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

[David Majnemer]

On Wed, Jul 29, 2015 at 11:00 AM, Pete Couperus <
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
>
>
>
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>
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