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

[Aleksey Shipilev]

On Wed, 10 Jan 2024 10:23:02 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> FYI the logic for immediate matching is:
>> 
>>     if (is_uimm32(imm)) {
>>         movl(dst, imm);
>>     } else if (is_simm32(imm)) {
>>         movq(dst, imm);
>>     } else {
>>         mov64(dst, imm);
>>     }
>> 
>> The reason is that `movl` is smaller than `movq` in code size.
>> 
>> Maybe we can change `movptr` to this. I hope that I do not miss anything here.
>
> Yeah, current `movptr` is sign-extending 32->64, which is not extra-efficient for 32-bit unsigned imms with highest bit set. I think we can indeed check for `is_uimm32` in `movptr` to cover that case too.

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?

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PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r1447200469