[cfe-commits] [PATCH] fix shifts that are defined in OpenCL but not in C99, etc

[David Tweed]

Following John McCall's comments about testing constant sized array expressions I found out that I also needed to modify the constant expression processing in the front end. The updated patch attached does this. Although the engineer in me is a little concerned that there's two widely different code-paths that need to implement the same subtle behavioural changes, I can't see a better way of doing this in the current architecture. The test case also includes both array expression tests and a brief note citing where the OpenCL spec talks about the different semantic.

Assuming no objections, I'll commit in a day or two.
________________________________________
From: cfe-commits-bounces at cs.uiuc.edu [cfe-commits-bounces at cs.uiuc.edu] On Behalf Of David Tweed [David.Tweed at arm.com]
Sent: Friday, December 21, 2012 5:51 PM
To: Jordan Rose
Cc: cfe-commits at cs.uiuc.edu
Subject: Re: [cfe-commits] [PATCH] fix shifts that are defined in OpenCL but not        in C99, etc

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 [jordan_rose at apple.com]
Sent: Friday, December 21, 2012 5:13 PM
To: David Tweed
Cc: cfe-commits at cs.uiuc.edu
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 <david.tweed at arm.com> 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
>
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