From: Mark Mitchell <mark@codesourcery.com>
To: kenner@vlsi1.ultra.nyu.edu
Cc: gcc@gcc.gnu.org
Subject: Re: SSA implementation
Date: Fri, 30 Jun 2000 09:14:00 -0000 [thread overview]
Message-ID: <20000630091356D.mitchell@codesourcery.com> (raw)
In-Reply-To: <10006301052.AA25097@vlsi1.ultra.nyu.edu>
>>>>> "Richard" == Richard Kenner <kenner@vlsi1.ultra.nyu.edu> writes:
Richard> We will be contributing a dead-code elimination pass
Richard> in the very near future that operates on the SSA form.
Richard> One big improvement is that this algorithm is a) very
Richard> fast and b) can eliminate loops in things like:
Richard> void f () { int i; for (i = 0; i < 100; ++i) ; }
Richard> We could always have eliminated such loops, but as I
Richard> understood it, we chose not to under the assumption they
Richard> were there for timing delay purposes. Has this policy
Richard> chaged?
Yes. This was debated to death a year or two ago. The manual has
said:
Historically, GCC has not deleted ``empty'' loops under the
assumption that the most likely reason you would put one in a program is
to have a delay, so deleting them will not make real programs run any
faster.
However, the rationale here is that optimization of a nonempty loop
cannot produce an empty one, which holds for C but is not always the
case for C++.
Moreover, with @samp{-funroll-loops} small ``empty'' loops are already
removed, so the current behavior is both sub-optimal and inconsistent
and will change in the future.
for quite some time.
--
Mark Mitchell mark@codesourcery.com
CodeSourcery, LLC http://www.codesourcery.com
next prev parent reply other threads:[~2000-06-30 9:14 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2000-06-30 3:39 Richard Kenner
2000-06-30 8:38 ` Bruce Korb
2000-06-30 9:14 ` Mark Mitchell [this message]
-- strict thread matches above, loose matches on Subject: below --
2000-06-29 7:30 David Dolan
2000-06-29 11:59 ` Geoff Keating
2000-06-29 12:11 ` Mark Mitchell
2000-06-29 12:24 ` Gerald Pfeifer
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