RFR: 8324123: aarch64: fix prfm literal encoding in assembler [v6]

[Wang Zhuo]

> Current prfm literal mode encoding in aarch64 assembler is not correct.
> The prfm_literal instruction requires 31 and 30 bits to be 0x11, while current assembler encodes the two bits to be 0x11, which is a ldr instruction, not prfm.
> For example, if adding the following code in stubGenerator
> __ prfm(Address(__ pc()))
> we get a ldr instruction like
>    ldr x0, 0x0000ffff83f8539c
> but it should be a prfm instruction like
>    prfm pldl1keep, 0x0000ffff8ff8539c
> 
> The bug is caused in ld_st2, literal mode, bit 31 and 30 bits are set to (size & 0b01), while for prfm instructions, 31 and 30 bits must be 0b11.
>   void ld_st2(Register Rt, const Address &adr, int size, int op, int V = 0) {
>     starti;
> 
>     f(V, 26); // general reg?
>     zrf(Rt, 0);
> 
>     // Encoding for literal loads is done here (rather than pushed
>     // down into Address::encode) because the encoding of this
>     // instruction is too different from all of the other forms to
>     // make it worth sharing.
>     if (adr.getMode() == Address::literal) {
>       assert(size == 0b10 || size == 0b11, "bad operand size in ldr");
>       assert(op == 0b01, "literal form can only be used with loads");
>       f(**size & 0b01, 31, 30**), f(0b011, 29, 27), f(0b00, 25, 24);
>       int64_t offset = (adr.target() - pc()) >> 2;
>       sf(offset, 23, 5);
>       code_section()->relocate(pc(), adr.rspec());
>       return;
>     }
> 
>     f(size, 31, 30);
>     f(op, 23, 22); // str
>     adr.encode(&current_insn);
>   }

Wang Zhuo has updated the pull request incrementally with two additional commits since the last revision:

 - Update assembler_aarch64.cpp, merge guarantee
   
   Co-authored-by: Andrew Haley <aph-open at littlepinkcloud.com>
 - Update assembler_aarch64.cpp delete some comments
   
   Co-authored-by: Andrew Haley <aph-open at littlepinkcloud.com>

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17482/files
  - new: https://git.openjdk.org/jdk/pull/17482/files/11d46ff7..62cce404

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17482&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17482&range=04-05

  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/17482.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17482/head:pull/17482

PR: https://git.openjdk.org/jdk/pull/17482