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

[hansieodendaal]

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

[ehuss]

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

[mati865]

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]

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]

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]

Closing as a duplicate of #71355.