Implementation of std::f64::powi function gives inconsistent answers in multiple Rust tool-chains #73920

hansieodendaal · 2020-07-01T13:52:59Z

Problem code

I tried this code (also in Rust playpen here):

fn main() {
    let base = 0.999_999_560_409_038_5f64;
    let i = [
        9182i32, 9430i32, 10855i32, 10856i32, 10857i32, 11708i32, 30335i32, 33923i32, 34947i32,
    ];
    // Reference results from:
    // - stable-x86_64-unknown-linux-gnu  (Debug)
    // - stable-x86_64-pc-windows-gnu     (Debug, Release)
    // - stable-x86_64-pc-windows-msvc    (Debug)
    // - nightly-x86_64-unknown-linux-gnu (Debug)
    // - nightly-x86_64-pc-windows-gnu    (Debug, Release)
    let result = [
        0.9959718099156295f64, 
        0.9958632363991619f64, 
        0.9952396058283226f64, 
        0.9952391683299874f64, 
        0.9952387308318444f64, 
        0.9948664896246138f64, 
        0.9867535223990311f64, 
        0.9851983894664985f64, 
        0.9847550108373471f64, 
    ];
    let iter = i.iter().zip(&result);
    for item in iter {
        let v = base.powi(*item.0);
        assert_eq!(v, *item.1)
    }
}

I expected to see this happen:

The code should always give consistent answers in Windows and Linux, either in release or debug mode, using stable or nightly.
The code should always give consistent answers in Rust playpen, either in release or debug mode, using stable, beta or nightly.

Instead, this happened:

Toolchains tested (Windows 10)

#	Tool-chain (Windows)	Debug/Release	Pass/Fail
1	stable-x86_64-pc-windows-gnu	Debug	Pass
	[1.44.1 (`c7087fe` 2020-06-17)]	Release	Pass
--	-----------------------------------------	-------------	---------
2	stable-x86_64-pc-windows-msvc	Debug	Pass
	[1.44.1 (`c7087fe` 2020-06-17)]	Release	Fail
--	-----------------------------------------	-------------	---------
3	nightly-x86_64-pc-windows-gnu	Debug	Pass
	[1.46.0-nightly (`16957bd` 2020-06-30)]	Release	Pass
--	-----------------------------------------	-------------	---------
4	nightly-x86_64-pc-windows-msvc	Debug	Fail
	[1.46.0-nightly (`16957bd` 2020-06-30)]	Release	Fail
--	-----------------------------------------	--------------	---------
5	nightly-2019-05-12-x86_64-pc-windows-msvc	Debug	Pass
	[1.36.0-nightly (`af98304` 2019-05-11)]	Release	Fail
--	-----------------------------------------	-------------	---------
6	nightly-2019-07-31-x86_64-pc-windows-msvc	Debug	Pass
	[1.38.0-nightly (`dddb7fc` 2019-07-30)]	Release	Fail
--	------------------------------------------	-------------	---------
7	nightly-2019-10-04-x86_64-pc-windows-msvc	Debug	Pass
	[1.40.0-nightly (`032a53a` 2019-10-03)]	Release	Fail
--	-----------------------------------------	-------------	---------
8	nightly-2020-01-08-x86_64-pc-windows-msvc	Debug	Pass
	[1.42.0-nightly (`8597644` 2020-01-07)]	Release	Fail
--	-----------------------------------------	-------------	---------
9	nightly-2020-02-23-x86_64-pc-windows-msvc	Debug	Pass
	[1.43.0-nightly (`436494b` 2020-02-22)]	Release	Fail
--	-----------------------------------------	-------------	---------
10	nightly-2020-06-10-x86_64-pc-windows-msvc	Debug	Fail
	[1.46.0-nightly (`feb3536` 2020-06-09)]	Release	Fail

Toolchains tested (Linux Ubuntu)

#	Tool-chain (Linux)	Debug/Release	Pass/Fail
1	stable-x86_64-unknown-linux-gnu	Debug	Pass
	[1.44.1 (`c7087fe` 2020-06-17)]	Release	Fail
--	-------------------------------------------	-------------	---------
2	nightly-2019-07-31-x86_64-unknown-linux-gnu	Debug	Pass
	[1.38.0-nightly (`dddb7fc` 2019-07-30)]	Release	Fail
--	-------------------------------------------	-------------	---------
3	nightly-2020-01-08-x86_64-unknown-linux-gnu	Debug	Pass
	[1.42.0-nightly (`8597644` 2020-01-07)]	Release	Fail
--	-------------------------------------------	-------------	---------
4	nightly-2020-06-10-x86_64-unknown-linux-gnu	Debug	Pass
	[1.46.0-nightly (`feb3536` 2020-06-09)]	Release	Fail
--	-------------------------------------------	-------------	---------
5	nightly-x86_64-unknown-linux-gnu	Debug	Pass
	[1.46.0-nightly (`16957bd` 2020-06-30)]	Release	Fail

Rust playpen

#	Tool-chain (Rust playpen)	Debug/Release	Pass/Fail
1	Stable channel	Debug	Pass
	[Stable version: 1.44.1]	Release	Fail
--	---------------------------------------	-------------	---------
2	Beta channel	Debug	Pass
	[1.45.0-beta.3 (2020-06-27 `8196407`)]	Release	Fail
--	--------------------------------------	-------------	---------
3	Nightly channel	Debug	Pass
	[1.46.0-nightly (2020-06-30 `16957bd`)]	Release	Fail
--	---------------------------------------	-------------	---------

Meta

rustc --version --verbose:
See detail information in the tables above.

Backtrace

C:\Code\f64_powi_issue>cargo run --release
warning: unused config key `build.RUSTFLAGS` in `C:\Users\.cargo\config`
    Finished release [optimized] target(s) in 0.05s
     Running `target\release\f64_powi_issue.exe`
thread 'main' panicked at 'assertion failed: `(left == right)`
  left: `0.9959718099154268`,
 right: `0.9959718099156295`', src\main.rs:22:9
stack backtrace:
   0: backtrace::backtrace::trace_unsynchronized
             at C:\Users\VssAdministrator\.cargo\registry\src\github.com-1ecc6299db9ec823\backtrace-0.3.46\src\backtrace\mod.rs:66
   1: std::sys_common::backtrace::_print_fmt
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\sys_common\backtrace.rs:78
   2: std::sys_common::backtrace::_print::{{impl}}::fmt
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\sys_common\backtrace.rs:59
   3: core::fmt::write
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libcore\fmt\mod.rs:1069
   4: std::io::Write::write_fmt<std::sys::windows::stdio::Stderr>
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\io\mod.rs:1504
   5: std::sys_common::backtrace::_print
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\sys_common\backtrace.rs:62
   6: std::sys_common::backtrace::print
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\sys_common\backtrace.rs:49
   7: std::panicking::default_hook::{{closure}}
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panicking.rs:198
   8: std::panicking::default_hook
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panicking.rs:218
   9: std::panicking::rust_panic_with_hook
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panicking.rs:511
  10: std::panicking::begin_panic_handler
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panicking.rs:419
  11: std::panicking::begin_panic_fmt
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panicking.rs:373
  12: __ImageBase
  13: __ImageBase
  14: std::rt::lang_start_internal::{{closure}}
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\rt.rs:52
  15: std::panicking::try::do_call
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panicking.rs:331
  16: std::panicking::try
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panicking.rs:274
  17: std::panic::catch_unwind
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\panic.rs:394
  18: std::rt::lang_start_internal
             at /rustc/c7087fe00d2ba919df1d813c040a5d47e43b0fe7\/src\libstd\rt.rs:51
  19: main
  20: invoke_main
             at d:\agent\_work\5\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:78
  21: __scrt_common_main_seh
             at d:\agent\_work\5\s\src\vctools\crt\vcstartup\src\startup\exe_common.inl:288
  22: BaseThreadInitThunk
  23: RtlUserThreadStart
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
error: process didn't exit successfully: `target\release\f64_powi_issue.exe` (exit code: 101)

C:\Code\f64_powi_issue>rustc --version --verbose
rustc 1.44.1 (c7087fe00 2020-06-17)
binary: rustc
commit-hash: c7087fe00d2ba919df1d813c040a5d47e43b0fe7
commit-date: 2020-06-17
host: x86_64-pc-windows-msvc
release: 1.44.1
LLVM version: 9.0

The text was updated successfully, but these errors were encountered:

ehuss · 2020-07-01T15:08:53Z

There is a bit of discussion about this in #71355 and #73420, which I think is pretty much the same.

mati865 · 2020-07-02T00:27:49Z

Quite interesting that #73420 affects both windows-gnu and windows-msvc targets but here only msvc output changed in the nightly.
#71355 on the other hand works fine on both Windows targets:

$ rustc -vV
rustc 1.46.0-nightly (16957bd4d 2020-06-30)
binary: rustc
commit-hash: 16957bd4d3a5377263f76ed74c572aad8e4b7e59
commit-date: 2020-06-30
host: x86_64-pc-windows-gnu
release: 1.46.0-nightly
LLVM version: 10.0

$ rustc powi2.rs && ./powi2
5e-324
5e-324
5e-324

$ rustc powi2.rs -O && ./powi2
5e-324
5e-324
5e-324

$ rustc powi2.rs --target x86_64-pc-windows-msvc && ./powi2
5e-324
5e-324
5e-324

$ rustc powi2.rs --target x86_64-pc-windows-msvc -O && ./powi2
5e-324
5e-324
5e-324

workingjubilee · 2021-03-28T02:51:45Z

Because LLVM generally turns powi into a call to libm, powi consistency is not possible without Rust bypassing this and shipping its own powi.

RalfJung · 2024-01-06T11:22:29Z

This should be closed as a duplicate of #71355 or vice versa.

So far it's not even clear to me that there is a bug here. Where is the exact rounding behavior of powi specified? Float operations behaving different across platforms and build modes is not unexpected; the exact precision of these operations is often left unspecified and hence should be considered non-deterministic. See rust-lang/rfcs#3514 for another source of non-determinism in float operations (the representation bits of a NaN value).

ehuss · 2024-01-06T14:39:09Z

Closing as a duplicate of #71355.

hansieodendaal added the C-bug Category: This is a bug. label Jul 1, 2020

hansieodendaal changed the title ~~Implementation of std::f64::powi function in Rust tool-chain nightly-2020-06-10 broken for Windows~~ Implementation of std::f64::powi function gives inconsistent answers in multiple Rust tool-chains Jul 1, 2020

hansieodendaal mentioned this issue Jul 1, 2020

Fix block validation failure in Windows for Rust tool-chain nightly-2020-06-10 tari-project/tari#2035

Merged

12 tasks

ecstatic-morse added A-floating-point Area: Floating point numbers and arithmetic A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. labels Jul 1, 2020

AaronKutch mentioned this issue Nov 28, 2020

Overhaul split integer handling and fix numerous issues rust-lang/compiler-builtins#384

Merged

workingjubilee mentioned this issue Mar 9, 2022

Clarify story on libm bindings #26350

Open

Nilstrieb added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Apr 5, 2023

ehuss closed this as not planned Won't fix, can't repro, duplicate, stale Jan 6, 2024

RalfJung mentioned this issue Jan 6, 2024

powi: Different rounding behaviour between debug and release mode, and across platforms #71355

Closed

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Implementation of std::f64::powi function gives inconsistent answers in multiple Rust tool-chains #73920

Implementation of std::f64::powi function gives inconsistent answers in multiple Rust tool-chains #73920

hansieodendaal commented Jul 1, 2020

ehuss commented Jul 1, 2020

mati865 commented Jul 2, 2020 •

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workingjubilee commented Mar 28, 2021

RalfJung commented Jan 6, 2024 •

edited

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ehuss commented Jan 6, 2024

Implementation of std::f64::powi function gives inconsistent answers in multiple Rust tool-chains #73920

Implementation of std::f64::powi function gives inconsistent answers in multiple Rust tool-chains #73920

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hansieodendaal commented Jul 1, 2020

Meta

ehuss commented Jul 1, 2020

mati865 commented Jul 2, 2020 • edited Loading

workingjubilee commented Mar 28, 2021

RalfJung commented Jan 6, 2024 • edited Loading

ehuss commented Jan 6, 2024

mati865 commented Jul 2, 2020 •

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RalfJung commented Jan 6, 2024 •

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