myc_llvm_safepoints

Crystal shard: optional LLVM pointer safepoints for a moving garbage collector on top of kostya/myc’s myc-llvm. Conservative / non-moving collectors do not need this.

It delivers a shard with open classes, required from src/cli/llvm.cr.

Optimization levels

Safepoints and LLVM optimization levels are independent. Declaring a root hook (or passing --gc-root) does not force --debug, does not add optnone, and does not replace myc’s pass pipeline. Daily compiles use myc-llvm default. Release compiles use --final. Both emit gc.statepoint when safepoints are on.

--debug is the emergency / unoptimized path, not a safepoint requirement. This shard’s --debug uses CodeGenOptLevel::None; stock myc-llvm --debug still uses Default codegen. That difference exists only for --debug.

Safepoints add these LLVM constraints under every flag, including default and --final. They are not “no optimizations”:

• FastISel off (it SIGSEGVs on gc.statepoint)
• gc "statepoint-example" and frame-pointer=all
• writable .llvm_stackmaps (SHF_WRITE) so a PIE has no DT_TEXTREL

MYC_LLVM_PASSES still overrides the pass pipeline, as in myc.

Consume

Library - in a stock myc tree:

# myc/shard.yml
dependencies:
  myc_llvm_safepoints:
    github: avant-lang/safepoints

# myc/src/cli/llvm.cr - after require "../backend/llvm/all"
require "myc_llvm_safepoints"

require "myc_llvm_safepoints" loads myc’s LLVM backend and then reopens those classes. It does not vendor myc. Overrides use Crystal previous_def and extra instance variables so myc can change its methods without this shard copying them (kostya/myc#10).

Then crystal build src/cli/llvm.cr -o myc-llvm as usual.

Binary (this repo) - does not edit myc:

shards install
crystal build src/cli.cr -o myc-llvm

Needs Crystal ≥ 1.19 and LLVM ≥ 15 (llvm-config on PATH, or LLVM_CONFIG).

How it turns on

Off unless the module (or header) declares the root hook (FUNC :gc_root by default, or --gc-root=NAME).

./myc-llvm c prog.myc out --gc-root=gc_root --gc-reload=gc_reload --gc-enter=gc_enter --gc-leave=gc_leave

--key=value is enough; myc’s CLI already stores unknown flags. --gc-root NAME (space form) is not registered in stock myc; use =.

Non-collecting CALL: printf, memset, memcpy, memmove, malloc, calloc, free, the hook names, and --gc-leaf. Everything else collects.

IR ATTRIBUTES ATTR :leaf is not part of this shard (it would need a myc core enum member). Use --gc-leaf.

QBE and C backends are not loaded by this CLI; they stay stock myc.

Tests

crystal spec

• gc.statepoint when gc_root is declared, and not without it
• --gc-root and --no-gc-safepoints
• --gc-leaf skips the statepoint
• object files have writable .llvm_stackmaps
• opt levels: default and --final are not forced to optnone / default<O0> when gc_root is present; --debug may be O0; --no-gc-safepoints --final matches stock --final

What this is not

• Not a myc fork. myc stays unmodified.
• Not QBE/C safepoints.
• After each gc.statepoint, gc.relocate results are stored back into the rooted allocas (volatile) so default / --final passes cannot keep a pre-CALL pointer SSA. Avant’s copying nursery is the proof case; --debug remains the emergency belt.