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.
Now, the comment says:
/* For memory the only cheap thing we can do is see if we
have a use of the default def of the virtual operand.
??? Not so cheap would be to use the alias oracle via
walk_aliased_vdefs, if we don't find any aliasing vdef
warn as is-used-uninitialized, if we don't find an aliasing
vdef that kills our use (stmt_kills_ref_p), warn as
may-be-used-uninitialized. But this walk is quadratic and
so must be limited which means we would miss warning
opportunities. */
I wonder if it isn't useful to walk even limited number of vdefs this way
Seems worth a try for gcc8, there are several PRs it could help with.
anyway (likely GCC8 material though), e.g. if we run into a clobber that
must (rather than just may) portion of the read ref (and of course when
seeing non-clobber stmt that could alias with the ref give up before that),
we could warn even if we are very far from the start of the function.
--
Marc Glisse
