tarantool / luajit / 16491528542

93%

tarantool/master: 93%

push

github

ARM64: Prevent STP fusion for conditional code emitted by TBAR.

Thanks to Peter Cawley.

(cherry picked from commit 7cc53f0b8)

Assume we have a trace for the several `setmetatable()` calls to the
same table. This trace contains the following IR:
| 0011          p64 FREF   0003  tab.meta
| ...
| 0018 x0    >  tab TNEW   0    0
| 0019          tab TBAR   0003
| 0020          tab FSTORE 0011  0018

The expected mcode to be emitted for the last two IRs is the following:
| 55626cffb0  ldrb  w30, [x19, 8] ; tab->marked
| 55626cffb4  tst   w30, 0x4      ; Is black?
| 55626cffb8  beq   0x626cffd0     ; Skip marking.
| 55626cffbc  ldr   x27, [x20, 128]
| 55626cffc0  and   w30, w30, 0xfffffffb
| 55626cffc4  str   x19, [x20, 128]
| 55626cffcc  strb  w30, [x19, 8]  ; tab->marked
| 55626cffc8  str   x27, [x19, 24] ; tab->gclist
| 55626cffd0  str   x0,  [x19, 32] ; tab->metatable

But the last 2 instructions are fused into the following `stp`:
| 55581dffd0  stp   x27, x0, [x19, 48]
Hence, the GC propagation frontier back is done partially, since
`str x27, [x19, 24]` is not skipped despite TBAR semantics. This leads
to the incorrect value in the `gclist` and the segmentation fault during
its traversal on GC step.

This patch prevents this fusion via switching instruction for
`tab->gclist` and `tab->marked` storing.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#11691

5710 of 6047 branches covered (94.43%)

Branch coverage included in aggregate %.

21783 of 23509 relevant lines covered (92.66%)

3874288.67 hits per line