From: Jamie Prescott <jpresss@yahoo.com>
To: Ian Lance Taylor <iant@google.com>
Cc: gcc@gcc.gnu.org
Subject: Re: nops
Date: Thu, 21 May 2009 15:47:00 -0000 [thread overview]
Message-ID: <633817.47788.qm@web111611.mail.gq1.yahoo.com> (raw)
In-Reply-To: <m3zld7xdn9.fsf@google.com>
----- Original Message ----
> From: Ian Lance Taylor <iant@google.com>
> To: Jamie Prescott <jpresss@yahoo.com>
> Cc: gcc@gcc.gnu.org
> Sent: Wednesday, May 20, 2009 9:50:50 PM
> Subject: Re: nops
>
> Jamie Prescott writes:
>
> > Under which conditions GCC generates nops?
>
> It depends entirely on the target. For many targets, gcc will never
> generate a nop instruction, except as a byproduct of alignment.
My target does not have anything special WRT alignment. I even set the function
alignment to 8, and it still issues gen_nop().
This seem to happen only with -O0, or at least I noticed it only under such condition
so far.
> > I noticed that with 4.4.0, gen_nop() is required, thing that wasn't with
> 4.3.3.
> > Can I just define an empty insn for nop, of GCC requires a one-byte insn for
> its
> > own alignment purposes?
>
> Normally gcc does alignment by issuing an assembler directive, and the
> assembler is responsible for generating nop instructions when necessary.
Did a quick grep and the one you mention (ASM_OUTPUT_ALIGN_WITH_NOP) is
not defined in my backend. And such directive does not even have a default.
So, in theory, I should not see any nops.
The problem is different. The error is at link-time, if gen_nop() is not defined, so it
means that the insn is generated programmatically.
Grepping the source, I noticed a few files that issue gen_nop() directly, thing that
was not happening with 4.3.3.
So my questions are. Is there a way to disable it? If not, can I define an empty-issue
instruction for 'nop'?
- Jamie
next prev parent reply other threads:[~2009-05-21 15:13 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2009-05-21 4:52 nops Jamie Prescott
2009-05-21 9:16 ` nops Ian Lance Taylor
2009-05-21 15:47 ` Jamie Prescott [this message]
2009-05-21 16:12 ` nops Andrew Pinski
2009-05-21 17:29 ` nops Jamie Prescott
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