Cycle collector is still turned off

[marijnh]

maybe_cc in rust_cc.cpp contains this:

    // FIXME: Needs a snapshot.
#if 0
    if (task->cc_counter++ > RUST_CC_FREQUENCY) {
        task->cc_counter = 0;
        do_cc(task);
    }
#endif

Removing the #if causes the stage1 compiler to crash with the following backtrace:

#0  0x00007ffff6026d05 in *__GI_raise (sig=6) at ../nptl/sysdeps/unix/sysv/linux/raise.c:64
#1  0x00007ffff602aab6 in *__GI_abort () at abort.c:92
#2  0x00007ffff69d7b5c in walk_vec2 (is_pod=<optimized out>, this=0x7ffff3f1b950, sp_size=<optimized out>) at ./src/rt/rust_cc.cpp:315
#3  walk_vec1 (sp_size=5, is_pod=true, this=0x7ffff3f1b950) at ./src/rt/rust_shape.h:878
#4  shape::ctxt<shape::data<cc::mark, shape::ptr> >::walk_vec0 (this=0x7ffff3f1b950) at ./src/rt/rust_shape.h:440
#5  0x00007ffff69d763e in walk_box_contents2 (box_dp=..., sub=..., this=<optimized out>) at ./src/rt/rust_cc.cpp:383
#6  shape::data<cc::mark, shape::ptr>::walk_box_contents1 (this=<optimized out>) at ./src/rt/rust_shape.h:947
#7  0x00007ffff69d77b2 in walk_box2 (this=0x7ffff3f1ba10) at ./src/rt/rust_cc.cpp:330
#8  walk_iface2 (this=0x7ffff3f1ba10) at ./src/rt/rust_cc.cpp:363
#9  walk_iface1 (this=0x7ffff3f1ba10) at ./src/rt/rust_shape.h:895
#10 shape::ctxt<shape::data<cc::mark, shape::ptr> >::walk (this=0x7ffff3f1ba10) at ./src/rt/rust_shape.h:381
#11 0x00007ffff69d7b88 in walk_struct2 (end_sp=0x7ffff7443c5d "\r\016", this=0x7ffff3f1ba10) at ./src/rt/rust_cc.cpp:388
#12 walk_struct1 (end_sp=0x7ffff7443c5d "\r\016", this=0x7ffff3f1ba10) at ./src/rt/rust_shape.h:874
#13 shape::ctxt<shape::data<cc::mark, shape::ptr> >::walk_struct0 (this=0x7ffff3f1ba10) at ./src/rt/rust_shape.h:510
#14 0x00007ffff69d763e in walk_box_contents2 (box_dp=..., sub=..., this=<optimized out>) at ./src/rt/rust_cc.cpp:383
#15 shape::data<cc::mark, shape::ptr>::walk_box_contents1 (this=<optimized out>) at ./src/rt/rust_shape.h:947
#16 0x00007ffff69d767e in walk_box2 (this=0x7ffff3f1bad0) at ./src/rt/rust_cc.cpp:330
#17 walk_box1 (this=0x7ffff3f1bad0) at ./src/rt/rust_shape.h:881
#18 shape::ctxt<shape::data<cc::mark, shape::ptr> >::walk_box0 (this=0x7ffff3f1bad0) at ./src/rt/rust_shape.h:488
#19 0x00007ffff69d7b88 in walk_struct2 (end_sp=0x7ffff7443c5d "\r\016", this=0x7ffff3f1bad0) at ./src/rt/rust_cc.cpp:388
#20 walk_struct1 (end_sp=0x7ffff7443c5d "\r\016", this=0x7ffff3f1bad0) at ./src/rt/rust_shape.h:874
#21 shape::ctxt<shape::data<cc::mark, shape::ptr> >::walk_struct0 (this=0x7ffff3f1bad0) at ./src/rt/rust_shape.h:510
#22 0x00007ffff69d763e in walk_box_contents2 (box_dp=..., sub=..., this=<optimized out>) at ./src/rt/rust_cc.cpp:383
#23 shape::data<cc::mark, shape::ptr>::walk_box_contents1 (this=<optimized out>) at ./src/rt/rust_shape.h:947
#24 0x00007ffff69d767e in walk_box2 (this=0x7ffff3f1bba0) at ./src/rt/rust_cc.cpp:330
#25 walk_box1 (this=0x7ffff3f1bba0) at ./src/rt/rust_shape.h:881
#26 shape::ctxt<shape::data<cc::mark, shape::ptr> >::walk_box0 (this=0x7ffff3f1bba0) at ./src/rt/rust_shape.h:488
#27 0x00007ffff69d7b88 in walk_struct2 (end_sp=0x7ffff7443c6f "", this=0x7ffff3f1bba0) at ./src/rt/rust_cc.cpp:388
#28 walk_struct1 (end_sp=0x7ffff7443c6f "", this=0x7ffff3f1bba0) at ./src/rt/rust_shape.h:874
#29 shape::ctxt<shape::data<cc::mark, shape::ptr> >::walk_struct0 (this=0x7ffff3f1bba0) at ./src/rt/rust_shape.h:510
#30 0x00007ffff69d683f in cc::mark::do_mark (task=0x626170, roots=..., marked=...) at ./src/rt/rust_cc.cpp:444
#31 0x00007ffff69d6e31 in cc::do_cc (task=0x626170) at ./src/rt/rust_cc.cpp:654
#32 0x00007ffff69c84b8 in upcall_s_malloc (args=0x6551d0) at ./src/rt/rust_upcall.cpp:126
#33 0x00007ffff69dc139 in __morestack () from /home/marijn/src/rust/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustrt.so
#34 0x00007ffff69c8878 in call_shim_on_c_stack (fn_ptr=0x7ffff69c8450, args=0x6551d0, this=<optimized out>) at ./src/rt/arch/x86_64/context.h:59
#35 call_upcall_on_c_stack (fn_ptr=0x7ffff69c8450, args=0x6551d0) at ./src/rt/rust_upcall.cpp:52
#36 upcall_malloc (td=<optimized out>) at ./src/rt/rust_upcall.cpp:144
#37 0x00007ffff71be06b in syntax::parse::parser::mk_expr::_6c35a4f2e274aebe ()
   from /home/marijn/src/rust/x86_64-unknown-linux-gnu/stage1/bin/../lib/librustc-4171d83aef249987-0.1.so
#38 0x00007ffff71be9d2 in syntax::parse::parser::mk_pexpr::_f82e5d709d2cbe83 ()
[more]

[nikomatsakis]

I've been wanting to look into this... the last time we talked about it, it seemed like there might some fundamental problem with collecting in that we could not usefully establish safe points. But after reading the code more, I see that we always zero out all structures on the heap, so I think that cc ought to work.

[nikomatsakis]

(Though of course I'd prefer not to zero out all boxes upon allocation if we could avoid it.)

[catamorphism]

Cycle collector has been removed, closing.