[LLVMdev] About LLVM switch instruction

[Milind Chabbi]

Hongbin

Can you elaborate more on your suggestion? I am not sure I fully
understand what you suggested.

-Milind

On Wed, Jul 17, 2013 at 11:11 PM, Hongbin Zheng <etherzhhb at gmail.com> wrote:
> Hi Milind,
>
> Maybe you could annotate the default case value as metadata to the swith
> instruction.
>
> Thanks
> Hongbin
>
>
> On Thu, Jul 18, 2013 at 1:09 PM, Milind Chabbi <Milind.Chabbi at rice.edu>
> wrote:
>>
>> Hi Mark,
>>
>> This will workaround the problem of "default" branch restriction on
>> the switch instruction. The trouble with this technique is that it
>> will trump later optimization phases such as constant propagation.
>> When a block was part of a case, because of the knowledge of the case
>> value, the block was a candidate for better optimization. However,
>> when we move the body of the case into the default, the knowledge of
>> the case value is lost and the body is less optimizable.
>>
>> -Milind
>>
>>
>> On Wed, Jul 17, 2013 at 9:29 PM, Mark Lacey <mark.lacey at apple.com> wrote:
>> > On Jul 17, 2013, at 9:01 PM, Milind Chabbi <Milind.Chabbi at rice.edu>
>> > wrote:
>> >> I am performing a transformation that requires changing the targets of
>> >> a basic block ending with a switch instruction.
>> >> In particular, I need to delete the edge that goes to the "default"
>> >> basic block.
>> >> But, LLVM switch instruction always wants a default target basic block
>> >> for a switch instruction.
>> >> It is not clear how to accomplish this, since I don't have a
>> >> replacement default target block.
>> >> I could potentially fake that edge to be one of the other case label
>> >> targets, but that is an ugly hack and I don't want to do that.
>> >> I would appreciate if you can suggest better alternatives.
>> >
>> > Hi Milind,
>> >
>> > If you make the "default" branch to a block that has an UnreachableInst
>> > as a terminator, the SimplifyCFG pass will remove one of the switch cases
>> > and replace the block that the default branches to with the block that this
>> > removed case branches to. This sounds a lot like the "ugly hack" that you
>> > would like to avoid. Would it be a reasonable solution for what you are
>> > trying to accomplish?
>> >
>> > Mark
>> >
>> >
>>
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>