Re: [Qemu-devel] [PATCH] tci: Support deposit operations

[Stefan Weil <sw@weilnetz.de>]

Am 10.09.2012 16:08, schrieb Richard Henderson:
> On Sat, 2012-09-08 at 16:12 +0200, Stefan Weil wrote:
>> +            tmp16 = *tb_ptr++;
>> +            tmp8 = *tb_ptr++;
>> +            tmp32 = (((1<<  tmp8) - 1)<<  tmp16);
>> +            tci_write_reg32(t0, (t1&  ~tmp32) | ((t2<<  tmp16)&
>> tmp32));
>
> Use the deposit* functions from bitops.h?
>
>
> r~


Well, that would justify their existence in bitops.h, wouldn't it?

When I wrote deposit for TCI (November 2011), there still
was no bitops.h, and I was just too lazy to modify and test the
code again.

Now I tested code with both my version (with a small correction for
the 64 bit variant) and with the deposit* functions.

Both variants give identical results. The deposit* variant
creates larger (and I expect also less efficient) code.
I doubt that the effect will be measurable in a system emulation.

Results with 64 bit Debian Linux:

my variant
    6909       0     128    7037    1b7d 
bin/ndebug/x86-tci/i386-linux-user/tci.o
    6975       0     128    7103    1bbf 
bin/ndebug/x86-tci/i386-softmmu/tci.o
    6917       0     128    7045    1b85 
bin/ndebug/x86-tci/x86_64-linux-user/tci.o
    6972       0     128    7100    1bbc 
bin/ndebug/x86-tci/x86_64-softmmu/tci.o

deposit*
    6935       0     128    7063    1b97 
bin/ndebug/x86-tci/i386-linux-user/tci.o
    7000       0     128    7128    1bd8 
bin/ndebug/x86-tci/i386-softmmu/tci.o
    6963       0     128    7091    1bb3 
bin/ndebug/x86-tci/x86_64-linux-user/tci.o
    6997       0     128    7125    1bd5 
bin/ndebug/x86-tci/x86_64-softmmu/tci.o

Other emulation targets show similar differences.

On a Windows host, the difference was smaller, but still in favour of my 
code.

Regards,
Stefan