RFR: 8315545: C1: x86 cmove can use short branches

[Aleksey Shipilev]

On Fri, 1 Sep 2023 14:38:59 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Spotted this minor inefficiency when looking at profiling code. C1 profiling code uses `cmove`-s often, and often with immediates, so `LIR_Assembler::cmove` ends up emitting the real jump over the alternative branch. But the alternative branch is just `{reg,stack,const}2reg` conversion, and AFAICS is guaranteed to be short. Therefore, we can use the short branch for that forward jump. 
> 
> Sample `cmov` disassembly:
> 
> 
> # Before
>    0.19%          0x00007f1c98cc2d1d:   cmp    $0x0,%eax
>               ╭   0x00007f1c98cc2d20:   jne    0x00007f1c98cc2d2b
>               │   0x00007f1c98cc2d26:   mov    $0x2,%ecx
>               ↘   0x00007f1c98cc2d2b:   and    $0x3ffe,%ecx
>    0.54%          0x00007f1c98cc2d31:   cmp    $0x0,%ecx
> 
> # After
>    0.17%          0x00007fcf18dda199:   cmp    $0x0,%eax
>               ╭   0x00007fcf18dda19c:   jne    0x00007fcf18dda1a3
>               │   0x00007fcf18dda19e:   mov    $0x2,%ecx
>               ↘   0x00007fcf18dda1a3:   and    $0x3ffe,%ecx
>    1.24%          0x00007fcf18dda1a9:   cmp    $0x0,%ecx
> 
> 
> There are some code space savings, visible even on trivial `-Xcomp -XX:TieredStopAtLevel=... HelloWorld`.
> 
> 
> # Before
> tier1: nmethod total size        :   430104 bytes
> tier2: nmethod total size        :   467336 bytes
> tier3: nmethod total size        :   923368 bytes
> 
> # After
> tier1: nmethod total size        :   429000 bytes (-0.25%)
> tier2: nmethod total size        :   466016 bytes (-0.28%)
> tier3: nmethod total size        :   915632 bytes (-0.84%)
> 
> 
> Additional testing:
>  - [x] Linux x86_64 fastdebug `tier1 tier2 tier3`

Testing passes. Reviews are here. Thanks all!

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PR Comment: https://git.openjdk.org/jdk/pull/15537#issuecomment-1705068507