[LLVMdev] About LLVM switch instruction

[Hongbin Zheng]

Hi Milind,

My suggestion just for your concern that if you eliminate the default
block, a block associated with a case value will become the default block
of the swhich instruction, since a switch instruction always requires a
default block.
But when a block associated with a case value become the default block, the
associated case value is lost and may confuse the later optimizations such
as constant propagation.

To prevent such information lost when you eliminate the default block and
make a block associated with a case value will become the default block,
you can attach a metadata[1] to the switch instruction to provide the case
value of the default block.

In order to take the advantage of the attached metadata for the default
case of the switch instruction you also need to modify the later
optimization accordingly.

Thanks
Hongbin

[1]http://blog.llvm.org/2010/04/extensible-metadata-in-llvm-ir.html


On Thu, Jul 18, 2013 at 2:30 PM, Milind Chabbi <Milind.Chabbi at rice.edu>wrote:

> 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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> >
> >
>
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