[LLVMdev] Enabling the vectorizer for -Os

[David Tweed]

On 5 June 2013 13:32, David Tweed <david.tweed at arm.com> wrote:

This is what I'd like to know about: what specific potential to change
results have you seen in the vectorizer?

 

Ø  No changes, just conceptual. AFAIK, the difference between the passes on
O2 and O3 are minimal (looking at the code where this is chosen) and they
don't seem to be particularly amazing to warrant their special place in On
land.

 

Ø  If the argument for having auto-vec on O2 is that O3 makes no difference,
than, why have O3 in the first place? Why not make O3 an alias to O2 and
solve all problems?

 

I think I'm managing to express myself unclearly again L For me the
practical definition of "O2" is "do transformations which are pretty much
guaranteed to actually be optimizations" rather than "do all optimizations
which don't carry a risk of disaster".  In which case the argument for or
against vectorizing at O2 is whether it's " pretty much guaranteed to
actually be an optimization" or not rather than whether it's an aggressive
optimization or not. I wouldn't say the argument for auto-vec on O2 isn't
that O3 makes no difference, it's whether the intrinsic properties of
auto-vec pass fit with the criteria which one uses for enabling passes at
O2. I think you were suggesting that "aggressive" transforms don't belong in
O2  and auto-vec  is "aggressive", while I tend to think of
simplicity/performance-relaiblity as the criteria for O2 and it's unclear if
auto-vec fits that.

 

Cheers,

Dave
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