RFR: JDK-8323497: On x64, use 32-bit immediate moves for narrow klass base if possible

[Thomas Stuefe]

On Wed, 10 Jan 2024 11:01:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> This seems to work:
>> 
>> 
>> diff --git a/src/hotspot/cpu/x86/macroAssembler_x86.cpp b/src/hotspot/cpu/x86/macroAssembler_x86.cpp
>> index 88296656485..ba4b089c7aa 100644
>> --- a/src/hotspot/cpu/x86/macroAssembler_x86.cpp
>> +++ b/src/hotspot/cpu/x86/macroAssembler_x86.cpp
>> @@ -2568,7 +2568,9 @@ void MacroAssembler::movptr(Register dst, Address src) {
>>  // src should NEVER be a real pointer. Use AddressLiteral for true pointers
>>  void MacroAssembler::movptr(Register dst, intptr_t src) {
>>  #ifdef _LP64
>> -  if (is_simm32(src)) {
>> +  if (is_uimm32(src)) {
>> +    movl(dst, checked_cast<uint32_t>(src));
>> +  } else if (is_simm32(src)) {
>>      movq(dst, checked_cast<int32_t>(src));
>>    } else {
>>      mov64(dst, src);
>> diff --git a/src/hotspot/share/asm/assembler.hpp b/src/hotspot/share/asm/assembler.hpp
>> index 7b7dbd4ede7..a533b963844 100644
>> --- a/src/hotspot/share/asm/assembler.hpp
>> +++ b/src/hotspot/share/asm/assembler.hpp
>> @@ -359,6 +359,7 @@ class AbstractAssembler : public ResourceObj  {
>>    }
>> 
>>    static bool is_uimm12(uint64_t x) { return is_uimm(x, 12); }
>> +  static bool is_uimm32(uint64_t x) { return is_uimm(x, 32); }
>> 
>>    // Accessors
>>    CodeSection*  code_section() const   { return _code_section; }
>> 
>> 
>> Does that work with all narrow klass bases now, @tstuefe?
>
> That change seems to hold water on its own, saving quite a bit of code. Filed: [JDK-8323503](https://bugs.openjdk.org/browse/JDK-8323503).

@shipilev I like this. I'll wait for your patch to go in.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r1447358646