From: Daniel Berlin <dberlin@dberlin.org>
To: Michael Matz <matz@suse.de>
Cc: Diego Novillo <dnovillo@redhat.com>,
Andrew Macleod <amacleod@redhat.com>,
gcc mailing list <gcc@gcc.gnu.org>
Subject: Re: [tree-ssa] Out of SSA status and issues
Date: Tue, 13 May 2003 15:01:00 -0000 [thread overview]
Message-ID: <B466F7A9-8553-11D7-9ECC-000A95A34564@dberlin.org> (raw)
In-Reply-To: <Pine.LNX.4.44.0305131450310.15564-100000@wotan.suse.de>
On Tuesday, May 13, 2003, at 08:56 AM, Michael Matz wrote:
> Hi,
>
> On 13 May 2003, Diego Novillo wrote:
>
>> Why?
>>
>> 1. foo()
>> 2. {
>> 3. int i, *p;
>> 4.
>> 5. p = malloc();
>> 6. i = *p;
>> 7. return i + 9;
>> 8. }
>>
>> I see nothing wrong in replacing 'i + 9' with '*p + 9'.
>
> I do. *p could have been changed in between 6 and 7. The useful
> thing of
> SSA is, that there is exactly one definition of an entity, and that is
> the
> reason that you can copy-propagate extremely easily (because you know,
> that the source of the copy can't be possibly changed after that copy
> insn, because the def must have reached it, and that was the only def).
>
> To be able to propagate *p you have to explicitely proove by some means
> that it still is unchanged at the places of the uses of the copied-into
> entity. This would be exactly detrimental to the usefullness of copy
> propagation.
>
> I.e. *p is not a register but an expression, and a copy instruction
> copies
> registers to registers.
>
It would also ruin sparse register promotion that load PRE will do
soon, by unpromoting things.
>> Not that tree-ssa will do anything with this code, the default
>> type-based aliasing is too conservative, but PTA may disambiguate
>> this.
>
> Of course you _can_ optimize such cases, but it's not a subset of copy
> propagation but, hmm, un-PRE or something like that.
>
It's SSUSPRE (Static Single Use Store PRE).
Store PRE builds a single static use form, ...
>
> Ciao,
> Michael.
>
next prev parent reply other threads:[~2003-05-13 15:01 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2003-05-12 14:42 Andrew MacLeod
2003-05-12 15:38 ` Diego Novillo
2003-05-12 15:57 ` Andrew MacLeod
2003-05-12 16:05 ` Michael Matz
2003-05-12 16:10 ` Andrew MacLeod
2003-05-12 16:16 ` law
2003-05-12 17:08 ` law
2003-05-12 17:12 ` Andrew MacLeod
2003-05-12 17:26 ` law
2003-05-12 18:57 ` Andrew MacLeod
2003-05-13 9:07 ` Michael Matz
2003-05-13 12:42 ` Diego Novillo
2003-05-13 12:50 ` Andrew MacLeod
2003-05-13 13:05 ` Diego Novillo
2003-05-13 13:29 ` Andrew MacLeod
2003-05-13 13:57 ` Diego Novillo
2003-05-13 12:57 ` Michael Matz
2003-05-13 13:11 ` Diego Novillo
2003-05-13 13:18 ` Andrew MacLeod
2003-05-14 17:19 ` Jan Vroonhof
2003-05-14 18:05 ` Andrew MacLeod
2003-05-14 18:33 ` Diego Novillo
2003-05-14 19:11 ` Daniel Berlin
2003-05-13 15:01 ` Daniel Berlin [this message]
2003-05-13 12:33 ` Diego Novillo
2003-05-13 12:49 ` Andrew MacLeod
2003-05-13 12:58 ` Diego Novillo
2003-05-13 13:17 Richard Kenner
2003-05-13 13:27 ` Diego Novillo
2003-05-13 13:40 ` Michael Matz
2003-05-13 15:08 ` Michael S. Zick
2003-05-13 13:42 Richard Kenner
2003-05-13 15:23 Richard Kenner
2003-05-13 18:50 ` Geoff Keating
2003-05-13 23:28 ` Michael S. Zick
2003-05-17 17:19 ` Michael S. Zick
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