llvm dev - Sep 2019 - [cfe-dev] CFG simplification question, and preservation of branching in the original code

Actually, this should go to llvm-dev.

On Sat, Sep 21, 2019 at 3:48 PM Roman Lebedev <lebedev.ri at gmail.com>
wrote:
>
> On Sat, Sep 21, 2019 at 3:18 PM Joan Lluch via cfe-dev
> <cfe-dev at lists.llvm.org> wrote:
> >
> > Hi all,
> >
> > For my custom architecture, I want to relax the CFG simplification
pass, and any other passes replacing conditional branches.
> >
> > I found that the replacement of conditional branches by “select"
and other instructions is often too aggressive, and this causes inefficient code
for my target as in most cases branches would be cheaper.
> >
> > For example, considering the following c code:
> >
> > long test (long a, long b)
> > {
> >   int neg = 0;
> >   long res;
> >
> >   if (a < 0)
> >   {
> >     a = -a;
> >     neg = 1;
> >   }
> >
> >   res = a*b;
> >
> >   if (neg)
> >     res = -res;
> >
> >   return res;
> > }
> >
> >
> > This code can be simplified in c, but it’s just an example to show the
point.
> >
> > The code above gets compiled like this (-Oz flag):
> >
> > ; Function Attrs: minsize norecurse nounwind optsize readnone
> > define dso_local i32 @test(i32 %a, i32 %b) local_unnamed_addr #0 {
> > entry:
> >   %cmp = icmp slt i32 %a, 0
> >   %sub = sub nsw i32 0, %a
> >   %a.addr.0 = select i1 %cmp, i32 %sub, i32 %a
> >   %mul = mul nsw i32 %a.addr.0, %b
> >   %sub2 = sub nsw i32 0, %mul
> >   %res.0 = select i1 %cmp, i32 %sub2, i32 %mul
> >   ret i32 %res.0
> > }
> >
> >
> > All branching was removed and replaced by ‘select’ instructions. For
my architecture, it would be desirable to keep the original branches in most
cases, because even simple 32 bit operations are too expensive to speculatively
execute them, and branches are cheap.
> >
> > Setting  'phi-node-folding-threshold’ to 1 or even 0 (instead of
the default 2), definitely improves the situation in many cases, but Clang still
creates many instances of ‘select’ instructions, which are detrimental to my
target. I am unsure about where are they created, as I believe that the
simplifycfg pass does not longer create them.
> You definitively can't ban llvm passes/clang from creating
select's.
>
> > So the question is: Are there any other hooks in clang, or custom code
that I can implement, to relax the creation of ’select’ instructions and make it
preserve branches in the original c code?
> I think this is backwards.
> Sure, you could maybe disable most of the folds that produce selects.
> That may be good for final codegen, but will also affect other passes
> since not everything deals with 2-node PHI as good as wit selects.
>
> But, what happens if you still get the select-y IR?
> Doesn't matter how, could be hand-written.
>
> I think you might want to instead aggressively convert selects into
> branches in backend.
>
> > Thanks,
> >
> > John
> Roman
>
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