From: Richard Henderson <rth@redhat.com>
To: Bernd Schmidt <bernds@codesourcery.com>
Cc: Andrew Stubbs <ams@codesourcery.com>, gcc-patches@gcc.gnu.org
Subject: Re: [PATCH 1/3] SH-2A FDPIC: New pattern: use_initial_val
Date: Thu, 19 Aug 2010 18:55:00 -0000 [thread overview]
Message-ID: <4C6D7B19.9080003@redhat.com> (raw)
In-Reply-To: <4C6D7831.1080802@codesourcery.com>
On 08/19/2010 11:30 AM, Bernd Schmidt wrote:
>> Looking forward to 3/3, it seems you emit a fancy USE insn, whose
>> purpose appears to be simply to extend the lifetime of the pseudo,
>> without actually using it in a specific pattern.
>
> Yes. It's an unspec which later gets turned into a no-op.
Looking at the split, it really does turn in to a nop insn, not
being optimized away entirely. Is that really intended? If so,
I think that deserves a comment.
What advantage does this unspec have over a (USE reg) insn?
I'm somewhat surprised that a free-floating use or unspec does
what you want better than actually using the pseudo in some
insn pattern that actually requires it. E.g. by adding it to
the CALL_INSN_FUNCTION_USAGE where optimize_mode_switching can
find it.
Can you tell me which part of mode switching ends up requiring
the pseudo? Scanning the 3/3 patch, it's non-obvious.
r~
next prev parent reply other threads:[~2010-08-19 18:42 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-08-19 16:12 Andrew Stubbs
2010-08-19 18:28 ` Richard Henderson
2010-08-19 18:54 ` Bernd Schmidt
2010-08-19 18:55 ` Richard Henderson [this message]
2010-08-19 19:34 ` Bernd Schmidt
2010-08-19 19:42 ` Daniel Jacobowitz
2010-08-19 19:55 ` Richard Henderson
2010-08-19 19:51 ` Richard Henderson
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