RFR 8229382 [lworld][c1] TestLWorld.java crashes with -XX:+UseSerialGC

Ioi Lam ioi.lam at oracle.com
Tue Aug 13 03:50:23 UTC 2019


https://bugs.openjdk.java.net/browse/JDK-8229382
http://cr.openjdk.java.net/~iklam/valhalla/8229382-c1-dont-cache-constants-in-conditional-code.v01/

Background
----------

When c1_LIRGenerator.cpp translates the HIR of a basic block to LIR, and it
loads a constant C into a virtual register R, it assumes that this must 
happen
for all possible paths through this basic block. Thus, it uses a cache
of {constant} -> {register} to avoid loading the same constant C again
in the same basic block. See LIRGenerator::load_constant().

However, when translating aastore, we end up having code like this:

  0x7f46c544a260: test $0x1,%bl            ; is this array flattened?
  0x7f46c544a263: jne 0x7f46c544a376       ; call store_flattened_array
  ....
  0x7f46c544a28b: mov $0x7f03335dc000,%rsi ; card_table_base
  0x7f46c544a295: movb $0x0,(%rax,%rsi,1)  ; mark card table

so the constant card_table_base is stored into %rsi only if the array
is not flattened.

When a subsequent aastore is translated, we mistakenly assume that
%rsi already contains the desired value. The code will crash when
using %rsi to mark the card table.


Fix
---

The fix is to avoid caching constants used in conditionally executed LIR.

I also added assertions so we don't call LIRItem::load_item() while
generating conditionally executed LIR. Otherwise we will create
{Value} -> {LIR_Opr} mappings (i.e., from HIR values to LIR virtual
registers) that are not always valid.

I looked at every line of "__ branch_destination" added by the
valhalla code and hopefully have caught all the bugs introduced by me.

Thanks
- Ioi



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