[LLVMdev] Motivation for 'select' instruction

[Chris Lattner]

On Thu, 22 Apr 2004, Vladimir Prus wrote:

> > >     int %logsch(int %ih, int %nbh) {
> > >     entry:
> > >              %tmp.1 = setlt int %nbh, 0
> > >              %ih_addr.1 = select bool %tmp.1, int 10, int %ih
> > >              %nbh_addr.1 = select bool %tmp.1, int 0, int %nbh
> > >              %tmp.4 = setgt int %nbh_addr.1, 22528
> > >              %ih_addr.0 = select bool %tmp.4, int 7, int %ih_addr.1
> > >              %nbh_addr.0 = select bool %tmp.4, int 22528, int %nbh_addr.1
> > >              %tmp.8 = add int %nbh_addr.0, %ih_addr.0
> > >              ret int %tmp.8
> > >     }
> > >
> > > Has 'select' instructions and a single path, which makes my task harder.
> >
> > What are you trying to do?  It looks like there is some correlation in the
> > branches that could be optimized away, is that what you're going after?
>
> No, not exactly. I need to find the path with the maximum execution time (in
> assembler for specific processor). Do to so, I must also find paths which
> cannot be executed no matter what input is (like one of the paths in the C
> example). I'm trying to do that using LLVM intermediate representation.

In that case, the above is a simple straight-line sequence of code, which
has exactly one path through it.  I don't think you should need to break
it up into multiple paths.  If the control flow has been flattened like
this, does it matter if some combination of predicates is impossible?

> > In the very worst case, you can always use the SelectLowering pass to turn
> > the select instructions into branches, but it would be preferable to find
> > another way if possible.  :)
>
> In fact, I think this might be enough for me, since for now I'm only going to
> evaluate the algorithms for detecting infeasible paths.

Okay, that works. :)

-Chris

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