cpumodel

ffmasm examples - cpumodel

This is an example of ffmasm to obtain CPU model from CPUID instruction in AMD64.

Requirements

• Java 24
• AMD64 Linux or Windows
• Maven

How to run

1. Install ffmasm

$ mvn clean install

2. Build cpumodel

$ cd examples/cpumodel
$ mvn clean package

3. Run via Maven

$ mvn exec:java

4. [EXTRA] check assembly in GDB

Run GDB if you want to check assembled code.

You can see PID and address of CodeSegment on the console when you run java -jar cpumodel-0.1.3.jar --stop in below.

PID: 928
Addr: 0x7f4c42704000
Intel(R) Core(TM) i3-8145U CPU @ 2.10GHz

Attach to JVM process (928 in this case), and run disas on GDB.

$ gdb -p 928

  :

(gdb) disas 0x7f4c42704000, 0x7f4c42704020
Dump of assembler code from 0x7f4c42704000 to 0x7f4c42704020:
   0x00007f4c42704000:  push   %rbp
   0x00007f4c42704001:  mov    %rsp,%rbp
   0x00007f4c42704004:  push   %rbx
   0x00007f4c42704005:  mov    %rdi,%rax
   0x00007f4c42704008:  mov    %rsi,%r11
   0x00007f4c4270400b:  cpuid
   0x00007f4c4270400d:  mov    %eax,(%r11)
   0x00007f4c42704010:  mov    %ebx,0x4(%r11)
   0x00007f4c42704014:  mov    %ecx,0x8(%r11)
   0x00007f4c42704018:  mov    %edx,0xc(%r11)
   0x00007f4c4270401c:  pop    %rbx
   0x00007f4c4270401e:  leave
   0x00007f4c4270401f:  ret
End of assembler dump.

Calling convention

Calling convention is different between Linux AMD64 and Windows x64. Linux AMD64 conform to System V Application Binary Interface, on the other hand Windows conform to here. You have to conform to them when you assemble.

Expected assembly code in this example on Linux

# prologue
push %rbp
mov  %rsp, %rbp

# Evacuate callee-saved register
push %rbx

# Copy arguments to temporary registers
mov %rdi,%rax
mov %rsi,%r11

# Call CPUID
cpuid

# Store result to given memory
mov %eax,   (%r11)
mov %ebx,  4(%r11)
mov %ecx,  8(%r11)
mov %edx, 12(%r11)

# Restore callee-saved register
pop %rbx

# Epilogue
leave
ret

Play with AOT

Java 24 introduced JEP 483: Ahead-of-Time Class Loading & Linking. It can cache InvokeDynamic link, so first call in FFM could be faster.

pom.xml generates AOT cache in package phase, so you can feel acceralation with AOT as following. Note that you have to move target directory because AOT identifies class path.

$ cd target


$ time $JAVA_HOME/bin/java -jar cpumodel-0.1.3.jar
PID: 3147
Addr: 0x7f8d1fc37000
AMD Ryzen 3 3300X 4-Core Processor

real    0m0.140s
user    0m0.196s
sys     0m0.049s


$ time $JAVA_HOME/bin/java -XX:AOTCache=ffmasm-cpumodel.aot -jar cpumodel-0.1.3.jar
PID: 3167
Addr: 0x7f9000001000
AMD Ryzen 3 3300X 4-Core Processor

real    0m0.095s
user    0m0.121s
sys     0m0.041s