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From: Richard Biener <>
To: Jeff Law <>
Subject: Re: [PATCH] PR tree-optimization/109417 - Check if dependency is valid before using in may_recompute_p.
Date: Tue, 11 Apr 2023 11:21:22 +0200	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Apr 5, 2023 at 11:53 PM Jeff Law via Gcc-patches
<> wrote:
> On 4/5/23 14:10, Andrew MacLeod via Gcc-patches wrote:
> > When a statement is first processed, any SSA_NAMEs that are dependencies
> > are cached for quick future access.
> >
> > if we ;later rewrite the statement (say propagate a constant into it),
> > its possible the ssa-name in this cache is no longer active.   Normally
> > this is not a problem, but the changed to may_recompute_p forgot to take
> > that into account, and was checking a dependency from the cache that was
> > in the SSA_NAME_FREE_LIST. It thus had no SSA_NAME_DEF_STMT when we were
> > expecting one.
> >
> > This patch simply rejects dependencies from consideration if they are in
> > the free list.
> >
> > Bootstrapping on x86_64-pc-linux-gnu  and presuming no regressio0ns, OK
> > for trunk?
> eek.  So you've got a released name in the cache?  What happens if the
> name gets released, then re-used?  Aren't you going to get bogus results
> in that case?

We never re-use SSA names from within the pass releasing it.  But if
the ranger cache
persists across passes this could be a problem.  See
flush_ssaname_freelist which
for example resets the SCEV hash table which otherwise would have the
same issue.

IIRC ranger never outlives a pass so the patch should be OK.

_But_ I wonder how ranger gets at the tree SSA name in the first place - usually
caches are indexed by SSA_NAME_VERSION (because that's cheaper and
better than a pointer to the tree) and ssa_name (ver) will return NULL
for released
SSA names.  So range_def_chain::rdc could be just

  struct rdc {
   int ssa1;           // First direct dependency
   int ssa2;           // Second direct dependency
   bitmap bm;           // All dependencies
   bitmap m_import;

and ::depend1 return ssa_name (m_def_chain[v].ssa1) and everything would
work automatically (and save 8 bytes of storage).


> jeff

  reply	other threads:[~2023-04-11  9:22 UTC|newest]

Thread overview: 8+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-04-05 20:10 Andrew MacLeod
2023-04-05 21:52 ` Jeff Law
2023-04-11  9:21   ` Richard Biener [this message]
2023-04-24 13:51     ` Andrew MacLeod
2023-04-25  7:17       ` Richard Biener
2023-04-26  2:34       ` Jeff Law
2023-04-26 19:26         ` [COMMITTED 1/5] PR tree-optimization/109417 - Don't save ssa-name pointer in dependency cache Andrew MacLeod
2023-04-06 10:38 ` [PATCH] PR tree-optimization/109417 - Check if dependency is valid before using in may_recompute_p Jakub Jelinek

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