RFR: JDK-8297660: x86: Redundant test+jump in C1 allocateArray

[Thomas Stuefe]

In `C1_MacroAssembler::initialize_body()` we test the input array length for 0. We do this again in `MacroAssembler::zero_memory()`. This results in a redundant test+jump instruction:


 13442   0x00007f58a8ca2f02:   sub    $0x10,%rsi                 
 13443   0x00007f58a8ca2f06:   je     0x00007f58a8ca2f26   <<
 13444   0x00007f58a8ca2f0c:   test   %rsi,%rsi                     
 13445   0x00007f58a8ca2f0f:   je     0x00007f58a8ca2f26    <<
 13446   0x00007f58a8ca2f15:   xor    %rbx,%rbx
 13447   0x00007f58a8ca2f18:   shr    $0x3,%rsi
 13448   0x00007f58a8ca2f1c:   mov    %rbx,0x8(%rax,%rsi,8)
 13449   0x00007f58a8ca2f21:   dec    %rsi
 13450   0x00007f58a8ca2f24:   jne    0x00007f58a8ca2f1c           ;*anewarray {reexecute=0 rethrow=0 return_oop=0}
 13451                                                             ; - java.lang.invoke.MethodHandles::<clinit>@24 (line 5109)


Since `MacroAssembler::zero_memory()` is only ever called from `C1_MacroAssembler::initialize_body()`, it does not need to test for len=0, since its caller already does.

---

Patch removes one test+jump and adds an assertion for len>0 to zero_memory.

Patch ran through SAP nightlies and GHAs.

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Commit messages:
 - remove-redundant-test-from-zeromemory

Changes: https://git.openjdk.org/jdk/pull/11372/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11372&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8297660
  Stats: 11 lines in 1 file changed: 7 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11372.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11372/head:pull/11372

PR: https://git.openjdk.org/jdk/pull/11372