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document --many-seeds; set the default range to 0..64

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Ralf Jung 2024-06-13 20:02:02 +02:00
parent 4345379369
commit cfcea21074
3 changed files with 20 additions and 20 deletions
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32
src/tools/miri/README.md
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@ -151,6 +151,21 @@ platform. For example `cargo miri test --target s390x-unknown-linux-gnu`
will run your test suite on a big-endian target, which is useful for testing
endian-sensitive code.
### Testing multiple different executions
Certain parts of the execution are picked randomly by Miri, such as the exact base address
allocations are stored at and the interleaving of concurrently executing threads. Sometimes, it can
be useful to explore multiple different execution, e.g. to make sure that your code does not depend
on incidental "super-alignment" of new allocations and to test different thread interleavings.
This can be done with the `--many-seeds` flag:
```
cargo miri test --many-seeds # tries the seeds in 0..64
cargo miri test --many-seeds=0..16
```
The default of 64 different seeds is quite slow, so you probably want to specify a smaller range.
### Running Miri on CI
When running Miri on CI, use the following snippet to install a nightly toolchain with the Miri
@ -183,23 +198,6 @@ Here is an example job for GitHub Actions:
The explicit `cargo miri setup` helps to keep the output of the actual test step
clean.
### Testing for alignment issues
Miri can sometimes miss misaligned accesses since allocations can "happen to be"
aligned just right. You can use `-Zmiri-symbolic-alignment-check` to definitely
catch all such issues, but that flag will also cause false positives when code
does manual pointer arithmetic to account for alignment. Another alternative is
to call Miri with various values for `-Zmiri-seed`; that will alter the
randomness that is used to determine allocation base addresses. The following
snippet calls Miri in a loop with different values for the seed:
```
for SEED in $(seq 0 255); do
echo "Trying seed: $SEED"
MIRIFLAGS=-Zmiri-seed=$SEED cargo miri test || { echo "Failing seed: $SEED"; break; };
done
```
### Supported targets
Miri does not support all targets supported by Rust. The good news, however, is

4
src/tools/miri/cargo-miri/src/phases.rs
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@ -34,6 +34,8 @@ Examples:
";
const DEFAULT_MANY_SEEDS: &str = "0..64";
fn show_help() {
println!("{CARGO_MIRI_HELP}");
}
@ -171,7 +173,7 @@ pub fn phase_cargo_miri(mut args: impl Iterator<Item = String>) {
ArgSplitFlagValue::from_string_iter(&mut args, "--target-dir").filter_map(Result::err)
{
if arg == "--many-seeds" {
many_seeds = Some(format!("0..256"));
many_seeds = Some(DEFAULT_MANY_SEEDS.to_owned());
} else if let Some(val) = arg.strip_prefix("--many-seeds=") {
many_seeds = Some(val.to_owned());
} else {

4
src/tools/miri/miri-script/src/main.rs
View file

@ -98,7 +98,7 @@ Build miri, set up a sysroot and then run the test suite.
Build miri, set up a sysroot and then run the driver with the given <flags>.
(Also respects MIRIFLAGS environment variable.)
If `--many-seeds` is present, Miri is run many times in parallel with different seeds.
The range defaults to `0..256`.
The range defaults to `0..64`.
./miri fmt <flags>:
Format all sources and tests. <flags> are passed to `rustfmt`.
@ -180,7 +180,7 @@ fn main() -> Result<()> {
dep = true;
} else if args.get_long_flag("verbose")? || args.get_short_flag('v')? {
verbose = true;
} else if let Some(val) = args.get_long_opt_with_default("many-seeds", "0..256")? {
} else if let Some(val) = args.get_long_opt_with_default("many-seeds", "0..64")? {
let (from, to) = val.split_once("..").ok_or_else(|| {
anyhow!("invalid format for `--many-seeds`: expected `from..to`")
})?;