[llvm-dev] Question about demanded bits analysis

[James Molloy via llvm-dev]

Hi,

We also use DemandedBits for a similar purpose during loop vectorisation
(see computeMinimalBitwidths in VectorUtils.cpp).

James
On Thu, 18 May 2017 at 17:28, Craig Topper via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> The demanded bits pass is an analysis pass. By itself it won't make any
> changes to the IR. It is used by other transformation passes that care
> about this type of information.
>
> The main place I know where we look to see if we can perform an operation
> truncated is in InstCombine. See canEvaluateTruncated in
> InstCombineCasts.cpp. But I believe we have to be able to completely remove
> the trunc before we will move it. So we'll need to find something like an
> earlier sign extend or zero extend that extended from the same type we'd be
> truncatng to.
>
> ~Craig
>
> On Thu, May 18, 2017 at 8:04 AM, Raul Garcia via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hello All,
>>
>>
>> I am trying to understand the demanded-bits pass. The example in the
>> source code (below) seems quite explicit. In the example, only 16 bits from
>> a 32 bit variable in IR are demanded and therefore the variable is
>> truncated to a 16 bit variable.
>>
>> %1 = add i32 %x, %y%2 = trunc i32 %1 to i16
>>
>>
>> However I was wondering if  for example, the addition demanded, say 18
>> bits, would this pass generate a i18 integer variable? I have not done the
>> experiment, but I would like to know beforehand if this is something that
>> this pass can do.
>>
>>
>> Regards,
>> Raul.
>>
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170518/636eda1e/attachment.html>