On 11/19/20 11:14 AM, Jan Hubicka wrote:
On 11/16/20 12:20 AM, Jan Hubicka wrote:
This is controlled by -fipa-icf-alias-sets
The patch drops too early, so we may end up processing function twice. Also if
merging is not performed we lose code quality for no win (this is rare case).
My original plan was to remember the mismatched parameter and apply them only
after merging decisions are finished, but I was not sure how to do that in
ipa-icf. In particular we need to ensure transitivity. In particular if
function foo is merged to bar, we also need to be sure that we dropped
base alias setsin functions tht are called by bar even if they themselves
are not merged. Martin, is there easy way to implement this on top of current
ICF?
Well, you will need to create a set of merged functions and then traverse all
callers of these (via cgraph_node callers). It should not be so difficult, or?
Hey.
Well, imagine you have function A1 and A2
and calls A1->B2
and A2->B3
and there is also B3.
You likely mean A1->B1 and A2->B2, right?
Now A1 is ICF equivalent to A2
and also B1,B2,B3 are ICF equivalent if some TBAA info is dropped.
ICF works in a way that we have classes of functions (groups) that we know
that are equivalent. And we we do, we subdivide these classes. Then when
e.g. foo1 and foo2 are known to be different then a class
with bar1->foo1 and bar2->foo2 is split as well.
ICF merges A2 to A1
it also considers to merge B2,B3 to B1 but concludes it is not benefical
at the very end (because some of them have address taken and
constructing wrapper is too expensive)
We never do a revert of a decision!
The comparsions done are
B1:B2
B1:B3
A1:A2
So after comparing we have info what to drop in B1 to make merging B2->B1
and B3->B1 valid. We also have info what to drop in B2 to make B1->B2
valid and in B3 to make B3->B1 valid.
But we meed to drop info in B2 to make B3->B2 valid to make call path
alias A2 of A1->B2 safe.
So what you need is to skip a division of groups based on the "strict aliasing"
and mark all functions that will need a drop operation.
Hope it helps?
Martin
Honza
