RFR: 8259822: [PPC64] Support the prefixed instruction format added in POWER10 [v2]

[Corey Ashford]

On Tue, 26 Jan 2021 19:07:40 GMT, Kazunori Ogata <ogatak at openjdk.org> wrote:

>> src/hotspot/cpu/ppc/assembler_ppc.hpp line 1533:
>> 
>>> 1531:      int32_t* p_inst = (int32_t*)p;
>>> 1532: 
>>> 1533:      if (is_aligned(reinterpret_cast<uintptr_t>(p_inst+1), 64) && is_nop(*p_inst)) {
>> 
>> This test is a bit confusing.  Shouldn't is_paddi return false if p points at a nop (even if it precedes the paddi)?
>
> "is_<instr>()" are the functions for testing instructions in the generated code and they are usually called in "assert()" or complex combination of if statements.  So if this function returns false when p points at nop, the assertions or the if-conditions need to handle the nop case and the code will become difficult to read.  Since the padding nop is a non-common case, I'd like to hide the existence of nop in this function.

Ok, got it.  Thanks for the explanation.  Maybe an extra comment in the code saying essentially what you said here would be appropriate?

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PR: https://git.openjdk.java.net/jdk/pull/2095