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

[Aleksey Shipilev]

On Wed, 10 Jan 2024 10:47:35 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> 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?

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).

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PR Review Comment: https://git.openjdk.org/jdk/pull/17340#discussion_r1447221010