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From "Zach Amsden (Code Review)" <>
Subject [Impala-ASF-CR] IMPALA-2020: Add rounding for decimal casts
Date Wed, 15 Feb 2017 23:53:20 GMT
Zach Amsden has posted comments on this change.

Change subject: IMPALA-2020: Add rounding for decimal casts

Patch Set 9:

Commit Message:

PS10, Line 10: Still writing tests,
             : but this is ready for human eyes to look at.
> Not needed in the commit message anymore, right ?
will cull
File be/src/exprs/

PS9, Line 303: round
> Yeah, I got that.  What I'm saying is why not remove the 'round' paramter s
Killing it with fire
File be/src/exprs/

Line 1503:      { true, 0, 3, 0 }}},
> okay.  i missed that the scale=1 case covers that.
It doesn't hurt to do anyways, and does apply some pressure to other corners.  Would probably
not hurt to add + 0.55 tests as well.
File be/src/runtime/decimal-value.h:

PS10, Line 237: /// Returns an approximate double for this decimal.
> long line
Will fix
File be/src/runtime/decimal-value.inline.h:

PS9, Line 43: precise
> sure, fixing in a separate change sounds fine.  If there's no jira for that
Freakily enough, 100000000000000000000000000000000000000 has exactly 53 significant digits
in binary.  So it should be represented exactly as a double precision floating point value.
 This means conversions from float to decimal(38, 38) should be well behaved.
File be/src/runtime/decimal-value.inline.h:

PS10, Line 38:   if (round) {
             :     // Decimal V2 behavior
             :     // Multiply the double by the scale.
             :     //
             :     // TODO: this could be made more precise by doing the computation in
             :     // 64 bit with 128 bit immediate result instead of doing an additional
             :     // multiply in 53-bit double precision floating point
             :     d *= DecimalUtil::GetScaleMultiplier<double>(scale);
             :     d = std::round(d);
             :     const T max_value = DecimalUtil::GetScaleMultiplier<T>(precision);
             :     if (abs(d) >= max_value) {
             :       *overflow = true;
             :       return DecimalValue();
             :     }
             :     // Return the rounded integer part.
             :     return DecimalValue(static_cast<T>(d));
             :   } else {
             :     // TODO: IMPALA-4924: remove DECIMAL V1 code
             :     // Check overflow.
             :     const T max_value = DecimalUtil::GetScaleMultiplier<T>(precision
- scale);
             :     if (abs(d) >= max_value) {
             :       *overflow = true;
             :       return DecimalValue();
             :     }
             :     // Multiply the double by the scale.
             :     d *= DecimalUtil::GetScaleMultiplier<double>(scale);
             :     // Truncate and just take the integer part.
             :     return DecimalValue(static_cast<T>(d));
             :   }
> Appears to me that we should be able to share the code for the most part:
Sort of.  I didn't want to change the original at all in this change though.  We can do this
if we can prove the results will be identical.

PS10, Line 108: typename RESULT_T::underlying_type_t
> May help to add a comment that underlying_type_t is defined only for the *I

Line 113:   if (divisor == 1) {
> If it's for optimization, constant propagation in codegen would have reaped
Things like dcheck, the divide, modulus and rounding are completely unnecessary in this case.

PS10, Line 117: const T remainder = v % divisor;
> Do we not need to compute this if round is false ?
No, but I was worried that moving the computation further away from the divide would cause
the compiler to do the divide twice.  We'll try it and see.  I haven't even gotten to looking
at the release binary yet.

PS10, Line 119: DCHECK(
Can't do it or GCC goes completely insane as it can't find ostream operators for int128_t.

PS10, Line 127: typename RESULT_T::underlying_type_t
> May be consider doing a typedef typename RESULT_T::underlying_type_t result
I can only get rid of this one, not the other, so it seems more confusing just to do that
for only one use.

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Gerrit-MessageType: comment
Gerrit-Change-Id: I2daf186b4770a022f9cb349d512067a1dd624810
Gerrit-PatchSet: 9
Gerrit-Project: Impala-ASF
Gerrit-Branch: master
Gerrit-Owner: Zach Amsden <>
Gerrit-Reviewer: Dan Hecht <>
Gerrit-Reviewer: Michael Ho
Gerrit-Reviewer: Michael Ho <>
Gerrit-Reviewer: Zach Amsden <>
Gerrit-HasComments: Yes

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