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

Author: RalfJung

src/tools/miri/README.md

@@ -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

src/tools/miri/cargo-miri/src/phases.rs

@@ -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 {

src/tools/miri/miri-script/src/main.rs

@@ -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`")
                     })?;