From: Dave Brolley <brolley@redhat.com>
To: "Frank Ch. Eigler" <fche@redhat.com>
Cc: Chuan-Hua Chang <chuanhua.chang@gmail.com>, cgen@sourceware.org
Subject: Re: generated decoder code question
Date: Fri, 23 Mar 2007 14:13:00 -0000 [thread overview]
Message-ID: <4603DD87.9090002@redhat.com> (raw)
In-Reply-To: <20070323113053.GR27273@redhat.com>
Frank Ch. Eigler wrote:
>Hi -
>
>
>
>>In the utils-sim.scm file, the "-gen-decode-insn-entry" function has
>>the following lines:
>> ; Generate code to check that all of the opcode bits
>> for this insn match
>>[...]
>>It generates an IF-statement to check the opcode again inside the case
>>statement before doing the real extraction. This seems really
>>redundant
>>
>>
>
>
>I believe that this check is done because the switch statement by
>itself is not necessarily sufficient. The switch may simply
>disambiguate the instructions (so no two candidate instructions fall
>into the same switch case), but that could leave some additional
>decodeable bits. Those bits would need to be tested to tell apart the
>remaining candidate instruction from illegal opcodes.
>
>
This is correct. The switch only disambiguates the valid insns.
Sometimes when a case has been reduced to only one insn, there are still
some opcodes bits which have not been tested. In this case only, the
additional test is generated.
>
>
>>and make the decoder inefficient.
>>
>>
>
>Do you have a sense of how frequently such a test is completely
>redundant, and how much additional time this test takes?
>
>
Please point out any cases where the test is not needed and I will see
if I can eliminate it in those cases.
Thanks,
Dave
next prev parent reply other threads:[~2007-03-23 14:13 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-03-23 8:40 Chuan-Hua Chang
2007-03-23 11:31 ` Frank Ch. Eigler
2007-03-23 14:13 ` Dave Brolley [this message]
2007-03-26 5:50 ` Chuan-Hua Chang
2007-03-23 13:02 ` Dave Brolley
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