[webkit-dev] jit for arm

Gavin Barraclough barraclough at apple.com
Tue Nov 17 15:59:49 PST 2009

On Nov 4, 2009, at 8:37 AM, ll Jefferry wrote:

> Hi,
> when i reading the jit for arm source code, i am not very clear the  
> functionality of the flowing functions:
>         ctiTrampoline

This code is used when entering from the C runtime into JIT generated  
code.  JIT generated code does not necessarily respect C calling  
conventions, so this routine sets up the stack frame, preserves  
registers, etc, as necessary to allow the JIT code to be run.

>         ctiVMThrowTrampoline

To perform certain operations the JIT will call back into C code.   
Usually the C callback can just return in a perfectly normal fashion  
and continue execution once it has completed, however in the case that  
an exception is thrown special handling is required to change the  
control flow.  The return address of the C callback is instead changed  
to point to this, and this piece of code handles looking up the  
exception handler at which execution will be resumed.

>         ctiOpThrowNotCaught

This is used to from within cti_op_throw, which implements the 'throw'  
keyword in JavaScript.  The cti_op_throw method will attempt to look  
up a handler routine that catches the exception.  However if the  
exception is not caught it is necessary to force an early termination  
of JIT execution.  The cti_op_throw C callback always modifies its  
return address, either to point to the code for the appropriate  
exception handler to catch the exception, or to ctiOpThrowNotCaught if  
no handler is found.

> could you explain to me?
> and another question is that:  in cacheFlush function, why the  
> system call number is 0xf0002? if it is defined by the toolchain?

Zoltan, Gabor?

> thanks!
> BR,
> Jeff
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