Master Thesis Research Advice. JIT

[Vladimir Ivanov]

Hi Vladyslav,

C2 has a number of aggressive optimizations which heavily rely on 
profiling data. It leads to numerous uncommon traps in the generated 
code. You can disable some of such optimizations, but there's no way to 
completely eliminate uncommon traps in the generated code: they are a 
core piece of the design.

Have you tried switching to C1 instead? C1 doesn't rely on profiling 
data that much and use code patching techniques in place of uncommon 
traps. So, the generated code usually has complete coverage of the 
compiled method.

Best regards,
Vladimir Ivanov

On 16.04.2020 01:05, Vladyslav Tkachuk wrote:
> Hello,
> 
> I am a Master's student at the University of Passau, Germany.
> My master thesis research is concerned with detecting equivalent mutants in
> Java.
> The main research question is to use the Trivial Compiler Equivalency
> technique. This means that we acquire Assembly code produced by Java JIT
> compiler for initial and mutated source and then compare them.
> 
> I have previously contacted Tobias Hartmann, who advised me to write here
> regarding technical questions. I would like to ask you if there is any
> solution to a problem I have.
> 
> Last time Tobias recommended me to use Opto-Assembly to achieve my purpose.
> It was a good hint and it helped me to get more precise data.
> However, after doing some research I noticed that in some cases C2 compiler
> unloaded the method code which I expected to find in assembly. As I found
> out this was a part of deoptimization and the method code was meant to be
> executed by the interpreter.
> Here is an example of what I mean:
> 
> {method}
>   - this oop:          0x000000000d2319c8
>   - method holder:     'Rational'
>   - constants:         0x000000000d230cf8 constant pool [85]
> {0x000000000d230d00} for 'Rational' cache=0x000000000d231cd8
>   - access:            0x81000001  public
>   - name:              'toString'
>   - signature:         '()Ljava/lang/String;'
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> some setup code
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 02c    movq    RBP, RDX # spill
> 02f    movl    RDX, #11 # int
>        nop # 3 bytes pad for loops and calls
> *037    call,static  wrapper for: uncommon_trap(reason='unloaded'
> action='reinterpret' index='11')*
> *        # Rational::toString @ bci:0  L[0]=RBP L[1]=_ L[2]=_ L[3]=_ L[4]=_
> L[5]=_ L[6]=_ L[7]=_*
> *        # OopMap{rbp=Oop off=60}*
> 03c    int3 # ShouldNotReachHere
> 03c
> 
> 
> This is a 'toString' method and as I could see and understand, there is no
> actual method code, but only a call to it.
> 
> I would like to know if it is possible to completely disable any
> deoptimizations and consistently receive the full asm code? I consent that
> it is not practical and hurts performance, but it is not a goal in this
> scope. According to my observations, in most cases the method code is full,
> but strangely here it did not work. I have tried to google any useful info,
> unfortunately, I did not see anything helpful, despite the explanations
> about what deoptimization is and its types.
> 
> I would be grateful if you could shed some light on the issue.
> Thanks in advance for any useful information.
> 
> Best regards,
> Vladyslav Tkachuk
>