Upgrade to LLVM11 caused a codegen regression on Windows

[jrmuizel]

The following code is panicing for me in CI on Win64 and Win32 (not other platforms). I haven't been able to reproduce it locally, but we bisected it to #73526 (the LLVM 11 upgrade).

Code

    fn GetRgbInputBufferImpl(kSwapRB: bool, kHasAlpha: bool) -> (usize, Vec<u8>) {
        let colorSamples = [0, 5, 16, 43, 101, 127, 182, 255];
        let colorSampleMax = colorSamples.len();
        let pixelSize = if kHasAlpha { 4 } else { 3 };
        let pixelCount = colorSampleMax * colorSampleMax * 256 * 3;

        let mut outBuffer = vec![0; pixelCount * pixelSize];

        let kRIndex = if kSwapRB { 2 } else { 0 };
        let kGIndex = 1;
        let kBIndex = if kSwapRB { 0 } else { 2 };
        let kAIndex = 3;

        // Sample every red pixel value with a subset of green and blue.
        let mut color = &mut outBuffer[..];
        for r in 0..=255 {
            for &g in colorSamples.iter() {
                for &b in colorSamples.iter() {
                    color[kRIndex] = r;
                    color[kGIndex] = g;
                    color[kBIndex] = b;
                    if kHasAlpha {
                        color[kAIndex] = 0x80;
                    }
                    color = &mut color[pixelSize..];
                }
            }
        }

        // Sample every green pixel value with a subset of red and blue.
        let mut color = &mut outBuffer[..];
        for &r in colorSamples.iter() {
            for g in 0..=255 {
                for &b in colorSamples.iter() {
                    color[kRIndex] = r;
                    color[kGIndex] = g;
                    color[kBIndex] = b;
                    if kHasAlpha {
                        color[kAIndex] = 0x80;
                    }
                    color = &mut color[pixelSize..];
                }
            }
        }

        // Sample every blue pixel value with a subset of red and green.
        let mut color = &mut outBuffer[..];
        let mut i = 0;
        for &r in colorSamples.iter() {
            for &g in colorSamples.iter() {
                for b in 0..=255 {
                    if color.len() == 0 {
                        dbg!((i, r, g, b));
                    }
                    color[kRIndex] = r;
                    color[kGIndex] = g;
                    color[kBIndex] = b;
                    if kHasAlpha {
                        color[kAIndex] = 0x80;
                    }
                    i += pixelSize;
                    color = &mut color[pixelSize..];
                }
            }
        }

        (pixelCount, outBuffer)
    }

[task 2020-10-23T03:49:58.569Z] 03:49:58     INFO -  ---- gtest::test::sRGB_to_sRGB_precache stdout ----
[task 2020-10-23T03:49:58.569Z] 03:49:58     INFO -  [gfx\qcms\src\gtest.rs:455] (i, r, g, b) = (
[task 2020-10-23T03:49:58.570Z] 03:49:58     INFO -      147456,
[task 2020-10-23T03:49:58.571Z] 03:49:58     INFO -      0,
[task 2020-10-23T03:49:58.571Z] 03:49:58     INFO -      0,
[task 2020-10-23T03:49:58.572Z] 03:49:58     INFO -      0,
[task 2020-10-23T03:49:58.573Z] 03:49:58     INFO -  )
[task 2020-10-23T03:49:58.573Z] 03:49:58     INFO -  thread 'gtest::test::sRGB_to_sRGB_precache' panicked at 'index out of bounds: the len is 0 but the index is 0', gfx\qcms\src\gtest.rs:457:21
[task 2020-10-23T03:49:58.574Z] 03:49:58     INFO -  stack backtrace:
[task 2020-10-23T03:49:58.574Z] 03:49:58     INFO -     0:     0x7ff7b5a789de - std::backtrace_rs::backtrace::dbghelp::trace
[task 2020-10-23T03:49:58.575Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\..\..\backtrace\src\backtrace\dbghelp.rs:108
[task 2020-10-23T03:49:58.576Z] 03:49:58     INFO -     1:     0x7ff7b5a789de - std::backtrace_rs::backtrace::trace_unsynchronized
[task 2020-10-23T03:49:58.576Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\..\..\backtrace\src\backtrace\mod.rs:66
[task 2020-10-23T03:49:58.577Z] 03:49:58     INFO -     2:     0x7ff7b5a789de - std::sys_common::backtrace::_print_fmt
[task 2020-10-23T03:49:58.578Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\sys_common\backtrace.rs:67
[task 2020-10-23T03:49:58.578Z] 03:49:58     INFO -     3:     0x7ff7b5a789de - std::sys_common::backtrace::_print::{{impl}}::fmt
[task 2020-10-23T03:49:58.579Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\sys_common\backtrace.rs:46
[task 2020-10-23T03:49:58.580Z] 03:49:58     INFO -     4:     0x7ff7b5a8da2b - core::fmt::write
[task 2020-10-23T03:49:58.580Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\core\src\fmt\mod.rs:1076
[task 2020-10-23T03:49:58.581Z] 03:49:58     INFO -     5:     0x7ff7b5a388c8 - std::io::Write::write_fmt<test::helpers::sink::Sink>
[task 2020-10-23T03:49:58.581Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\io\mod.rs:1516
[task 2020-10-23T03:49:58.582Z] 03:49:58     INFO -     6:     0x7ff7b5a71fe1 - std::io::impls::{{impl}}::write_fmt<Write>
[task 2020-10-23T03:49:58.583Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\io\impls.rs:179
[task 2020-10-23T03:49:58.583Z] 03:49:58     INFO -     7:     0x7ff7b5a7bc8d - std::sys_common::backtrace::_print
[task 2020-10-23T03:49:58.584Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\sys_common\backtrace.rs:49
[task 2020-10-23T03:49:58.585Z] 03:49:58     INFO -     8:     0x7ff7b5a7bc8d - std::sys_common::backtrace::print
[task 2020-10-23T03:49:58.585Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\sys_common\backtrace.rs:36
[task 2020-10-23T03:49:58.586Z] 03:49:58     INFO -     9:     0x7ff7b5a7bc8d - std::panicking::default_hook::{{closure}}
[task 2020-10-23T03:49:58.586Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\panicking.rs:208
[task 2020-10-23T03:49:58.587Z] 03:49:58     INFO -    10:     0x7ff7b5a7b807 - std::panicking::default_hook
[task 2020-10-23T03:49:58.588Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\panicking.rs:224
[task 2020-10-23T03:49:58.588Z] 03:49:58     INFO -    11:     0x7ff7b5a7c53f - std::panicking::rust_panic_with_hook
[task 2020-10-23T03:49:58.589Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\panicking.rs:577
[task 2020-10-23T03:49:58.589Z] 03:49:58     INFO -    12:     0x7ff7b5a7c0a5 - std::panicking::begin_panic_handler::{{closure}}
[task 2020-10-23T03:49:58.590Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\panicking.rs:484
[task 2020-10-23T03:49:58.591Z] 03:49:58     INFO -    13:     0x7ff7b5a7931f - std::sys_common::backtrace::__rust_end_short_backtrace<closure-0,!>
[task 2020-10-23T03:49:58.591Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\sys_common\backtrace.rs:141
[task 2020-10-23T03:49:58.592Z] 03:49:58     INFO -    14:     0x7ff7b5a7c059 - std::panicking::begin_panic_handler
[task 2020-10-23T03:49:58.592Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\panicking.rs:483
[task 2020-10-23T03:49:58.593Z] 03:49:58     INFO -    15:     0x7ff7b5a8c1f0 - core::panicking::panic_fmt
[task 2020-10-23T03:49:58.594Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\core\src\panicking.rs:85
[task 2020-10-23T03:49:58.594Z] 03:49:58     INFO -    16:     0x7ff7b5a8c1b7 - core::panicking::panic_bounds_check
[task 2020-10-23T03:49:58.595Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\core\src\panicking.rs:62
[task 2020-10-23T03:49:58.596Z] 03:49:58     INFO -    17:     0x7ff7b5a2818d - qcms::gtest::test::GetRgbInputBufferImpl
[task 2020-10-23T03:49:58.596Z] 03:49:58     INFO -                                 at z:\build\build\src\gfx\qcms\src\gtest.rs:457
[task 2020-10-23T03:49:58.597Z] 03:49:58     INFO -    18:     0x7ff7b5a2213c - qcms::gtest::test::GetRgbInputBuffer
[task 2020-10-23T03:49:58.597Z] 03:49:58     INFO -                                 at z:\build\build\src\gfx\qcms\src\gtest.rs:473
[task 2020-10-23T03:49:58.598Z] 03:49:58     INFO -    19:     0x7ff7b5a2213c - qcms::gtest::test::QcmsProfileTest::SetBuffers
[task 2020-10-23T03:49:58.598Z] 03:49:58     INFO -                                 at z:\build\build\src\gfx\qcms\src\gtest.rs:610
[task 2020-10-23T03:49:58.599Z] 03:49:58     INFO -    20:     0x7ff7b5a2213c - qcms::gtest::test::QcmsProfileTest::TransformPrecache
[task 2020-10-23T03:49:58.599Z] 03:49:58     INFO -                                 at z:\build\build\src\gfx\qcms\src\gtest.rs:695
[task 2020-10-23T03:49:58.600Z] 03:49:58     INFO -    21:     0x7ff7b5a2213c - qcms::gtest::test::sRGB_to_sRGB_precache
[task 2020-10-23T03:49:58.601Z] 03:49:58     INFO -                                 at z:\build\build\src\gfx\qcms\src\gtest.rs:797
[task 2020-10-23T03:49:58.601Z] 03:49:58     INFO -    22:     0x7ff7b5a2213c - qcms::gtest::test::sRGB_to_sRGB_precache::{{closure}}
[task 2020-10-23T03:49:58.602Z] 03:49:58     INFO -                                 at z:\build\build\src\gfx\qcms\src\gtest.rs:791
[task 2020-10-23T03:49:58.602Z] 03:49:58     INFO -    23:     0x7ff7b5a2213c - core::ops::function::FnOnce::call_once<closure-0,tuple<>>
[task 2020-10-23T03:49:58.603Z] 03:49:58     INFO -                                 at Z:\task_1603423771\fetches\rustc\lib\rustlib\src\rust\library\core\src\ops\function.rs:227
[task 2020-10-23T03:49:58.603Z] 03:49:58     INFO -    24:     0x7ff7b5a68b16 - core::ops::function::FnOnce::call_once
[task 2020-10-23T03:49:58.604Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\core\src\ops\function.rs:227
[task 2020-10-23T03:49:58.605Z] 03:49:58     INFO -    25:     0x7ff7b5a68b16 - test::__rust_begin_short_backtrace<fn()>
[task 2020-10-23T03:49:58.606Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\test\src\lib.rs:517
[task 2020-10-23T03:49:58.606Z] 03:49:58     INFO -    26:     0x7ff7b5a6645e - alloc::boxed::{{impl}}::call_once
[task 2020-10-23T03:49:58.607Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\alloc\src\boxed.rs:1042
[task 2020-10-23T03:49:58.607Z] 03:49:58     INFO -    27:     0x7ff7b5a6645e - std::panic::{{impl}}::call_once
[task 2020-10-23T03:49:58.608Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panic.rs:308
[task 2020-10-23T03:49:58.608Z] 03:49:58     INFO -    28:     0x7ff7b5a6645e - std::panicking::try::do_call
[task 2020-10-23T03:49:58.609Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panicking.rs:381
[task 2020-10-23T03:49:58.610Z] 03:49:58     INFO -    29:     0x7ff7b5a6645e - std::panicking::try
[task 2020-10-23T03:49:58.610Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panicking.rs:345
[task 2020-10-23T03:49:58.611Z] 03:49:58     INFO -    30:     0x7ff7b5a6645e - std::panic::catch_unwind
[task 2020-10-23T03:49:58.612Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panic.rs:382
[task 2020-10-23T03:49:58.612Z] 03:49:58     INFO -    31:     0x7ff7b5a6645e - test::run_test_in_process
[task 2020-10-23T03:49:58.613Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\test\src\lib.rs:544
[task 2020-10-23T03:49:58.613Z] 03:49:58     INFO -    32:     0x7ff7b5a6645e - test::run_test::run_test_inner::{{closure}}
[task 2020-10-23T03:49:58.614Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\test\src\lib.rs:450
[task 2020-10-23T03:49:58.614Z] 03:49:58     INFO -    33:     0x7ff7b5a37bd6 - std::sys_common::backtrace::__rust_begin_short_backtrace<closure-0,tuple<>>
[task 2020-10-23T03:49:58.615Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\sys_common\backtrace.rs:125
[task 2020-10-23T03:49:58.615Z] 03:49:58     INFO -    34:     0x7ff7b5a37bd6 - std::sys_common::backtrace::__rust_begin_short_backtrace<closure-0,tuple<>>
[task 2020-10-23T03:49:58.616Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\sys_common\backtrace.rs:125
[task 2020-10-23T03:49:58.617Z] 03:49:58     INFO -    35:     0x7ff7b5a3e93e - std::thread::{{impl}}::spawn_unchecked::{{closure}}::{{closure}}
[task 2020-10-23T03:49:58.617Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\thread\mod.rs:470
[task 2020-10-23T03:49:58.618Z] 03:49:58     INFO -    36:     0x7ff7b5a3e93e - std::panic::{{impl}}::call_once
[task 2020-10-23T03:49:58.618Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panic.rs:308
[task 2020-10-23T03:49:58.619Z] 03:49:58     INFO -    37:     0x7ff7b5a3e93e - std::panicking::try::do_call
[task 2020-10-23T03:49:58.620Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panicking.rs:381
[task 2020-10-23T03:49:58.621Z] 03:49:58     INFO -    38:     0x7ff7b5a3e93e - std::panicking::try
[task 2020-10-23T03:49:58.622Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panicking.rs:345
[task 2020-10-23T03:49:58.623Z] 03:49:58     INFO -    39:     0x7ff7b5a3e93e - std::panic::catch_unwind
[task 2020-10-23T03:49:58.623Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\panic.rs:382
[task 2020-10-23T03:49:58.624Z] 03:49:58     INFO -    40:     0x7ff7b5a3e93e - std::thread::{{impl}}::spawn_unchecked::{{closure}}
[task 2020-10-23T03:49:58.625Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\std\src\thread\mod.rs:469
[task 2020-10-23T03:49:58.626Z] 03:49:58     INFO -    41:     0x7ff7b5a3e93e - core::ops::function::FnOnce::call_once<closure-0,tuple<>>
[task 2020-10-23T03:49:58.627Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\core\src\ops\function.rs:227
[task 2020-10-23T03:49:58.627Z] 03:49:58     INFO -    42:     0x7ff7b5a85627 - alloc::boxed::{{impl}}::call_once
[task 2020-10-23T03:49:58.628Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\alloc\src\boxed.rs:1042
[task 2020-10-23T03:49:58.629Z] 03:49:58     INFO -    43:     0x7ff7b5a85627 - alloc::boxed::{{impl}}::call_once
[task 2020-10-23T03:49:58.630Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\library\alloc\src\boxed.rs:1042
[task 2020-10-23T03:49:58.630Z] 03:49:58     INFO -    44:     0x7ff7b5a85627 - std::sys::windows::thread::{{impl}}::new::thread_start
[task 2020-10-23T03:49:58.631Z] 03:49:58     INFO -                                 at /rustc/31530e5d132ebcc3654baf2e5460599681520af0\/library\std\src\sys\windows\thread.rs:56
[task 2020-10-23T03:49:58.631Z] 03:49:58     INFO -    45:     0x7ffe280e13d2 - BaseThreadInitThunk
[task 2020-10-23T03:49:58.631Z] 03:49:58     INFO -    46:     0x7ffe2a1c54e4 - RtlUserThreadStart

It still fails on Nightly.

https://bugzilla.mozilla.org/show_bug.cgi?id=1672813 has some more details.

[Mark-Simulacrum]

cc @rust-lang/wg-llvm

[jrmuizel]

I can now reproduce this locally. I'll try to come up with a standalone test case.

[jrmuizel]

Running the following on win64 with --release and codegen-units=1 panics.

fn inner() -> Vec<u8> {
    let color_sample_max = 4;
    let pixel_size = 4;
    let mut out = vec![0; color_sample_max * 256 * pixel_size];

    let mut color = &mut out[..];
    for _ in 0..color_sample_max {
        for b in 0..=255 {
            color[0] = 0x80;
            color[1] = 0x80;
            color[2] = b;
            color[3] = 0x80;

            color = &mut color[pixel_size..];
        }
    }
    out
}

fn f1()  {
    inner();
}

fn f2()  {
    inner();
}

fn main() {
    f1();
    f2();
}

[mati865]

Affects only MSVC:

$ rustc -O -C codegen-units=1 test.rs && ./test

$ rustc -O -C codegen-units=1 test.rs --target x86_64-pc-windows-msvc && ./test
thread 'main' panicked at 'index out of bounds: the len is 0 but the index is 0', test.rs:21:17
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

@rustbot modify labels: +O-windows-msvc

[rustbot]

Error: Label Windows-msvc can only be set by Rust team members

Please let @rust-lang/release know if you're having trouble with this bot.