[PATCH] Teach instcombine how to combine multiply of constant splat of known power-of-2

[Andrea_DiBiagio at sn.scee.net]

Hi,

Here is an updated patch that deals with the general case where a multiply 
by a vector of constants known
power-of-2 are combined into shifts.
My original patch only allowed combining multiply by splat of known 
constant powers of 2.

Thanks,
Andrea Di Biagio
SN Systems - Sony Computer Entertainment Group



Andrea DiBiagio/SN R&D/BS/UK/SCEE wrote on 24/05/2013 13:46:41:

> Hi Rafael,
> 
> Rafael Espíndola <rafael.espindola at gmail.com> wrote on 23/05/2013 
18:13:29:
> 
> > you explicitly test that things like
> > 
> > define i32 @MulTest3_i8(i32 %InVec)  {
> > entry:
> >   %0 = bitcast i32 %InVec to <4 x i8>
> >   %mul = mul <4 x i8> %0, <i8 4, i8 4, i8 2, i8 2>
> >   %1 = bitcast <4 x i8> %mul to i32
> >   ret i32 %1
> > }
> > 
> > are not optimized. Why is it not profitable to use
> > 
> > shl <4 x i8> %0, <i8 2, i8 2, i8 1, i8 1>
> > 
> > ?

> My original test simply tried to verify that multiply by constant 
> splats of known 
> power-of-2 were correctly optimized into shifts.
> However, I think that converting a mul into a shift is profitable even 
in the
> case you reported and probably my patch should be rewritten to 
> address the general case
> (i.e. not only the case of constant splats).
> 
> I will update this thread as soon as I have a new patch available.

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