Fix compacting GC bug when chaining tagged and non-tagged fields together

Authored by osa1 on Oct 24 2019, 4:11 AM.


Fix compacting GC bug when chaining tagged and non-tagged fields together

Currently compacting GC has the invariant that in a chain all fields are tagged
the same. However this does not really hold: root pointers are not tagged, so
when we thread a root we initialize a chain without a tag. When the pointed
objects is evaluated and we have more pointers to it from the heap, we then add
*tagged* fields to the chain (because pointers to it from the heap are tagged),
ending up chaining fields with different tags (pointers from roots are NOT
tagged, pointers from heap are). This breaks the invariant and as a result
compacting GC turns tagged pointers into non-tagged.

This later causes problem in the generated code where we do reads assuming that
the pointer is aligned, e.g.

0x7(%rax) -- assumes that pointer is tagged 1

which causes misaligned reads. This caused Trac #17088.

We fix this using the "pointer tagging for large families" patch (Trac #14373,

  • With the pointer tagging patch the GC can know what the tagged pointer to a CONSTR should be (previously we'd need to know the family size -- large families are always tagged 1, small families are tagged depending on the constructor).
  • Since we now know what the tags should be we no longer need to store the pointer tag in the info table pointers when forming chains in the compacting GC.

As a result we no longer need to tag pointers in chains with 1/2 depending on
whether the field points to an info table pointer, or to another field: an info
table pointer is always tagged 0, everything else in the chain is tagged 1. The
lost tags in pointers can be retrieved by looking at the info table.

Finally, instead of using tag 1 for fields and tag 0 for info table pointers, we
use two different tags for fields:

  • 1 for fields that have untagged pointers
  • 2 for fields that have tagged pointers

When unchaining we then look at the pointer to a field, and depending on its tag
we either leave a tagged pointer or an untagged pointer in the field.

This allows chaining untagged and tagged fields together in compacting GC.

Fixes Trac #17088


bgamariDec 6 2019, 5:22 PM
rGHCb911c53237b2: Implement pointer tagging for big families (#14373)