[Simh] Simh execution speed improvement - about 45% improvement.

[Davis Johnson]

Jan-Benedict Glaw wrote:

>On Tue, 2008-01-22 20:21:00 -0500, Davis Johnson <davis at frizzen.com> wrote:
>  
>
>>improvement from inlining alone. Function call overhead is relatively 
>>small, after all. I did expect it to be at least measurable. I expected 
>>    
>>
>
>Calling a function is quite some thing! I wouldn't call it `small'
>after all!  RAM accessing is one of the most used operations and as a
>hot path, every single CPU instruction used to access it really counts
>a lot...
>
>MfG, JBG
>
>  
>
Function call overhead is relatively small.

I ran my numbers on an ultra-SPARC system where there is generally no 
need to save any registers, or restore them on exit. The call itself is 
little more expensive than a regular branch. The return is just another 
branch. Parameters are generally passed in registers, so setup before 
the call is making sure the right values are in the right registers. A 
similar situation exists on other RISC architectures (PPC, Alpha -- less 
so for MIPS).

While every cycle does count, the few used by the call itself is small 
compared to the body of  all but the smallest functions. A simulated 
memory access in SIMH typically needs to worry about one or more of 
memory relocation, protection (read, write, execute protect), access 
levels, possible page faults, dirty bits, byte order issues, and 
possibly other minutia.

I'd say that the function call overhead is _relatively_ small. Probably 
even on Intel CPUs.

Inlining the functions is still a powerful idea, just not because of 
eliminated function call overhead. It is powerful because it allows the 
optimizer to apply information from the context of the call to the 
optimization of the function body. The function body gets optimized in 
the same register allocation context as the caller. Common sub 
expressions can be found that exist in both called and caller. If tests 
and branches can frequently be eliminated. And so on and so on. Almost 
every optimization trick works better.

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