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[SR-9759] JSONEncoder doesn't round-trip floating point values on i386 #3548

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stephentyrone opened this issue Jan 25, 2019 · 8 comments
Open

[SR-9759] JSONEncoder doesn't round-trip floating point values on i386 #3548

stephentyrone opened this issue Jan 25, 2019 · 8 comments
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@stephentyrone
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Previous ID SR-9759
Radar None
Original Reporter @stephentyrone
Type Bug
Additional Detail from JIRA
Votes 0
Component/s Foundation
Labels Bug
Assignee bendjones (JIRA)
Priority Medium

md5: 8d8255d7caa873b18de3534ebf594aa2

Issue Description:

No real investigation, but we're seeing some SIMD Codable failures on i386 only:

00:10:37 [ RUN      ] SIMDCodable.roundTrip
00:10:37 stdout>>> check failed at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/test/stdlib/SIMD.swift, line 34
00:10:37 stdout>>> expected: SIMD2<Float>(0.26298332, -2.4110584) (of type Swift.SIMD2<Swift.Float>)
00:10:37 stdout>>> actual: SIMD2<Float>(0.2629833, -2.411058) (of type Swift.SIMD2<Swift.Float>)
00:10:37 stdout>>> check failed at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/test/stdlib/SIMD.swift, line 34
00:10:37 stdout>>> expected: SIMD3<Float>(-12.835825, 1.2614937, -0.21712112) (of type Swift.SIMD3<Swift.Float>)
00:10:37 stdout>>> actual: SIMD3<Float>(-12.83582, 1.261494, -0.2171211) (of type Swift.SIMD3<Swift.Float>)
00:10:37 stdout>>> check failed at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/test/stdlib/SIMD.swift, line 34
00:10:37 stdout>>> expected: SIMD4<Float>(3.3120403, 10.199102, 9.498707, 7.4242783) (of type Swift.SIMD4<Swift.Float>)
00:10:37 stdout>>> actual: SIMD4<Float>(3.31204, 10.1991, 9.498707, 7.424278) (of type Swift.SIMD4<Swift.Float>)
00:10:37 stdout>>> check failed at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/test/stdlib/SIMD.swift, line 34
00:10:37 stdout>>> expected: SIMD2<Double>(15.633335777865131, -5.08971682968474) (of type Swift.SIMD2<Swift.Double>)
00:10:37 stdout>>> actual: SIMD2<Double>(15.63333577786513, -5.08971682968474) (of type Swift.SIMD2<Swift.Double>)
00:10:37 stdout>>> check failed at /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/test/stdlib/SIMD.swift, line 34
00:10:37 stdout>>> expected: SIMD3<Double>(6.682962227179768, 2.238796173347538, 10.422819490078087) (of type Swift.SIMD3<Swift.Double>)
00:10:37 stdout>>> actual: SIMD3<Double>(6.682962227179768, 2.238796173347538, 10.42281949007809) (of type Swift.SIMD3<Swift.Double>)
00:10:37 [     FAIL ] SIMDCodable.roundTrip

which suggest that Float and Double values are rounded incorrectly by either JSONEncoder or JSONDecoder only on i386. Maybe an x87 excess-precision bug.
@stephentyrone
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There may be nothing that we can do about this on the Swift side, but I want a bug here to track the issue. It does appear to be i386 only, which makes it less painful.

@belkadan
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It would be really weird if NSNumber were truncating things to word size, though. @itaiferber, @phausler?

@stephentyrone
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@belkadan It occurs even with `Float`, so it can't be that simple (also the `Double` errors would be larger if that were what's going on).

@itaiferber
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Likely a precision issue with how JSONSerialization writes out floating-point values; we use snprintf on Darwin.

@stephentyrone Is this on Darwin, or a different platform? (If Darwin, can we please get a Radar? bendjones (JIRA User) will investigate.)

@stephentyrone
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@itaiferber i386-simulator, so Darwin.

@stephentyrone
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<rdar://problem/47551161> JSONEncoder doesn't round-trip floats on i386 Simulator (maybe other platforms too?)

@itaiferber
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Excellent, thanks!

@swift-ci
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Comment by Ben D. Jones (JIRA)

Yeah thanks @stephentyrone I'll take a look...

@swift-ci swift-ci transferred this issue from apple/swift-issues Apr 25, 2022
@shahmishal shahmishal transferred this issue from apple/swift May 5, 2022
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@itaiferber @stephentyrone @belkadan @swift-ci