On 02/21/2017 02:01 AM, Richard Biener wrote:
On Mon, 20 Feb 2017, Marc Glisse wrote:
On Mon, 20 Feb 2017, Jakub Jelinek wrote:
As mentioned by Jason in the PR, we've regressed on the following testcase
since we started emitting CLOBBERs at the start of ctors (and we warn as
before with -fno-lifetime-dse -Wuninitialized).
With -fno-lifetime-dse, the vuse on the b.x read stmt is default def
and thus we warn, but if there are clobbers before that, that is not the
case and we don't warn. The patch is quick hack to bypass the initial
clobbers as long as there aren't really many. If we wanted to handle
all initial clobbers, I bet the first time we run into this we could
recursively walk vop uses from the default def and build say a bitmap
of vop SSA_NAMEs which are vdefs of clobbers that only have clobbers
before it as vdef stmts.
MEM[(struct &)&b] ={v} {CLOBBER};
_4 = b.x;
if (_4 != 0)
It would be nice (at some point in a distant future) to turn that into
MEM[(struct &)&b] ={v} {CLOBBER};
if (_4(D) != 0)
i.e. replace reads from clobbered memory with a default def (with a gimple_nop
defining statement).
IIRC I tried to do it in FRE once, but failed.
Yeah, FRE isn't too happy about VN_TOP in the lattice (which is what
you'd use here). I also have/had patches trying to make it (more) happy
about VN_TOP but it still broke stuff like for example tail-merging...
Need to dissect PRE from tail-merging again, and also possibly PRE
from value-numbering (run eliminate() before doing PRE).
Couldn't this be easily modeled as redundant load elimination? Pretend
the clobber is a read, which would make the read of b.x redundant.
I haven't prototyped it, but that'd be the first approach I'd try.
jeff
