Waiting messages for a process may be stored in a queue outside of any heap or heap fragment belonging to the process. This is an optimization added in a recent major release to avoid garbage collection messages again and again if there is a long message queue. Until such message has been matched and accepted by the remove_message/0 instruction, the message must not be included in the root set for a garbage collection, as that would corrupt the message. The loop_rec/2 instruction explicitly turns off garbage collection of the process as long messages are being matched. However, if the compiler were to put references to a message outside of the heap in an Y register (on the stack) and there happened to be a GC when the process had been scheduled out, the message would be corrupted and the runtime system would crash sooner or later. To ensure that doesn't happen, teach beam_validator to check for references on the stack to messages outside of the heap.
Fix unsafe optimization of record test
beam_record would make an unsafe optimization for the
not_used_p/4 function added to beam_utils_SUITE in this
commit. The bug is in beam_utils, which would falsely
report that {x,4} was unused when it in fact was used.
The bug was in the function not_used/1. The purpose of
not_used/1 is to return a 'not_used' result unless the
actual result is 'used'. Unfortunately it was not
implemented in that way. It would let a 'transparent'
result slip through, which the caller in this case would
convert to 'killed' (because the register was killed on
all other paths).
Reported-by: Richard Carlsson
Compile external fun expressions to literals OTP-15003
Stop the compiler from overflowing the atom table
The expressions fun M:F/A, when all elements are literals are also treated as a literal. Since they have consistent representation and don't depend on the code currently loaded in the VM, this is safe. This can provide significant performance improvements in code using such functions extensively - a full function call to erlang:make_fun/3 is replaced by a single move instruction and no register shuffling or saving registers to stack is necessary. Additionally, compound data types that contain such external functions as elements can be treated as literals too. The commit also changes the representation of external funs to be a valid Erlang syntax and adds support for literal external funs to core Erlang.
Use integer variable names instead of atoms in v3_core, sys_core_fold, and v3_kernel to avoid overflowing the atom table. It is a deliberate design decision to calculate the first free integer variable name (in sys_core_fold and v3_kernel) instead of somehow passing it from one pass to another. I don't want that kind of dependency between compiler passes. Also note that the next free variable name is not easily available after running the inliner.
The way variables created by make_template() are used, it is necessary that the names are unique in the entire function. This has not happened to cause any problems in the past because all other compiler passes created atom variable names, not integer variable names. If other passes start to create integer variable names, this bug is exposed.
During compilation, the bs_save2 and bs_restore2 instructions contain a match context reference. That reference is the variable name that holds the match context. beam_clean assumes that the reference always is an atom, which is not a safe assumption since integers are legal variable names in Core Erlang.