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From "Tim Armstrong (Code Review)" <ger...@cloudera.org>
Subject [Impala-ASF-CR] IMPALA-4939, IMPALA-4939: Decimal V2 multiplication
Date Tue, 01 Aug 2017 18:48:47 GMT
Tim Armstrong has posted comments on this change.

Change subject: IMPALA-4939, IMPALA-4939: Decimal V2 multiplication
......................................................................


Patch Set 4:

(4 comments)

http://gerrit.cloudera.org:8080/#/c/7438/4//COMMIT_MSG
Commit Message:

Line 7: IMPALA-4939, IMPALA-4939: Decimal V2 multiplication
The same JIRA is listed twice.


Line 75: DECIMAL_V2 disabled: 2.65s
This is scary and I don't think we should consider merging this until it's fixed (or we have
a strong reason why the regression is unavoidable and justified). Do we understand why the
int256_t is so much slower?


http://gerrit.cloudera.org:8080/#/c/7438/1/be/src/runtime/decimal-value.inline.h
File be/src/runtime/decimal-value.inline.h:

Line 301:       // The following is frought with apparent difficulty, as there is only 1 bit
> Added a TODO.
It seems most efficient development-wise if we give this a try now while we're working on
and benchmarking this code instead of leaving a TODO.

This could be a big win - I know the div here is a big contributor to runtime for some queries,
e.g. TPC-H Q1


http://gerrit.cloudera.org:8080/#/c/7438/1/be/src/util/bit-util.h
File be/src/util/bit-util.h:

Line 65:     return value < 0 ? -1 : 1;
> I included the benchmark result in the commit message.
Does it compile to the same assembly (or better assembly) as before? If it's adding a branch,
the benchmark may be misleading since the worst case would be a 50/50 mix of positive and
negative types, leading to branch mispredicts.

Another option is to keep the existing implementation and specialise the template for int256.


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Gerrit-MessageType: comment
Gerrit-Change-Id: I37ad6232d7953bd75c18dc86e665b2b501a1ebe1
Gerrit-PatchSet: 4
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Gerrit-Reviewer: Dan Hecht <dhecht@cloudera.com>
Gerrit-Reviewer: Michael Ho <kwho@cloudera.com>
Gerrit-Reviewer: Taras Bobrovytsky <tbobrovytsky@cloudera.com>
Gerrit-Reviewer: Tim Armstrong <tarmstrong@cloudera.com>
Gerrit-Reviewer: Zach Amsden <zamsden@cloudera.com>
Gerrit-HasComments: Yes

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