond the range of the data-type). On the other hand, with OpenCL you want to warn that 1<<37 is actually going to give the value 1<<5 (or 0x20) which might not be what is expected. So AFAICS you either need completely separate code with new warnings for this case, or you've got to reduce the shift value (noting that you've done it) prior to doing the checking, and I gather changing values is something which shouldn't be being done in the Sema layer.
Regards, Dave ________________________________________ From: Jordan Rose [[email protected]] Sent: Friday, December 21, 2012 5:13 PM To: David Tweed Cc: [email protected] Subject: Re: [cfe-commits] [PATCH] fix shifts that are defined in OpenCL but not in C99, etc I don't have much context here, but it seems like the warnings are still valid in OpenCL (because of the surprise factor); they might just be off by default. Or, since we don't like off-by-default warnings, they would at least have different warning text to make it clear that it's not undefined behavior, just something the programmer might not expect. On Dec 21, 2012, at 0:27 , David Tweed <[email protected]> wrote: > Hi, > > One of the corner cases for C family languages is shifts (<< and >>) where > the shift size is not with 0 to "word width". In C99 (and I think most other > C variants) these are undefined while OpenCL defines the meaning to be along > the lines of "integer promote shift variable as required, promote the shift > amount the same way and use the bottom promoted-type-width bits as the shift > amount" (this applies even if the shift amount is originally a negative > number). (Full technical description is in section 6.3j of OpenCL spec.) This > patch implements two parts to this; actually generating the IR in clang is > straightforward. The difficult bit is the front-end semantic diagnostics when > values are known statically. If in OpenCL I something expands to "1<<37" or > "1<<(-4)" it's definitely a well defined program fargment, but should any > judgement be made as to if it is likely to do what the programmer intended? > At the moment I've taken the view that, particularly since Sema shouldn't ac! tually change values by reducing the shift, it's not possible to generate meaningful warnings for this construct in OpenCL, so it returns before most of the bad shift checking in SemaExpr.cpp's DiagnoseBadShiftValues? > > Could this be reviewed and then I'll commit it please? > > Cheers, > Dave > > -- IMPORTANT NOTICE: The contents of this email and any attachments are > confidential and may also be privileged. If you are not the intended > recipient, please notify the sender immediately and do not disclose the > contents to any other person, use it for any purpose, or store or copy the > information in any medium. Thank > you.<shiftSize2.diff>_______________________________________________ > cfe-commits mailing list > [email protected] > http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits -- IMPORTANT NOTICE: The contents of this email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please notify the sender immediately and do not disclose the contents to any other person, use it for any purpose, or store or copy the information in any medium. Thank you. _______________________________________________ cfe-commits mailing list [email protected] http://lists.cs.uiuc.edu/mailman/listinfo/cfe-commits
