Is D right for me?

[Jacob Carlborg]

On 2010-10-15 01:22, Sean Kelly wrote:
> Jacob Carlborg Wrote:
>>
>> Thread.getThis() calls pthread_getspecific which is just three
>> instructions on Mac OS X, so I guess that's not why it's so slow. The
>> only thing I can think of is first moving the if statement into the
>> assert and then trying to inline as much of the function calls.
>
> Swapping the assert and the executable code would save you a jump, but inlining the call to ___tls_get_addr would be be a bit trickier.  We'd probably have to expose Thread.sm_this as an extern (C) symbol, move the function to object.d and explicitly do the pthread_getspecific call there.  If that would be enough for the compiler to inline the calls then it shouldn't be too hard to test, but I'm worried that the call generation may happen too late.  I guess it wouldn't be too hard to figure out from looking at the asm output though (PIC code weirdness notwithstanding).

I think it would save more than just a jump. When compiling in release 
mode the compiler have to generate code for the if statement but with an 
assert it can just skip it. See the assembly at the bottom.

I was thinking about inlining Thread.getThis() as a first step. Then 
inlining pthread_getspecific as a second step. I don't know if we can 
inline pthread_getspecific due to license issues but at least there is 
an inline version available. Then of course inlining the call to 
__tls_get_addr could help as well.

Both of the following versions are compiled with "dmd -c -O -release".

___tls_get_addr in thread.d compiled with if statement:

___tls_get_addr:
		push	EBP
		mov	EBP,ESP
		push	EAX
		mov	EDX,EAX
		push	EBX
		call	  L2000
L2000:		pop	EBX
		cmp	03AAh[EBX],EDX
		ja	L2011
		cmp	03AEh[EBX],EDX
		ja	L2012
L2011:		hlt
L2012:		mov	-4[EBP],EDX
		call	  L217E
		mov	EAX,054h[EAX]
		add	EAX,-4[EBP]
		sub	EAX,03AAh[EBX]
		pop	EBX
		mov	ESP,EBP
		pop	EBP
		ret
		nop

22 lines for the above code, the same compiled with an assert instead:

___tls_get_addr:
		push	EBP
		mov	EBP,ESP
		sub	ESP,8
		mov	-4[EBP],EAX
		call	  L216E
		mov	EAX,054h[EAX]
		add	EAX,-4[EBP]
		call	  L200D
L200D:		pop	ECX
		sub	EAX,038Dh[ECX]
		mov	ESP,EBP
		pop	EBP
		ret

This is just 13 lines of code, I can tell you that I don't know assembly 
but I can see the number of instructions are a lot more in the version 
with the if statement than the one with the assert.

-- 
/Jacob Carlborg