RFR: 8263425: AArch64: two potential bugs in C1 LIRGenerator::generate_address()

[Nick Gasson]

Around line 177 we have:

  LIR_Opr tmp = new_pointer_register();
  if (Assembler::operand_valid_for_add_sub_immediate(large_disp)) {
    __ add(tmp, tmp, LIR_OprFact::intptrConst(large_disp));    <----
    index = tmp;
   } else {

This is supposed to be calculating "tmp = index + large_disp" where tmp
is a freshly allocated register but it actually does "large_disp = tmp +
tmp".

On line 155 we check if index is a constant and if so accumulate its
value into large_disp. Then on line 194:

  // at this point we either have base + index or base + displacement
  if (large_disp == 0) {
    return new LIR_Address(base, index, type);      <----
  } else {

LIR_Address::verify() asserts that index is either a register or an
illegal value, but if the displacement and constant index sum to zero we
pass a constant in here.

C1 in mainline JDK doesn't hit these code paths, but I don't see any
reason why it couldn't in the future, and indeed it does on the lworld
branch. Tested tier1 on AArch64 with TieredStopAtLevel=1.

-------------

Commit messages:
 - 8263425: AArch64: two potential bugs in C1 LIRGenerator::generate_address()

Changes: https://git.openjdk.java.net/jdk/pull/2961/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2961&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263425
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2961.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2961/head:pull/2961

PR: https://git.openjdk.java.net/jdk/pull/2961