From: David Edelsohn <dje@watson.ibm.com>
To: Gabriel Paubert <paubert@iram.es>
Cc: Nathan Sidwell <nathan@codesourcery.com>, gcc <gcc@gcc.gnu.org>,
Geoffrey Keating <geoffk@apple.com>
Subject: Re: PR target/17101: question about powerpc s<cond> expanders
Date: Tue, 16 Nov 2004 15:38:00 -0000 [thread overview]
Message-ID: <200411161524.iAGFO7D27942@makai.watson.ibm.com> (raw)
In-Reply-To: Message from Gabriel Paubert <paubert@iram.es> of "Tue, 16 Nov 2004 00:41:06 +0100." <20041115234106.GB8995@iram.es>
>>>>> Gabriel Paubert writes:
Gabriel> Wouldn't the best solution in this case be?
Gabriel> addi tmp,val,-56
Gabriel> nor tmp,tmp,val
Gabriel> srwi tmp,tmp,31
Gabriel> now when comparing two variables that is different. But when one
Gabriel> of the operands is a constant there are lots of tricks to play
Gabriel> with using the complete set of Power/PPC logical instructions on
Gabriel> the sign bit.
Straight-line, simple integer sequences are better.
>> > The number of instructions do not correspond to the cost.
>> >Compares, moving bits from condition registers, and bit extraction is
>> >slower on newer PowerPC processors.
Gabriel> I'm quite surprised that integer compares have been affected, that's a
Gabriel> frequent operation and quite critical in a lot of applications. I was
Gabriel> not aware that rlwinm (and I suppose rlwimi/cntlzw/sraw/srawi and their
Gabriel> 64 bit equivalents) are becoming slower (relative to add/subtract and
Gabriel> logical), but I suspect that this mostly affects high-end 64 bit
Gabriel> processors and not 32 bit variants for embedded systems.
I would appreciate if you would avoid hyperbole and spreading
misinformation. You can look at the scheduling description in GCC to see
that your assumptions and statements are incorrect.
David
next prev parent reply other threads:[~2004-11-16 15:24 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2004-11-12 17:16 Nathan Sidwell
2004-11-12 18:37 ` David Edelsohn
2004-11-12 19:30 ` Nathan Sidwell
2004-11-12 20:57 ` David Edelsohn
2004-11-16 8:26 ` Gabriel Paubert
2004-11-16 15:38 ` David Edelsohn [this message]
2004-11-12 19:45 ` Geoffrey Keating
2004-11-12 22:45 ` Nathan Sidwell
2004-11-15 23:39 ` Gabriel Paubert
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