[cfe-dev] OpenCL shift on short/char types

[Pete Couperus]

Hello David,

Thanks for your answer here.
This point is clarified in the current OpenCL spec (I’m not sure what version I was looking at originally).
Thanks!

Pete


From: David Majnemer [mailto:david.majnemer at gmail.com]
Sent: Wednesday, July 29, 2015 3:39 PM
To: Pete Couperus
Cc: cfe-dev at cs.uiuc.edu
Subject: Re: [cfe-dev] OpenCL shift on short/char types



On Wed, Jul 29, 2015 at 11:00 AM, Pete Couperus <Peter.J.Couperus at synopsys.com<mailto:Peter.J.Couperus at synopsys.com>> wrote:
Hello,

OpenCL defines shifts so the requested shift size is masked.
I’m looking at the IR generated by clang for shifts in OpenCL, and had a question.
For int shifts, clang creates a mask and does the shift on the mask of the requested shift, like so:
  %shr.mask = and i32 %b, 31
  %shr = lshr i32 %a, %shr.mask
This is basically what I would expect.

For shifts on shorts/chars:
uchar
s(uchar a, uchar b) {
  return a >> b;
}

Clang generates:
define zeroext i8 @s(i8 zeroext %a, i8 zeroext %b) #0 {
entry:
  %conv = zext i8 %a to i32
  %conv1 = zext i8 %b to i32
  %shr.mask = and i32 %conv1, 31
  %shr = lshr i32 %conv, %shr.mask
  %conv2 = trunc i32 %shr to i8
  ret i8 %conv2
}

I expected the mask in this case to be done “and 7” rather than “and 31”.
Is this a bug in clang, or is my expectation/understanding wrong?
Thanks!

It isn't possible, in C, to shift around types whose rank is smaller than 'int'.  The operands will be promoted to int and the shift operation will apply to the promoted values.


Pete


_______________________________________________
cfe-dev mailing list
cfe-dev at cs.uiuc.edu<mailto:cfe-dev at cs.uiuc.edu>
http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/cfe-dev/attachments/20150729/ce3782eb/attachment.html>