Previously, compile_fail and ignore code examples displayed a tooltip
indicating this in the documentation. This tooltip has now also been
added to should_panic examples.
[RISC-V] Do not force frame pointers
We have been seeing some very inefficient code that went away when using
`-Cforce-frame-pointers=no`. For instance `core::ptr::drop_in_place` at
`-Oz` was compiled into a function which consisted entirely of saving
registers to the stack, then using the frame pointer to restore the same
registers (without any instructions between the prolog and epilog).
The RISC-V LLVM backend supports frame pointer elimination, so it makes
sense to allow this to happen when using Rust. It's not clear to me that
frame pointers have ever been required in the general case.
In rust-lang/rust#61675 it was pointed out that this made reassembling
stack traces easier, which is true, but there is a code generation
option for forcing frame pointers, and I feel the default should not be
to require frame pointers, given it demonstrably makes code size worse
(around 10% in some embedded applications).
The kinds of targets mentioned in rust-lang/rust#61675 are popular, but
should not dictate that code generation should be worse for all RISC-V
targets, especially as there is a way to use CFI information to
reconstruct the stack when the frame pointer is eliminated. It is also
a misconception that `fp` is always used for the frame pointer. `fp` is
an ABI name for `x8` (aka `s0`), and if no frame pointer is required,
`x8` may be used for other callee-saved values.
---
I am partly posting this to get feedback from @fintelia who introduced the change to require frame pointers, and @hanna-kruppe who had issues with the original PR. I would understand if we wanted to remove this setting on only a subset of RISC-V targets, but my preference would be to remove this setting everywhere.
There are more details on the code size savings seen in Tock here: https://github.com/tock/tock/pull/1660
If config.toml `profiler = false`, the test/mir-opt/instrument_coverage
test is ignored. Otherwise, this patch ensures the profiler_runtime is
loaded when -Zinstrument-coverage is enabled. Confirmed that this works
for MacOS.
Update LLVM submodule
Only includes one commit:
- [D80759](https://reviews.llvm.org/D80759): Fix FastISel dropping srcloc metadata from InlineAsm
Fixes#40555
Rollup of 8 pull requests
Successful merges:
- #73237 (Check for overflow in DroplessArena and align returned memory)
- #73339 (Don't run generator transform when there's a TyErr)
- #73372 (Re-order correctly the sections in the sidebar)
- #73373 (Use track caller for bug! macro)
- #73380 (Add more info to `x.py build --help` on default value for `-j JOBS`.)
- #73381 (Fix typo in docs of std::mem)
- #73389 (Use `Ipv4Addr::from<[u8; 4]>` when possible)
- #73400 (Fix forge-platform-support URL)
Failed merges:
r? @ghost
