RFR: 8139457: Array bases are aligned at HeapWord granularity

[Fei Yang]

On Tue, 8 Nov 2022 20:18:09 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> See [JDK-8139457](https://bugs.openjdk.org/browse/JDK-8139457) for details.
> 
> Basically, when running with -XX:-UseCompressedClassPointers, arrays will have a gap between the length field and the first array element, because array elements will only start at word-aligned offsets. This is not necessary for smaller-than-word elements.
> 
> Also, while it is not very important now, it will become very important with Lilliput, which eliminates the Klass field and would always put the length field at offset 8, and leave a gap between offset 12 and 16.
> 
> Testing:
>  - [x] runtime/FieldLayout/ArrayBaseOffsets.java (x86_64, x86_32, aarch64, arm, riscv, s390)
>  - [x] bootcycle (x86_64, x86_32, aarch64, arm, riscv, s390)
>  - [x] tier1 (x86_64, x86_32, aarch64, riscv)
>  - [x] tier2 (x86_64, aarch64, riscv)
>  - [x] tier3 (x86_64, riscv)

Hi, you might need one extra change for riscv in order to pass this test: ./test/hotspot/jtreg/runtime/FieldLayout/ArrayBaseOffsets.java
But I haven't perform full test for all these changes on riscv.


diff --git a/src/hotspot/cpu/riscv/c1_MacroAssembler_riscv.cpp b/src/hotspot/cpu/riscv/c1_MacroAssembler_riscv.cpp
index 5989d5ab809..9dced7c53e9 100644
--- a/src/hotspot/cpu/riscv/c1_MacroAssembler_riscv.cpp
+++ b/src/hotspot/cpu/riscv/c1_MacroAssembler_riscv.cpp
@@ -177,6 +177,13 @@ void C1_MacroAssembler::initialize_body(Register obj, Register len_in_bytes, int
   sub(len_in_bytes, len_in_bytes, hdr_size_in_bytes);
   beqz(len_in_bytes, done);

+  // Zero first 4 bytes, if start offset is not word aligned.
+  if (!is_aligned(hdr_size_in_bytes, BytesPerWord)) {
+    sw(zr, Address(obj, hdr_size_in_bytes));
+    sub(len_in_bytes, len_in_bytes, BytesPerInt);
+    hdr_size_in_bytes += BytesPerInt;
+  }
+
   // Preserve obj
   if (hdr_size_in_bytes) {
     add(obj, obj, hdr_size_in_bytes);

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

PR: https://git.openjdk.org/jdk/pull/11044