RFR: 8324123: aarch64: fix prfm literal encoding in assembler

[Andrew Haley]

On Thu, 18 Jan 2024 10:02:59 GMT, Wang Zhuo <wzhuo at openjdk.org> wrote:

> 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);
>   }

src/hotspot/cpu/aarch64/assembler_aarch64.hpp line 1585:

> 1583:       int64_t offset = (adr.target() - pc()) >> 2;              \
> 1584:       sf(offset, 23, 5);                                        \
> 1585:     } else {                                                    \

This looks reasonable, but we don't need it to be inline. See the examples of `adr` and `_adrp`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17482#discussion_r1458602894