[llvm-commits] [Review request] Have Correlated Value Propagation handle instruction operands.

[Duncan Sands]

Hi Frits,

>> Do you done any measurements of the compile-time impact of this?
>
> No, sorry. Duncan Sands was worried about this too...

it is very expensive.  I ran some comparisons and it is very visible that with
your patch compilation is much slower.  The obvious thing to do is to keep your
logic but only analyse a small set of instructions, for example the ones it used
to analyse (Select, PHI, I/FCmp, Load, Store) plus branch instructions.  Sadly
the result is disappointing: it catches way less stuff than my pass.  I then
tried with your original patch, i.e. analysing all instructions.  It still
catches way less stuff than my pass if I judge by the number of lines of bitcode
removed.  I don't understand this since indeed LVI knows lots of tricks, so it
is probably worth trying to understand why correlated value propagation/LVI is
not doing a better job.

Ciao, Duncan.

PS: Way less = order of magnitude.