From: "H.J. Lu" <hjl.tools@gmail.com>
To: Richard Guenther <rguenther@suse.de>
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH] Fix one part of PR42108
Date: Sat, 04 Jun 2011 16:38:00 -0000 [thread overview]
Message-ID: <BANLkTimtX-S1x=UmW8yuqBJU2GcXj0YL6A@mail.gmail.com> (raw)
In-Reply-To: <6dc9ffc80912191050n227a42fbm74b5af9ce1be93c1@mail.gmail.com>
On Sat, Dec 19, 2009 at 10:50 AM, H.J. Lu <hjl.tools@gmail.com> wrote:
> On Fri, Dec 18, 2009 at 3:29 PM, Richard Guenther <rguenther@suse.de> wrote:
>>
>> This fixes one part of PR42108, the missed discovery of a full
>> redundant load. The issue is that the SSA SCC value-numberer
>> does not visit loads and stores in a defined order. The (or rather
>> one) fix is to properly canonicalize the virtual operand SSA names
>> we record in the expression hash tables. The proper canonical
>> virtual operand is the def of the first dominating may-definition
>> (or a PHI node vdef, but we can as well choose a non-may-definition
>> without loss of precision and generality).
>>
>> The patch possibly slows down SCCVN a bit for examples like
>>
>> # .MEM_2 = VDEF <.MEM_1(D)>
>> may-def
>> ....
>> # VUSE <.MEM_120>
>> ... = X;
>> # VUSE <.MEM_120>
>> ... = X;
>>
>> where discovering the redundant load of X needs to canonicalize
>> their VUSE SSA name twice (previously we entered the expression
>> into the hashtable with .MEM_120 so it would be found immediately).
>> Now if we had
>>
>> # VUSE <.MEM_60>
>> ... = X;
>>
>> inbetween the may-def and the other loads we previously discovered
>> the full redundancy only if we first visited the load with .MEM_60
>> and only after that the loads with .MEM_120. But nothing guarantees
>> this - this is the case the patch fixes. There are about 0.5%
>> more redundant loads discovered in tramp3d with this patch.
>>
>> This is a regression of the alias-improvements branch merge as
>> previously we had different virtual operands and thus in more
>> cases the canonical vuses were automagically chosen.
>>
>> Bootstrapped and tested on x86_64-unknown-linux-gnu. I have patched
>> one of our SPEC / C++ testers for more testing coverage.
>>
>
> This may have caused:
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42435
>
This also caused:
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49279
--
H.J.
prev parent reply other threads:[~2011-06-04 16:38 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-12-19 0:13 Richard Guenther
2009-12-20 2:21 ` H.J. Lu
2011-06-04 16:38 ` H.J. Lu [this message]
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