From: Jeffrey A Law <law@cygnus.com>
To: Jim Wilson <wilson@cygnus.com>
Cc: egcs@cygnus.com, Torbjorn Granlund <tege@pdc.kth.se>,
"David S. Miller" <davem@jenolan.rutgers.edu>
Subject: Re: This was fun to track down
Date: Wed, 24 Sep 1997 23:15:00 -0000 [thread overview]
Message-ID: <19356.875168270@hurl.cygnus.com> (raw)
In-Reply-To: <199709241957.MAA01872@cygnus.com>
In message < 199709241957.MAA01872@cygnus.com >you write:
> I see no reason why expand_divmod needs to call convert_modes in this case.
> If we are passed in a constant value, then we should keep the value as a
> constant, so that we have a chance of emitting optimized code for the divide.
> There is no need to emit insns to perform an actual conversion on the value.
> All we need to do is convert the RTL itself, and I don't even think that is
> necessary, since a CONST_INT is valid regardless of the mode.
>
> Hence, it looks like expmed.c should be calling gen_lowpart (or something
> similar) instead of convert_modes if op1 is a constant. This will give
> better code than the current patch by retaining the constant.
Note the following comment in convert_modes; it may apply to any attempt
to use gen_lowpart:
/* There is one case that we must handle specially: If we are converting
a CONST_INT into a mode whose size is twice HOST_BITS_PER_WIDE_INT and
we are to interpret the constant as unsigned, gen_lowpart will do
the wrong if the constant appears negative. What we want to do is
make the high-order word of the constant zero, not all ones. */
next prev parent reply other threads:[~1997-09-24 23:15 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
1997-09-22 19:37 David S. Miller
1997-09-23 0:25 ` Jeffrey A Law
1997-09-23 1:49 ` Torbjorn Granlund
1997-09-23 14:09 ` David S. Miller
1997-09-23 13:41 ` Torbjorn Granlund
1997-09-23 13:50 ` David S. Miller
1997-09-23 13:57 ` Torbjorn Granlund
1997-09-23 14:16 ` David S. Miller
1997-09-23 19:05 ` Jeffrey A Law
1997-09-24 12:58 ` Jim Wilson
1997-09-24 23:15 ` Jeffrey A Law [this message]
1997-09-27 13:40 ` Torbjorn Granlund
1997-09-27 23:38 ` Jeffrey A Law
1997-09-29 11:53 ` Jim Wilson
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