This is the mail archive of the
mailing list for the GCC project.
Re: [patch] conditional store elimination
On Thu, 11 Oct 2007, Tehila Meyzels wrote:
> >I tweaked the patch a bit to only do the transformation by default if
> >the target has conditional moves.
> Sorry, but I didn't find where in the patch you are checking this. Could
> you please enlighten?
In the hunk of toplev.c:
if (flag_tree_cselim == AUTODETECT_VALUE)
flag_tree_cselim = 1;
flag_tree_cselim = 0;
> There is another option, that will prevent the need of speculative load
> (and hence all the trapping/non trapping check):
> You can transform:
> if (cond)
> *p = X;
> q (points to a dummy stack slot)
> if (cond)
> *q = X;
> i.e., choose the address (that you store to) instead of the value.
Oh, that's an interesting idea, indeed. Shouldn't be hard to do that if
non-trappingness can't be proven.
> Few other points...
> What do you think about extending this to handle few conditional stores
> (and not only one)?
That's a logical extension and I think it would be nice to have. The
problem is, that the more stores you transform in this way, the more
important the correct cost model is. For instance it quickly becomes only
worthwhile if we then really can if-convert the conditional block, which
currently just so happens to be the case, as all these single-insn blocks
are trivially convertible. So if we want to the the former (and I agree
that we want that), we should first tackle the latter. And it's probable
that this transformation then should rather be integrated in a real
if-converter instead of being a separate pass doing this enabling
> What is your opinion that also conditional load hoisting will be included
Wasn't there disagreement if conditional load hoisting weren't better done
by some other pass? I don't really remember, and I also can't think of
any other current pass which could do that. Because in redundancy terms
this transformation is a pessimization (if only one if arm contains a
load), hence no current pass would do this transformation. But a working
cost model for hoisting cond loads would be even more important.
So if we would integrate these enabling transformations into the
if-converter itself we prabably have a higher chance of succeeding.