Comments (16)
@rustbot label +F-f16_and_f128 +A-ABI +T-libs +C-bug -needs-triage +A-llvm
from rust.
If we define f128 to be the corresponding IEEE float format, then the transmute is by definition correct. That float format defines the representation in memory and I think it's endianess must work out exactly the same as u128? Cc @eddyb @jcranmer @Muon
So at first this looks like a platform bug to me. Certainly we'd have to change a lot more than just from_bits / to_bits if this was deemed correct behavior -- codegen and Miri also need to know how to convert between the raw bits an a floating-point value. Do LLVM optimizations do anything to handle this?
from rust.
Are you sure this is not an ABI bug, a mismatch between caller and callee in how the arguments are interpreted? That seems more likely to me than an in-memory float format with mixed endianess.
Does it make any difference if you remove the from_bits
call from the equation, and do the transmute inline via a union instead?
#![feature(f128)]
#[no_mangle]
#[inline(never)]
fn add_entry(a: f128, b: f128) -> f128 {
a + b
}
union U {
i: u128,
f: f128,
}
fn main() { unsafe {
let a = U { i: 0x0 };
let b = U { i: 0x1 };
dbg!(a.f, b.f);
let c = add_entry(a.f, b.f);
dbg!(c);
} }
from rust.
Yeah there is definitely something more going on here, setting opt-level=3 makes the problem go away
from rust.
Slightly reduced
$ cat transmute.rs
#![feature(f128)]
fn main() {
let a = f128::from_bits(0x1);
dbg!(a);
}
$ rustc transmute.rs --target powerpc64-unknown-linux-gnu -C linker=powerpc64-linux-gnu-gcc -Ctarget-cpu=pwr9 -o transmute.rs.ppc64.pwr9
$ qemu-ppc64 -L /usr/powerpc64-linux-gnu/ transmute.rs.ppc64.pwr9
[transmute.rs:5:5] a = 0x00000000000000010000000000000000
Any idea how to narrow further?
from rust.
Your suggestion does indeed fix the issue:
$ cat transmute_union.rs 24-05-14 08:15:29
#![feature(f128)]
union U {
i: u128,
f: f128,
}
fn main() {
let a = U { i: 0x1 };
dbg!(unsafe { a.f });
}
$ rustc transmute_union.rs --target powerpc64-unknown-linux-gnu -C linker=powerpc64-linux-gnu-gcc -Ctarget-cpu=pwr9 -o transmute_union.rs.ppc64.pwr9
$ qemu-ppc64 -L /usr/powerpc64-linux-gnu/ transmute_union.rs.ppc64.pwr9
[transmute_union.rs:10:5] unsafe { a.f } = 0x00000000000000000000000000000001
edit: but turn on -O and itβs borked again. Interesting.
from rust.
Any idea how to narrow further?
I'd define from_bits
locally in this crate and add more debugging to see if it's passing arguments to that function where things break, or returning them from that function.
from rust.
If we define f128 to be the corresponding IEEE float format, then the transmute is by definition correct. That float format defines the representation in memory and I think it's endianess must work out exactly the same as u128?
IEEE 754 does not specify memory layout. It just gives interpretations for 128-bit integers, but nothing prevents hardware from having, say, little-endian integer arithmetic and big-endian floating-point arithmetic. That said, AFAICT, PowerPC is big-endian for everything. More relevantly, however, PowerPC does not have 128-bit floating-point support, so this is being emulated in software anyway.
from rust.
Correction: Power9 does in fact have native IEEE 754 binary128 support, as part of VSX, according to its architecture manual. And indeed it is big endian. Older PowerPC doesn't, but I did see that the compilation command line did request Power9.
from rust.
0x00000000000000010000000000000000
would indicate some sort of weird mixed-endian format. So I can't believe that that's the actual intended format of anything, it looks more like a bug (possible more than one bug) in argument passing... and possible optimizer bugs as well, given that with -O
even the union-based variant breaks.
In fact maybe it's all an optimizer bug; from_bits
is from the stdlib after all, so built with optimizations.
from rust.
@ecnelises or @lu-zero any chance you have any ideas here? It is looking like this has to be a bug in LLVM, just don't know exactly what.
from rust.
I saw this message from Qiu Chaofan on Zulip, and thought I'd test whether the memory representation of the smallest positive f128 differs from the representation of 1 as a u128.
I got an ICE due to unimplemented stuff in const eval in Rust, so I had to resort to C (gcc, clang). However, there were no differences in the representation.
This isn't conclusive yet since the two groups of 4 bytes in the middle could still be swapped around, so I did another test with no repeats there (gcc, clang). Again, no differences.
It seems that both GCC and Clang assume that u128 is big endian, just like f128, even if the loads and stores in that case are conducted as two lots of 64 bits.
from rust.
even if the loads and stores in that case are conducted as two lots of 64 bits.
But that should be an implementation detail, right, and not observable? When the two halves are strung together to form f128, endianess needs to be taken into account properly.
I think the best starting point is figuring out why the optimizer breaks
#![feature(f128)]
union U {
i: u128,
f: f128,
}
fn main() {
let a = U { i: 0x1 };
dbg!(unsafe { a.f });
}
I see no way that's not an LLVM bug, so it might be worth reporting there (after some further minimization to remove the debug machinery from the equation).
from rust.
even if the loads and stores in that case are conducted as two lots of 64 bits.
But that should be an implementation detail, right, and not observable? When the two halves are strung together to form f128, endianess needs to be taken into account properly.
Yes, that's right. I was just checking whether u128 was stored with the lower 64 bits before the higher 64 bits. That would mess it up and look mixed endian. But that's not the case. That is, u128 is stored big endian, so it's definitely an LLVM bug.
from rust.
Something must be quite wonky on the LLVM side here, accidentally tried to compile the same thing for 32-bit ppc with the pwr9 target-cpu and it gives me a SIGILL. Not sure if pwr9 is even valid or not but an error message would have been nice :)
$ rustc transmute.rs --target powerpc-unknown-linux-gnu -C linker=powerpc-linux-gnu-gcc -Ctarget-cpu=pwr9 -o transmute.rs.ppc.pwr9
Illegal instruction
For reference:
rustc 1.80.0-nightly (9c9b56879 2024-05-05)
binary: rustc
commit-hash: 9c9b568792ef20d8459c745345dd3e79b7c7fa8c
commit-date: 2024-05-05
host: x86_64-unknown-linux-gnu
release: 1.80.0-nightly
LLVM version: 18.1.4
bt via gdb:
(gdb) file rustc
Reading symbols from rustc...
(gdb) run transmute.rs --target powerpc-unknown-linux-gnu -C linker=powerpc-linux-gnu-gcc -Ctarget-cpu=pwr9 -o transmute.rs.ppc.pwr9
Starting program: /home/tmgross/.cargo/bin/rustc transmute.rs --target powerpc-unknown-linux-gnu -C linker=powerpc-linux-gnu-gcc -Ctarget-cpu=pwr9 -o transmute.rs.ppc.pwr9
This GDB supports auto-downloading debuginfo from the following URLs:
<https://debuginfod.ubuntu.com>
Enable debuginfod for this session? (y or [n]) y
Debuginfod has been enabled.
To make this setting permanent, add 'set debuginfod enabled on' to .gdbinit.
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
process 8177 is executing new program: /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/bin/rustc
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
[New Thread 0x7fffe95ff6c0 (LWP 8189)]
[New Thread 0x7fffe8dff6c0 (LWP 8190)]
[New Thread 0x7fffe36a66c0 (LWP 8191)]
[New Thread 0x7fffe2fff6c0 (LWP 8192)]
[New Thread 0x7fffe27ff6c0 (LWP 8193)]
[New Thread 0x7fffe1fff6c0 (LWP 8194)]
[Thread 0x7fffe27ff6c0 (LWP 8193) exited]
Thread 7 "opt cgu.0" received signal SIGILL, Illegal instruction.
[Switching to Thread 0x7fffe1fff6c0 (LWP 8194)]
0x00007fffeee2a599 in llvm::PPCTargetLowering::LowerOperation(llvm::SDValue, llvm::SelectionDAG&) const ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
(gdb) b
Breakpoint 1 at 0x7fffeee2a599
(gdb) bt
#0 0x00007fffeee2a599 in llvm::PPCTargetLowering::LowerOperation(llvm::SDValue, llvm::SelectionDAG&) const ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#1 0x00007ffff0227d99 in llvm::TargetLowering::LowerOperationWrapper(llvm::SDNode*, llvm::SmallVectorImpl<llvm::SDValue>&, llvm::SelectionDAG&) const ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#2 0x00007ffff0c68ac0 in llvm::DAGTypeLegalizer::ExpandIntegerOperand(llvm::SDNode*, unsigned int) [clone .cold] ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#3 0x00007ffff0389b36 in llvm::DAGTypeLegalizer::run() [clone .warm] () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#4 0x00007fffefb71b09 in llvm::SelectionDAGISel::CodeGenAndEmitDAG() () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#5 0x00007fffefbd92c2 in llvm::SelectionDAGISel::SelectAllBasicBlocks(llvm::Function const&) () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#6 0x00007fffefb167ea in llvm::SelectionDAGISel::runOnMachineFunction(llvm::MachineFunction&) () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#7 0x00007fffeede2b1e in (anonymous namespace)::PPCDAGToDAGISel::runOnMachineFunction(llvm::MachineFunction&) ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#8 0x00007fffefb0ee14 in llvm::FPPassManager::runOnFunction(llvm::Function&) () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#9 0x00007fffefb0e285 in llvm::FPPassManager::runOnModule(llvm::Module&) () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#10 0x00007ffff0074ca0 in llvm::legacy::PassManagerImpl::run(llvm::Module&) () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/libLLVM.so.18.1-rust-1.80.0-nightly
#11 0x00007ffff6b58d10 in LLVMRustWriteOutputFile () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-c823c964cb628bbb.so
#12 0x00007ffff6b58928 in rustc_codegen_llvm::back::write::write_output_file () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-c823c964cb628bbb.so
#13 0x00007ffff6b565af in rustc_codegen_llvm::back::write::codegen () from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-c823c964cb628bbb.so
#14 0x00007ffff6b56285 in rustc_codegen_ssa::back::write::finish_intra_module_work::<rustc_codegen_llvm::LlvmCodegenBackend> ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-c823c964cb628bbb.so
#15 0x00007ffff6b59377 in std::sys_common::backtrace::__rust_begin_short_backtrace::<<rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::spawn_named_thread<rustc_codegen_ssa::back::write::spawn_work<rustc_codegen_llvm::LlvmCodegenBackend>::{closure#0}, ()>::{closure#0}, ()> ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-c823c964cb628bbb.so
#16 0x00007ffff6b58e5b in <<std::thread::Builder>::spawn_unchecked_<<rustc_codegen_llvm::LlvmCodegenBackend as rustc_codegen_ssa::traits::backend::ExtraBackendMethods>::spawn_named_thread<rustc_codegen_ssa::back::write::spawn_work<rustc_codegen_llvm::LlvmCodegenBackend>::{closure#0}, ()>::{closure#0}, ()>::{closure#2} as core::ops::function::FnOnce<()>>::call_once::{shim:vtable#0} ()
from /home/tmgross/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/librustc_driver-c823c964cb628bbb.so
#17 0x00007ffff19997bb in alloc::boxed::{impl#48}::call_once<(), dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global> () at library/alloc/src/boxed.rs:2022
#18 alloc::boxed::{impl#48}::call_once<(), alloc::boxed::Box<dyn core::ops::function::FnOnce<(), Output=()>, alloc::alloc::Global>, alloc::alloc::Global> () at library/alloc/src/boxed.rs:2022
#19 std::sys::pal::unix::thread::{impl#2}::new::thread_start () at library/std/src/sys/pal/unix/thread.rs:108
#20 0x00007ffff1760b5a in start_thread (arg=<optimized out>) at ./nptl/pthread_create.c:444
#21 0x00007ffff17f15fc in clone3 () at ../sysdeps/unix/sysv/linux/x86_64/clone3.S:78
Edit: reported this one at llvm/llvm-project#92233
from rust.
I at least was able to reduce the original issue enough to remove dbg!
but it could probably be reduced further. Reported to LLVM at llvm/llvm-project#92246
from rust.
Related Issues (20)
- `f16`/`f128` fallback code is not getting inlined HOT 4
- #[diagnostic::on_unimplemented] hint not shown for associated type mismatch HOT 4
- Unexpected trait bound not satisfied related to `std::io::Write` HOT 1
- Different behavior for macro by example and procedural macro HOT 1
- Unexpected comment position after macro expansion HOT 1
- Empty where clauses don't have an unused warning HOT 5
- Vec<T> implements PartialEq<[T; N]> but the other way round doesn't work HOT 1
- rustc can't find the linker when `--sysroot` is set to a local build of the sysroot HOT 14
- `async closure does not implement `FnMut` because it captures state from its environment` for async closures, while closure returning async block works fine HOT 1
- ICE: `assertion failed: unique` HOT 1
- Suboptimal interaction between lifetime oriented and value oriented type system in the presence of closures. HOT 1
- rustc should magically detect repeating refactoring errors and compress them
- Tracking Issue for `push_str` returning `self` HOT 3
- Exponential time complexity when type checking code with equality constraints HOT 1
- Suggest `Some(&v)` rather than `Some(v.clone())` when calling generic owning methods
- endless loop by running `cargo test` with toolchain 2024-05-14 HOT 4
- Call to function broken by unrelated constraint on generic function containing the call HOT 1
- It's not clear from the documentation when particular features were inctoduces or deprecated HOT 2
- pre-commit hook for `./x.py fmt` appears to not always know which files were changed.
- SIGSEGV when std::time::Instant::now() called HOT 3
Recommend Projects
-
React
A declarative, efficient, and flexible JavaScript library for building user interfaces.
-
Vue.js
π Vue.js is a progressive, incrementally-adoptable JavaScript framework for building UI on the web.
-
Typescript
TypeScript is a superset of JavaScript that compiles to clean JavaScript output.
-
TensorFlow
An Open Source Machine Learning Framework for Everyone
-
Django
The Web framework for perfectionists with deadlines.
-
Laravel
A PHP framework for web artisans
-
D3
Bring data to life with SVG, Canvas and HTML. πππ
-
Recommend Topics
-
javascript
JavaScript (JS) is a lightweight interpreted programming language with first-class functions.
-
web
Some thing interesting about web. New door for the world.
-
server
A server is a program made to process requests and deliver data to clients.
-
Machine learning
Machine learning is a way of modeling and interpreting data that allows a piece of software to respond intelligently.
-
Visualization
Some thing interesting about visualization, use data art
-
Game
Some thing interesting about game, make everyone happy.
Recommend Org
-
Facebook
We are working to build community through open source technology. NB: members must have two-factor auth.
-
Microsoft
Open source projects and samples from Microsoft.
-
Google
Google β€οΈ Open Source for everyone.
-
Alibaba
Alibaba Open Source for everyone
-
D3
Data-Driven Documents codes.
-
Tencent
China tencent open source team.
from rust.