From: Ian Lance Taylor <ian@airs.com>
To: "Matt Lee" <reachmatt.lee@gmail.com>
Cc: gcc-help@gcc.gnu.org
Subject: Re: Question about RTL for bitwise AND
Date: Thu, 13 Apr 2006 20:29:00 -0000 [thread overview]
Message-ID: <m3mzepkyn7.fsf@gossamer.airs.com> (raw)
In-Reply-To: <e94302560604130957y4241c76cp48ffafbe7150af81@mail.gmail.com>
"Matt Lee" <reachmatt.lee@gmail.com> writes:
> I am using powerpc-eabi-gcc (version 3.4.1) and I have a question
> about the RTL that is produced for,
>
> test.c
> int a;
>
> if (a & 2) {
> // Do something
> } else
> // Do something else
> }
>
>
> I see in test.c.01.rtl,
>
> (insn 12 11 13 (set (reg:SI 122)
> (lshiftrt:SI (reg:SI 121)
> (const_int 1 [0x1]))) -1 (nil)
> (nil))
>
> (insn 13 12 14 (parallel [
> (set (reg:SI 123)
> (and:SI (reg:SI 122)
> (const_int 1 [0x1])))
> (clobber (scratch:CC))
> ]) -1 (nil)
> (nil))
>
>
> My question is, why is a logical shift right required? Wouldn't a
> direct bit-wise AND with const_int 2 suffice?
In general this kind of decision is made based on target specific
costs. See prefer_and_bit_test in dojump.c.
For the PowerPC, you should look at later optimization passes. It
seems possible that the two instructions above will get combined into
a single rlwinm instruction. Although I haven't tried it.
> This is causing problems in my (other) port where I can do only
> single-bit shifts. In the worst case, a & 0x80000000 the final
> assembly contains 31 right shifts. This is a big optimization problem.
Fix your costs to indicate this.
Ian
next prev parent reply other threads:[~2006-04-13 20:29 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2006-04-13 16:57 Matt Lee
2006-04-13 20:29 ` Ian Lance Taylor [this message]
2006-04-13 23:31 ` Matt Lee
2006-04-14 3:21 ` Ian Lance Taylor
2006-04-14 21:49 ` Matt Lee
2006-04-18 4:33 ` Ian Lance Taylor
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