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  1. JDK
  2. JDK-8256387

Unexpected result if patching an entire instruction on AArch64

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    • Subcomponent:
    • Resolved In Build:
      b26
    • CPU:
      aarch64
    • OS:
      generic

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        Description

        Shift operation is undefined if the shift count greater than or equals to the length in bits of the promoted left operand. https://stackoverflow.com/a/18918340/5262383

         

        Here is an example:

        static int test_shift(int nbit) { return 1U << nbit;}

        static int test_shift_32() {  return 1U << 32;}

        int main(int argc, char **argv) {
          int r;
          r = test_shift(32); 
          printf("%d\n", r);  // 1
          r = test_shift_32(); 
          printf("%d\n", r);  // 0
          return 0;
        }
         

        For c2, Instruction_aarch64::patch(https://github.com/openjdk/jdk/blob/master/src/hotspot/cpu/aarch64/assembler_aarch64.hpp#L218)&#xA0;shared the same problem. 

        // piece of code from assembler_aarch64.hpp
         static void patch(address a, int msb, int lsb, uint64_t val) {
            int nbits = msb - lsb + 1;
            guarantee(val < (1ULL << nbits), "Field too big for insn");
            assert_cond(msb >= lsb);
            unsigned mask = (1U << nbits) - 1;
            val <<= lsb;
            mask <<= lsb;
            unsigned target = *(unsigned *)a;
            target &= ~mask;
            target |= val;
            *(unsigned *)a = target;
          }
          When someone intends to patch an entire instruction with msb=31, lsb=0, there would be   an unexpected result.

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                Assignee:
                eliu Eric Liu
                Reporter:
                eliu Eric Liu
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                  Updated:
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