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8253409: Double-rounding possibility in float fma #2684

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@jddarcy jddarcy commented Feb 23, 2021

In floating-point, usually doing an operation to double precision and then rounding to float gives the right result in float precision. One exception to this is fused multiply add (fma) where "a * b + c" is computed with a single rounding. This requires the equivalent of extra intermediate precision inside the operation. If a float fma is implemented using a double fma rounded to float, for some well-chosen arguments where the final result is near a half-way result in float, an incorrect answer will be computed due to double rounding. In more detail, the double result will round up and then the cast to float will round up again whereas a single rounding of the exact answer to float would only round-up once.

The new float fma implementation does the exact arithmetic using BigDecimal where possible, with guard to handle the non-finite and signed zero IEEE 754 details.


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  • JDK-8253409: Double-rounding possibility in float fma

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$ git fetch https://git.openjdk.java.net/jdk pull/2684/head:pull/2684
$ git checkout pull/2684

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bridgekeeper bot commented Feb 23, 2021

👋 Welcome back darcy! A progress list of the required criteria for merging this PR into master will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

@openjdk openjdk bot added the rfr label Feb 23, 2021
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openjdk bot commented Feb 23, 2021

@jddarcy The following label will be automatically applied to this pull request:

  • core-libs

When this pull request is ready to be reviewed, an "RFR" email will be sent to the corresponding mailing list. If you would like to change these labels, use the /label pull request command.

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bplb approved these changes Feb 23, 2021
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@bplb bplb left a comment

Looks fine. Presumably the updated test fails without the source change.

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openjdk bot commented Feb 23, 2021

@jddarcy This change now passes all automated pre-integration checks.

ℹ️ This project also has non-automated pre-integration requirements. Please see the file CONTRIBUTING.md for details.

After integration, the commit message for the final commit will be:

8253409: Double-rounding possibility in float fma

Reviewed-by: bpb

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jddarcy commented Feb 23, 2021

Looks fine. Presumably the updated test fails without the source change.

Right; the added test case is the failing one from the bug report. It will fail if the old non-intrinsic implementation, that is the Java implementation is used.

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jddarcy commented Feb 23, 2021

/integrate

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@openjdk openjdk bot added integrated and removed ready rfr labels Feb 23, 2021
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openjdk bot commented Feb 23, 2021

@jddarcy Since your change was applied there have been 64 commits pushed to the master branch:

Your commit was automatically rebased without conflicts.

Pushed as commit e5304b3.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

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jddarcy commented Feb 25, 2021

/backport 16u

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@jddarcy Unknown command backport - for a list of valid commands use /help.

@jddarcy jddarcy deleted the 8253409 branch Jun 10, 2021
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