[aarch64-port-dev ] RFR: Fix out by one in writing array barriers

[Edward Nevill]

Hi,

The following patch fixes some problems I am seeing with GC in the JTReg hotspot tests, specifically compiler/8010927/Test8010927

The problem seems to be an out by one error in gen_write_ref_array_post_barrier

The original code reads

           __ lsr(start, start, CardTableModRefBS::card_shift);
           __ add(end, end, BytesPerHeapOop);
           __ lsr(end, end, CardTableModRefBS::card_shift);
           __ sub(end, end, start); // number of bytes to copy

          const Register count = end; // 'end' register contains bytes count now
          __ mov(scratch, (address)ct->byte_map_base);
          __ add(start, start, scratch);
          __ BIND(L_loop);
          __ strb(zr, Address(start, count));
          __ subs(count, count, 1);
          __ br(Assembler::HI, L_loop);

This seems to me to be broken. The last store in the loop is always

strb zr, [start, 1]

because of the BHI. However it must store to [start, 0] because for each HeapOOP it must do

[base + [addr >> card_shift]] = 0

However in the above the last store is done to

[base + [addr >> card_shift] + 1] = 0

The end condition of the loop should therefore be BHS, not BHI to include 0.

However, this sometimes stores 1 too many (and sometimes not). The problem is the

add end, end, BytesPerHeapOop

which converts the inclusive pointer to an exclusive pointer. However this is not what we want. What we want is to store 0 in all the bytes in the range

[base + [start >> card_shift]] to [base + [end >> card_shift]]

The following patch fixes this.

OK to push?

Ed.

--- CUT HERE ---
exporting patch:
# HG changeset patch
# User Edward Nevill edward.nevill at linaro.org
# Date 1386852655 0
#      Thu Dec 12 12:50:55 2013 +0000
# Node ID 36ec6f5b872338684a26d353b77d7b747558281d
# Parent  3c620760454c2c4ea1f871d178e7ca8700bf92d3
Fix out by 1 errors in writing array barriers

diff -r 3c620760454c -r 36ec6f5b8723 src/cpu/aarch64/vm/stubGenerator_aarch64.cpp
--- a/src/cpu/aarch64/vm/stubGenerator_aarch64.cpp	Wed Dec 11 09:06:48 2013 +0000
+++ b/src/cpu/aarch64/vm/stubGenerator_aarch64.cpp	Thu Dec 12 12:50:55 2013 +0000
@@ -905,7 +905,6 @@
           Label L_loop;
 
            __ lsr(start, start, CardTableModRefBS::card_shift);
-           __ add(end, end, BytesPerHeapOop);
            __ lsr(end, end, CardTableModRefBS::card_shift);
            __ sub(end, end, start); // number of bytes to copy
 
@@ -915,7 +914,7 @@
 	  __ BIND(L_loop);
 	  __ strb(zr, Address(start, count));
           __ subs(count, count, 1);
-          __ br(Assembler::HI, L_loop);
+          __ br(Assembler::HS, L_loop);
         }
         break;
       default:
--- CUT HERE ---