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From "xiaoming gu" <xiaoming...@gmail.com>
Subject Re: generalized multiplication replacement
Date Thu, 07 Aug 2008 15:44:45 GMT
There is a section "Signed division quotient rounded towards 0" in the
PLDI94 paper for the division in
Java. I'm going to implement the algorithm in peephole pass. Thanks for your
kindful help, Ian.

Xiaoming

On Wed, Aug 6, 2008 at 10:40 PM, Ian Rogers <rogers.email@gmail.com> wrote:

> xiaoming gu wrote:
>
>> Hi, all. Previously, Harmony Jira 5901 brought some benefits. Now I'm
>> working on generalized multiplication replacement with shift and addition.
>>
>> Today I checked with gcc and found whether a multiplication is replaced or
>> not in gcc mainly (not strictly) depends on the number of the replaced
>> operations. There are some exceptions in gcc and I couldn't summarize the
>> rule without reading the source code. So I'm planning to round it to a
>> simple decision as following: if it needs >=4 shift/add operations, don't
>> do
>> replacement; Otherwise, replace it with a combination of shift/add.
>>
>> For example x*20 (20=00010100) is replaced by (x<<2+x)<<2  and x*56
>> (56=00111000) is replaced by (x<<3-x)<<3 but x*29 (29=00011101) is not
>> optimized by ((x<<3)-x)<<2+x.
>>
>> Any comments?
>>
>> Another question - may I read the source code of gcc since it is
>> opensource
>> in GPL? Thanks.
>>
>> Xiaoming
>>
>>
> Hi Xiaoming,
>
> IANAL but I guess it's not allowed to base your implementation on GCC. I
> wrote the same code for Jikes RVM [1] and I'd be happy for this CPL code
> also to be available under the ASF. This code doesn't handle the use of
> subtraction and partially computed results from earlier stages, I would very
> much like to improve it to do this. There is a tracker for Jikes RVM for
> this [2], there are also papers/programs explaining how to do the better
> approach [3][4] and some other considerations are mentioned in [5].
>
> Regards,
> Ian Rogers
>
> [1]
> http://jikesrvm.svn.sourceforge.net/viewvc/jikesrvm/rvmroot/trunk/rvm/src/org/jikesrvm/compilers/opt/Simplifier.java?revision=14704&view=markup#l_1385
> [2] http://jira.codehaus.org/browse/RVM-256
> [3] http://portal.acm.org/citation.cfm?id=178249
> [4]
> http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/25112.PDFpages
160 and 186
> [5] http://www.intel.com/design/processor/manuals/248966.pdf
> --
> http://www.cs.man.ac.uk/~irogers <http://www.cs.man.ac.uk/%7Eirogers>
>

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