From: Ian Lance Taylor <iant@google.com>
To: Georg-Johann Lay <avr@gjlay.de>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Reload pass ignores constraints. Why?
Date: Wed, 15 Apr 2009 18:31:00 -0000 [thread overview]
Message-ID: <m3zlehu5u5.fsf@google.com> (raw)
In-Reply-To: <49E62055.1020007@gjlay.de> (Georg-Johann Lay's message of "Wed\, 15 Apr 2009 19\:58\:45 +0200")
Georg-Johann Lay <avr@gjlay.de> writes:
> iii) Expand into MOV+LSHIFTRT and allow the resulting const in
> *movsi_insn:
> CSE et. al. will reconstruct the original constant and
> replace MOV+LSHIFTRT with a single SET:
> Bad: expanding was in vain and we Goto i)
I think this is the option you should use. You should use
TARGET_RTX_COSTS to make the constants which may not be immediate
operands expensive. Then CSE will not reconstruct them.
>> I don't know exactly what is going on. But it is most likely just a
>> coincidence that it is failing when using SHIFTRT. There is probably
>> some way to make it fail in other ways as well.
>
> Would state it like this: If the movMM expander expands the move into
> several insns, each insn must be able to handle an alternative (which
> reload might select) without needing a reload.
Yes, but also when can_create_pseudo_p returns false the move expander
must not call force_reg or gen_reg_reg or anything else which creates a
new pseudo register. When can_create_pseudo_p returns true, it is often
preferable to generate a new pseudo-reg rather than reuse an existing
one.
Ian
next prev parent reply other threads:[~2009-04-15 18:31 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-04-14 9:35 Georg-Johann Lay
2009-04-14 14:43 ` Ian Lance Taylor
2009-04-14 18:12 ` Georg-Johann Lay
2009-04-14 20:54 ` Ian Lance Taylor
2009-04-15 12:00 ` Georg-Johann Lay
2009-04-15 14:28 ` Ian Lance Taylor
2009-04-15 16:55 ` Georg-Johann Lay
2009-04-15 18:31 ` Ian Lance Taylor [this message]
2009-04-17 15:34 ` Georg-Johann Lay
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