Functions sometimes disappear from coverage under MIR opts +Inline,+ConstProp

[Zalathar]

(Note that this is almost certainly a bug in coverage, not in MIR opts.)

Consider this coverage report, produced by x test run-coverage --bless on a new test file:

   LL|       |#![feature(coverage_attribute)]
   LL|       |// compile-flags: --edition=2021 -Copt-level=0 -Zmir-opt-level=1 -Zmir-enable-passes=+Inline,+ConstProp
   LL|       |
   LL|      8|#[derive(Debug, PartialEq, Eq)]
   LL|       |struct Foo(u32);
   LL|       |
   LL|      1|fn eq_good() {
   LL|      1|    assert_eq!(Foo(1), Foo(1));
   LL|      1|}
   LL|       |
   LL|      1|fn eq_good_message() {
   LL|      1|    assert_eq!(Foo(1), Foo(1), "message");
                                             ^0
   LL|      1|}
   LL|       |
   LL|       |fn ne_good() {
   LL|       |    assert_ne!(Foo(1), Foo(3));
   LL|       |}
   LL|       |
   LL|       |fn ne_good_message() {
   LL|       |    assert_ne!(Foo(1), Foo(3), "message");
   LL|       |}
   LL|       |
   LL|      1|fn eq_bad() {
   LL|      1|    assert_eq!(Foo(1), Foo(3));
   LL|      0|}
   LL|       |
   LL|       |fn eq_bad_message() {
   LL|       |    assert_eq!(Foo(1), Foo(3), "message");
   LL|       |}
   LL|       |
   LL|      1|fn ne_bad() {
   LL|      1|    assert_ne!(Foo(1), Foo(1));
   LL|      0|}
   LL|       |
   LL|      1|fn ne_bad_message() {
   LL|      1|    assert_ne!(Foo(1), Foo(1), "message");
   LL|      0|}
   LL|       |
   LL|       |#[coverage(off)]
   LL|       |fn main() {
   LL|       |    eq_good();
   LL|       |    eq_good_message();
   LL|       |    ne_good();
   LL|       |    ne_good_message();
   LL|       |
   LL|       |    let _ = std::panic::catch_unwind(eq_bad);
   LL|       |    let _ = std::panic::catch_unwind(eq_bad_message);
   LL|       |    let _ = std::panic::catch_unwind(ne_bad);
   LL|       |    let _ = std::panic::catch_unwind(ne_bad_message);
   LL|       |}

All of these functions should look roughly the same in the coverage report, but instead three of them (ne_good, ne_good_message, eq_bad_message) are completely missing, despite the fact that they are all executed.

The problem also occurs under -Copt-level=2 or -Zmir-opt-level=3, but I narrowed it down to just -Zmir-opt-level=1 plus the Inline and ConstProp passes. If I reduce the MIR opt level to 0, or remove either of those passes, I get the expected output where all functions are reported correctly.

[Zalathar]

@rustbot label +A-code-coverage

[Zalathar]

Mysteriously, if I run the same test file in tests/coverage-map, the mappings seem fine.

So far I haven't tracked down the precise differences between affected binaries and unaffected binaries.

[Zalathar]

The above test file was the result of reducing a much more complicated test file (tests/run-coverage/issue-84561.rs), in which the entire test3 function disappears from coverage.

Unlike my simplified examples above, that test3 function contains a bunch of print statements and also is responsible for panicking out of the entire program, which seems to rule out problems based entirely on small functions with no other effects.

[Zalathar]

OK, I think this is caused by the fact that map_data will emit expression operands containing counters that are not present in counters_seen.

[Zalathar]

Because I wasn't seeing anything missing/suspicious in the .cov-map files, I figured this might have something to do with llvm-cov.

I was able to apply printf debugging by:

• Adding #include <iostream> and std::cout << ... << std::endl; to the LLVM source to print extra information, which would then appear in the .cov-map files as part of the tool output.
• rm build/aarch64-apple-darwin/llvm/llvm-finished-building ; x build llvm to tell the build system to actually rebuild LLVM with my changes.