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From: "gjl at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/66511] [avr] whole-byte shifts not optimized away for uint64_t
Date: Sat, 27 Jun 2015 18:07:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66511

--- Comment #1 from Georg-Johann Lay <gjl at gcc dot gnu.org> ---
(In reply to Matthijs Kooijman from comment #0)
> I haven't found a readily available 5.x package yet to test.

It's the same.

> As you can see, the versions operating on 64 bit values preserve the
> 8-bit shift (which is very inefficient on AVR), while the versions
> running on 32 bit values simply copy the right registers.

Lib functions are used because users complained about bloated 64-bit
arithmetic.  

Notice that indide these 64-bit shift functions byte-shifts are used.

> The foo32_16 function still has some useless instructions (r27 and r26
> are not part of the return value, not sure why these are set) but that
> is probably an unrelated problem.

Yes.

> I've marked this with component "target", since I think these
> optimizations are avr-specific (or at least not applicable to bigger
> architectures).