Bug ID: JDK-8372285 G1: Micro-optimize x86 barrier code

Type: Enhancement
Component: hotspot
Sub-Component: gc
Affected Version: 25,26

Priority: P4
Status: Open
Resolution: Unresolved

Submitted: 2025-11-20
Updated: 2025-11-21

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Other
tbdUnresolved

I noticed in disassembly for G1 barriers that we have are missing a few easy things for code density in our hand-written assembly for G1 barriers:
 *) cmpptr(reg, 0) -> testptr(reg, reg)
 *) jcc -> jccb for forward branches, where obvious
 *) jmp -> jmpb for forward jumps, where obvious

A simple patch already gives me about 1.2% savings even without compiling a lot of code.

for I in `seq 1 3`; do build/linux-x86_64-server-release/images/jdk/bin/java -Xcomp -XX:+CITime 2>&1 | grep "nmethod code"; done 

# Before
  nmethod code size         :  5764304 bytes
  nmethod code size         :  5764336 bytes
  nmethod code size         :  5764480 bytes

# After
  nmethod code size         :  5691032 bytes
  nmethod code size         :  5691032 bytes
  nmethod code size         :  5691000 bytes

Relates :	JDK-8372284 - Deduplicate C2 stubs
Relates :	JDK-8334060 - Implementation of Late Barrier Expansion for G1