[LLVMdev] A small pass to constant fold branch conditions in destination blocks

[Jeff Kunkel]

Are you sure this is really advantageous? '%c' is only one variable, but
when you add the constant propagation, '%c' and false/true are two different
variables. Thus

 define i1 @t1(i1 %c) {
   br i1 %c, label %t, label %f
 t:
   ret i1 %c
 f:
   ret i1 %c
 }
should be
  br i1 R0, label %t, label %f
t:
  ret R0
f:
  ret R0

However, with your pass
 define i1 @t1(i1 %c) {
   br i1 %c, label %t, label %f
 t:
   ret i1 true
 f:
   ret i1 false
 }
will be
 define i1 @t1(i1 %c) {
   br i1 R0, label %t, label %f
 t:
   R1 = true
   ret i1 R1
 f:
   R1 = false
   ret i1 R1
 }

I am thinking X86 where '%c' would be allocated a register and the
false/true statement would be allocated a different register which would be
EAX/AX on the x86 machine.

Honestly, I believe this pattern could be conditional constant propagation
/ conditional re-materialization in the spiller. LLVM uses the spiller to
propagate constants. This pass would be useful to identify some conditional
re-materializations. You should look into hacking the spiller and see if
this can be added to it.

- My 2 cents,
Jeff Kunkel

On Mon, Feb 7, 2011 at 7:50 AM, Duncan Sands <baldrick at free.fr> wrote:

> Hi all, I wrote a little pass (attached) which does the following: if it
> sees a
> conditional branch instruction then it replaces all occurrences of the
> condition
> in the true block with "true" and in the false block with "false".  Well,
> OK, it
> is a bit more sophisticated (and a bit more careful!) than that but you get
> the
> idea.  It will turn this
>  define i1 @t1(i1 %c) {
>    br i1 %c, label %t, label %f
>  t:
>    ret i1 %c
>  f:
>    ret i1 %c
>  }
> into this
>  define i1 @t1(i1 %c) {
>    br i1 %c, label %t, label %f
>  t:
>    ret i1 true
>  f:
>    ret i1 false
>  }
> for example.  Curiously enough LLVM doesn't seem to have a pass that does
> this.
> I took a look at the effect on the testsuite by scheduling a run of this
> pass
> just after each run of -correlated-propagation.  In spite of being so
> simple
> (not to say simplistic) it has an enormous positive impact on Ada code and
> a
> substantial positive impact throughout the LLVM test-suite (I didn't check
> that
> programs still work after running the pass, so it could be that it has such
> a
> big effect because it is wrong!).
>
> So... should this kind of logic be incorporated into LLVM?  Perhaps as part
> of
> an existing pass like -correlated-propagation?
>
> It would be easy to make the pass a bit more powerful.  For example if the
> condition was "X == 0" then it could also replace X with 0 everywhere in
> the
> true block.
>
> Ciao, Duncan.
>
> PS: This was inspired by PR9004.
>
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