Default `repr(C)` enums to `c_int` size #107592

workingjubilee · 2023-02-02T11:17:38Z

This is what ISO C strongly implies this is correct, and
many processor-specific ABIs imply or mandate this size, so
"everyone" (LLVM, gcc...) defaults to emitting enums this way.
However, this is by no means guaranteed by ISO C,
and the bare-metal Arm targets show it can be overridden,
which rustc supports via c-enum-min-bits in a target.json.

The override is a flag named -fshort-enums in clang and gcc,
but introducing a CLI flag is probably unnecessary for rustc.
This flag can be used by non-Arm microcontroller targets,
like AVR and MSP430, but it is not enabled for them by default.
Rust programmers who know the size of a target's enums
can use explicit reprs, which also lets them match C23 code.

This change is most relevant to 16-bit targets: AVR and MSP430.
Most of rustc's targets use 32-bit ints, but ILP64 does exist.
Regardless, rustc should now correctly handle enums for
both very small and very large targets.

Thanks to William for confirming MSP430 behavior,
and to Waffle for better style and no-core size_of asserts.

Fixes #107361
Fixes #77806

rustbot · 2023-02-02T11:17:45Z

r? @lcnr

(rustbot has picked a reviewer for you, use r? to override)

rustbot · 2023-02-02T11:17:48Z

These commits modify compiler targets.
(See the Target Tier Policy.)

lcnr · 2023-02-02T12:37:38Z

r? compiler

workingjubilee · 2023-02-04T04:09:43Z

cc @Rahix re: vibe-checking this AVR-related PR since it will affect the pile of target-spec.jsons that you maintain in avr-hal.

Rahix · 2023-02-06T12:32:59Z

Change looks reasonable to me. Re. target-specs, we derive ours from the upstream one using a small python script. I'll just re-synchronize once we update to a more recent nightly. But for now we are still blocked on #106619, anyway...

jackh726 · 2023-02-10T01:17:46Z

Sorry, not my area of expertise...

r? compiler

cjgillot · 2023-02-11T14:26:28Z

I don't know anything about AVR either. @workingjubilee do you have somebody in mind for the review?
@jonas-schievink I saw you approved #76364, can you help?

workingjubilee · 2023-02-13T02:24:01Z

Jonas is an alumnus (and thank you).

I believe for one reason or another a lot of the people who are very active contributors regarding these targets (e.g. @dylanmckay, @cr1901, @Patryk27) are usually not formal Rust team members and of those who are more active with the Embedded WG they don't necessarily have a ton of active review cycles. Which is why I just tagged in Rahix.

WaffleLapkin

I'm not entirely sure what are the implications here, but from what I could find this seems like a correct solution.

I think this will be good to go after a couple of nitpicks and doc fixes.

r? @WaffleLapkin

compiler/rustc_target/src/spec/mod.rs

tests/ui/repr/16-bit-repr-c-enum.rs

cr1901 · 2023-02-13T17:34:24Z

people who are very active contributors regarding these targets... are usually not formal Rust team members

I'm not aware of the process of becoming a formal team member for maintaining msp430. I just open bug reports and bisect when I notice the compiler pessimizes or otherwise breaks msp430 codegen. I also have little bandwidth to fix LLVM errors at present. Is this enough to be considered a maintainer if I add target-specific documentation?

Anyways, this PR looks fine to me. The below sample code compiles on msp430-elf-gcc (will test clang once I get past my build scripts getting a bit stale, but I can't imagine that the behavior would differ...)

Makefile

include include.mk

all: enum.elf

elf-info: enum.elf
        msp430-elf-objdump -D enum.elf > enum.lst
        msp430-elf-readelf -a --wide enum.elf > enum.sym
        msp430-elf-size -A enum.elf

enum.S: enum.c
        $(CC) -c $(CFLAGS) -S -o enum.S -I$(MSPINC) enum.c

enum.o: enum.S
        $(CC) -c $(CFLAGS) -o enum.o -I$(MSPINC) enum.S

enum.elf: enum.o
        $(CC) $(LDFLAGS) -o enum.elf -L$(MSPINC) enum.o

prog: enum.elf
        mspdebug $(MSPDRV) "prog enum.elf"

clean:
        rm -rf enum.elf enum.o enum.lst enum.sym enum.S

include.mk.sample

MSPINC=/path/to/include # https://github.com/pftbest/msp430_svd
MSPMCU=msp430g2211 # Define to desired micro to build on.
MSPDRV=rf2500 # tilib, etc. Input to mspdebug.

# GCC invocation
CC=msp430-elf-gcc
CFLAGS=-Os -mmcu=$(MSPMCU)
LDFLAGS=-mcpu=msp430 -Wl,-T$(MSPMCU).ld # Work around possible driver bug with -mcpu=msp430.
# -mmcu=$(MSPMCU) also works in place of both these options.

# Clang invocation- requires existing GCC.
# GCC_TOOLCHAIN=/path/to/gcc/toolchain # This can't be automated if not installing clang.
# CC=clang --target=msp430 --gcc-toolchain=$(GCC_TOOLCHAIN) # Or /path/to/clang if using clang's build directory.
# CFLAGS=-Os -mmcu=$(MSPMCU)
# LDFLAGS=-Wl,-T$(MSPMCU).ld # No such option -mcpu=msp430.
# -mmcu=$(MSPMCU) also works in place of providing a linker script.

enum.c

#include <assert.h>

enum foo
{
    BAR
};


int main(void)
{
    static_assert(sizeof(enum foo) == 2);
}

workingjubilee · 2023-02-15T19:06:04Z

Fixup c_enum_min_bits to always work like that, not just when pulled from a target.json

WaffleLapkin

A couple of tiny nits and I think this is good to go, r=me when ready.

compiler/rustc_target/src/spec/mod.rs

This is what ISO C strongly implies this is correct, and many processor-specific ABIs imply or mandate this size, so "everyone" (LLVM, gcc...) defaults to emitting enums this way. However, this is by no means guaranteed by ISO C, and the bare-metal Arm targets show it can be overridden, which rustc supports via `c-enum-min-bits` in a target.json. The override is a flag named `-fshort-enums` in clang and gcc, but introducing a CLI flag is probably unnecessary for rustc. This flag can be used by non-Arm microcontroller targets, like AVR and MSP430, but it is not enabled for them by default. Rust programmers who know the size of a target's enums can use explicit reprs, which also lets them match C23 code. This change is most relevant to 16-bit targets: AVR and MSP430. Most of rustc's targets use 32-bit ints, but ILP64 does exist. Regardless, rustc should now correctly handle enums for both very small and very large targets. Thanks to William for confirming MSP430 behavior, and to Waffle for better style and no-core size_of asserts. Co-authored-by: William D. Jones <thor0505@comcast.net> Co-authored-by: Waffle Maybe <waffle.lapkin@gmail.com>

workingjubilee · 2023-02-17T00:37:13Z

@bors r=WaffleLapkin

bors · 2023-02-17T00:37:15Z

📌 Commit 2edf6c8 has been approved by WaffleLapkin

It is now in the queue for this repository.

…iaskrgr Rollup of 6 pull requests Successful merges: - rust-lang#107592 (Default `repr(C)` enums to `c_int` size) - rust-lang#107956 (Copy `bin/*` and `lib/*.dylib` files to `stage0-sysroot`) - rust-lang#108126 (fix a line, and do a consistency fix) - rust-lang#108144 (Add compiler-errors to a few more triagebot groups) - rust-lang#108149 (typo) - rust-lang#108154 (`BasicBlock::new(0)` -> `START_BLOCK` [no functional changes]) Failed merges: r? `@ghost` `@rustbot` modify labels: rollup

Pkgsrc changes: * Adjust patches and cargo checksums to new versions. * Sadly, the patch to reduce the cargo verbosity no longer applies, so I've asked upstream about the proper way to get the old result. (so the build log becomes Quite Bloated for now). Upstream changes: Version 1.69.0 (2023-04-20) ========================== Language -------- - [Deriving built-in traits on packed structs works with `Copy` fields.] (rust-lang/rust#104429) - [Stabilize the `cmpxchg16b` target feature on x86 and x86_64.] (rust-lang/rust#106774) - [Improve analysis of trait bounds for associated types.] (rust-lang/rust#103695) - [Allow associated types to be used as union fields.] (rust-lang/rust#106938) - [Allow `Self: Autotrait` bounds on dyn-safe trait methods.] (rust-lang/rust#107082) - [Treat `str` as containing `[u8]` for auto trait purposes.] (rust-lang/rust#107941) Compiler -------- - [Upgrade `*-pc-windows-gnu` on CI to mingw-w64 v10 and GCC 12.2.] (rust-lang/rust#100178) - [Rework min_choice algorithm of member constraints.] (rust-lang/rust#105300) - [Support `true` and `false` as boolean flags in compiler arguments.] (rust-lang/rust#107043) - [Default `repr(C)` enums to `c_int` size.] (rust-lang/rust#107592) Libraries --------- - [Implement the unstable `DispatchFromDyn` for cell types, allowing downstream experimentation with custom method receivers.] (rust-lang/rust#97373) - [Document that `fmt::Arguments::as_str()` may return `Some(_)` in more cases after optimization, subject to change.] (rust-lang/rust#106823) - [Implement `AsFd` and `AsRawFd` for `Rc`.] (rust-lang/rust#107317) Stabilized APIs --------------- - [`CStr::from_bytes_until_nul`] (https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html#method.from_bytes_until_nul) - [`core::ffi::FromBytesUntilNulError`] (https://doc.rust-lang.org/stable/core/ffi/struct.FromBytesUntilNulError.html) These APIs are now stable in const contexts: - [`SocketAddr::new`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.new) - [`SocketAddr::ip`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.ip) - [`SocketAddr::port`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.port) - [`SocketAddr::is_ipv4`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv4) - [`SocketAddr::is_ipv6`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv6) - [`SocketAddrV4::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.new) - [`SocketAddrV4::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.ip) - [`SocketAddrV4::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.port) - [`SocketAddrV6::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.new) - [`SocketAddrV6::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.ip) - [`SocketAddrV6::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.port) - [`SocketAddrV6::flowinfo`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.flowinfo) - [`SocketAddrV6::scope_id`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.scope_id) Cargo ----- - [Cargo now suggests `cargo fix` or `cargo clippy --fix` when compilation warnings are auto-fixable.] (rust-lang/cargo#11558) - [Cargo now suggests `cargo add` if you try to install a library crate.] (rust-lang/cargo#11410) - [Cargo now sets the `CARGO_BIN_NAME` environment variable also for binary examples.] (rust-lang/cargo#11705) Rustdoc ----- - [Vertically compact trait bound formatting.] (rust-lang/rust#102842) - [Only include stable lints in `rustdoc::all` group.] (rust-lang/rust#106316) - [Compute maximum Levenshtein distance based on the query.] (rust-lang/rust#107141) - [Remove inconsistently-present sidebar tooltips.] (rust-lang/rust#107490) - [Search by macro when query ends with `!`.] (rust-lang/rust#108143) Compatibility Notes ------------------- - [The `rust-analysis` component from `rustup` now only contains a warning placeholder.] (rust-lang/rust#101841) This was primarily intended for RLS, and the corresponding `-Zsave-analysis` flag has been removed from the compiler as well. - [Unaligned references to packed fields are now a hard error.] (rust-lang/rust#102513) This has been a warning since 1.53, and denied by default with a future-compatibility warning since 1.62. - [Update the minimum external LLVM to 14.] (rust-lang/rust#107573) - [Cargo now emits errors on invalid characters in a registry token.] (rust-lang/cargo#11600) - [When `default-features` is set to false of a workspace dependency, and an inherited dependency of a member has `default-features = true`, Cargo will enable default features of that dependency.] (rust-lang/cargo#11409) - [Cargo denies `CARGO_HOME` in the `[env]` configuration table. Cargo itself doesn't pick up this value, but recursive calls to cargo would, which was not intended.] (rust-lang/cargo#11644) - [Debuginfo for build dependencies is now off if not explicitly set. This is expected to improve the overall build time.] (rust-lang/cargo#11252) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Move `format_args!()` into AST (and expand it during AST lowering)] (rust-lang/rust#106745)

Pkgsrc changes: * Adjust patches and cargo checksums to new versions. Upstream changes: Version 1.69.0 (2023-04-20) ========================== Language -------- - [Deriving built-in traits on packed structs works with `Copy` fields.] (rust-lang/rust#104429) - [Stabilize the `cmpxchg16b` target feature on x86 and x86_64.] (rust-lang/rust#106774) - [Improve analysis of trait bounds for associated types.] (rust-lang/rust#103695) - [Allow associated types to be used as union fields.] (rust-lang/rust#106938) - [Allow `Self: Autotrait` bounds on dyn-safe trait methods.] (rust-lang/rust#107082) - [Treat `str` as containing `[u8]` for auto trait purposes.] (rust-lang/rust#107941) Compiler -------- - [Upgrade `*-pc-windows-gnu` on CI to mingw-w64 v10 and GCC 12.2.] (rust-lang/rust#100178) - [Rework min_choice algorithm of member constraints.] (rust-lang/rust#105300) - [Support `true` and `false` as boolean flags in compiler arguments.] (rust-lang/rust#107043) - [Default `repr(C)` enums to `c_int` size.] (rust-lang/rust#107592) Libraries --------- - [Implement the unstable `DispatchFromDyn` for cell types, allowing downstream experimentation with custom method receivers.] (rust-lang/rust#97373) - [Document that `fmt::Arguments::as_str()` may return `Some(_)` in more cases after optimization, subject to change.] (rust-lang/rust#106823) - [Implement `AsFd` and `AsRawFd` for `Rc`.] (rust-lang/rust#107317) Stabilized APIs --------------- - [`CStr::from_bytes_until_nul`] (https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html#method.from_bytes_until_nul) - [`core::ffi::FromBytesUntilNulError`] (https://doc.rust-lang.org/stable/core/ffi/struct.FromBytesUntilNulError.html) These APIs are now stable in const contexts: - [`SocketAddr::new`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.new) - [`SocketAddr::ip`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.ip) - [`SocketAddr::port`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.port) - [`SocketAddr::is_ipv4`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv4) - [`SocketAddr::is_ipv6`] (https://doc.rust-lang.org/stable/std/net/enum.SocketAddr.html#method.is_ipv6) - [`SocketAddrV4::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.new) - [`SocketAddrV4::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.ip) - [`SocketAddrV4::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV4.html#method.port) - [`SocketAddrV6::new`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.new) - [`SocketAddrV6::ip`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.ip) - [`SocketAddrV6::port`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.port) - [`SocketAddrV6::flowinfo`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.flowinfo) - [`SocketAddrV6::scope_id`] (https://doc.rust-lang.org/stable/std/net/struct.SocketAddrV6.html#method.scope_id) Cargo ----- - [Cargo now suggests `cargo fix` or `cargo clippy --fix` when compilation warnings are auto-fixable.] (rust-lang/cargo#11558) - [Cargo now suggests `cargo add` if you try to install a library crate.] (rust-lang/cargo#11410) - [Cargo now sets the `CARGO_BIN_NAME` environment variable also for binary examples.] (rust-lang/cargo#11705) Rustdoc ----- - [Vertically compact trait bound formatting.] (rust-lang/rust#102842) - [Only include stable lints in `rustdoc::all` group.] (rust-lang/rust#106316) - [Compute maximum Levenshtein distance based on the query.] (rust-lang/rust#107141) - [Remove inconsistently-present sidebar tooltips.] (rust-lang/rust#107490) - [Search by macro when query ends with `!`.] (rust-lang/rust#108143) Compatibility Notes ------------------- - [The `rust-analysis` component from `rustup` now only contains a warning placeholder.] (rust-lang/rust#101841) This was primarily intended for RLS, and the corresponding `-Zsave-analysis` flag has been removed from the compiler as well. - [Unaligned references to packed fields are now a hard error.] (rust-lang/rust#102513) This has been a warning since 1.53, and denied by default with a future-compatibility warning since 1.62. - [Update the minimum external LLVM to 14.] (rust-lang/rust#107573) - [Cargo now emits errors on invalid characters in a registry token.] (rust-lang/cargo#11600) - [When `default-features` is set to false of a workspace dependency, and an inherited dependency of a member has `default-features = true`, Cargo will enable default features of that dependency.] (rust-lang/cargo#11409) - [Cargo denies `CARGO_HOME` in the `[env]` configuration table. Cargo itself doesn't pick up this value, but recursive calls to cargo would, which was not intended.] (rust-lang/cargo#11644) - [Debuginfo for build dependencies is now off if not explicitly set. This is expected to improve the overall build time.] (rust-lang/cargo#11252) Internal Changes ---------------- These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools. - [Move `format_args!()` into AST (and expand it during AST lowering)] (rust-lang/rust#106745)

rustbot assigned lcnr Feb 2, 2023

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[AVR] Changing a struct size changes program behaviour #107293

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workingjubilee changed the title ~~Use 16-bit repr(C) enums on AVR, MSP430~~ Default repr(C) enums to c_int size Feb 17, 2023

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Rollup of 6 pull requests #108159

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LucasSte mentioned this pull request Feb 23, 2024

[SOL] Set 32 bits for SBF minimum enum size anza-xyz/rust#90

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Default `repr(C)` enums to `c_int` size #107592

Default `repr(C)` enums to `c_int` size #107592

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Default repr(C) enums to c_int size #107592

Default repr(C) enums to c_int size #107592

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Makefile

include.mk.sample

enum.c

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Default `repr(C)` enums to `c_int` size #107592

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