[pull] master from ruby:master#790
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Similar to the way ZJIT already folds +, -, and * operations. One
complication is that the % operator behaves differently in Ruby than in
Rust for negative values. For example in Ruby:
```
ruby -e "puts 11 % -3" # => -1
```
vs Rust:
```
println!("{}", 11 % -3); // => 2
```
Rust has the `#rem_euclid()` method; however, it also differs from Ruby.
My solution was to call into the cruby #rb_fix_mod_fix method to get the
correct behaviour.
Note that any `% 0` operations are not folded as they need to raise a
ZeroDivisionError.
One concern was an integer overflow. Fortunately that's not possible.
Given any two Fixnums `a`, and `b`, by definition the result of `a % b`
must be closer to 0 than `b`, making an overflow impossible.
One adjacent edge case is when calculating `RUBY_FIXNUM_MIN % -1`. The
result is 0 which is obviously fine; however, the related result of
`RUBY_FIXNUM_MIN / -1` would overflow. The #rb_fix_mod_fix method avoids
this problem by not calculating the `div` portion. See:
https://github.com/ruby/ruby/blob/07f5126c104507ec70c5bfb2eb5204ceb22b7ebd/internal/fixnum.h#L164
When --zjit-trace-exits is enabled, rb_zjit_record_exit_stack was called before compile_exit restored the VM state (cfp->pc, cfp->sp, stack, locals). This ccall clobbers caller-saved registers that may hold stack/local operands, causing garbage values to be written to the VM stack when compile_exit runs afterward. This led to crashes like `klass: T_FALSE` in the interpreter when those corrupted values were used as receivers. Fix by splitting compile_exit into compile_exit_save_state and compile_exit_return, and emitting the full exit inline when recording: save VM state first, then call rb_zjit_record_exit_stack (so rb_profile_frames sees valid cfp->pc), then return. Before this I was able to cause a crash with ``` ruby --zjit-trace-exits -rffi -e1 ```
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