RFR: 8319406: x86: Shorter movptr(reg, imm) for 32-bit immediates

[Aleksey Shipilev]

On Fri, 3 Nov 2023 19:46:26 GMT, Quan Anh Mai <qamai at openjdk.org> wrote:

>> Noticed this while doing C1 work, but the issue is more generic. If you look into x86 code, then sometimes you'll notice `movabs` with small immediates on x86. That `movabs` actually carries the full-blown 64-bit immediate.
>> 
>> Similar to [JDK-8255838](https://bugs.openjdk.org/browse/JDK-8255838), it would be useful to shorten movptr(reg, imm) when immediate fits in 32 bits. This would compact some code, notably the code in C1 profiling ([JDK-8315843](https://bugs.openjdk.org/browse/JDK-8315843)), but also other code, generically.
>> 
>> For example, sample branch profiling hunk from C1 tier3 on x86_64:
>> 
>> 
>> Before:
>>    0x00007f269065ed02:   test   %edx,%edx
>>    0x00007f269065ed04:   movabs $0x7f260a4ddd68,%rax  ;   {metadata(method data for {method} …
>>    0x00007f269065ed0e:   movabs $0x138,%rsi
>>  ╭ 0x00007f269065ed18:   je     0x00007f269065ed24
>>  │ 0x00007f269065ed1a:   movabs $0x148,%rsi
>>  ↘ 0x00007f269065ed24:   mov    (%rax,%rsi,1),%rdi
>>    0x00007f269065ed28:   lea    0x1(%rdi),%rdi
>>    0x00007f269065ed2c:   mov    %rdi,(%rax,%rsi,1)
>>    0x00007f269065ed30:   je     0x00007f269065ed4e     
>> 
>> After:
>>    0x00007f1370dcd782:   test   %edx,%edx
>>    0x00007f1370dcd784:   movabs $0x7f12f64ddd68,%rax   ;   {metadata(method data for {method} …
>>    0x00007f1370dcd78e:   mov    $0x138,%esi
>>  ╭ 0x00007f1370dcd793:   je     0x00007f1370dcd79a        
>>  │ 0x00007f1370dcd795:   mov    $0x148,%esi
>>  ↘ 0x00007f1370dcd79a:   mov    (%rax,%rsi,1),%rdi        
>>    0x00007f1370dcd79e:   lea    0x1(%rdi),%rdi
>>    0x00007f1370dcd7a2:   mov    %rdi,(%rax,%rsi,1)       
>>    0x00007f1370dcd7a6:   je     0x00007f1370dcd7c4 
>> 
>> 
>> We can use a shorter 32-bit immediate moves. In the hunk above, this saves about 8 bytes.
>> 
>> This is not limited to the profiling code. There is observable code space savings on larger tests in C2, e.g. on `-Xcomp -XX:TieredStopAtLevel=... HelloWorld`.
>> 
>> 
>> # Before
>>  nmethod code size         :   430328 bytes
>>  nmethod code size         :   467032 bytes
>>  nmethod code size         :   908936 bytes
>>  nmethod code size         :  1267816 bytes
>> 
>> # After
>>  nmethod code size         :   429616 bytes (-0.1%)
>>  nmethod code size         :   466344 bytes (-0.1%)
>>  nmethod code size         :   897144 bytes (-1.3%)
>>  nmethod code size         :  1256216 bytes (-0.9%)
>> 
>> 
>> There are two wrinkles:
>>   1. Current `movslq(Register, int32_t)` is broken and protected by `ShouldNotReachHere()`. I fixed it to make this...
>
> src/hotspot/cpu/x86/macroAssembler_x86.cpp line 2576:
> 
>> 2574: #ifdef _LP64
>> 2575:   if (is_simm32(src)) {
>> 2576:     movslq(dst, checked_cast<int32_t>(src));
> 
> Why not just `movq`? there is no `movslq r, i` so this is kind of confusing.

Right. There is no point in trying to fix `movslq`. Replaced with `movq`.

> src/hotspot/cpu/x86/macroAssembler_x86.hpp line 1818:
> 
>> 1816:   void mov_metadata(Address  dst, Metadata* obj, Register rscratch);
>> 1817: 
>> 1818:   void mov_ptrslot(Register dst, intptr_t val);
> 
> I believe the convention here would be `movptr_imm64`

Good name, renamed.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/16497#discussion_r1384844360
PR Review Comment: https://git.openjdk.org/jdk/pull/16497#discussion_r1384845944