[m-dev.] tail call to loop optimisation for low-level grades

[Zoltan Somogyi]

On 27-Jun-2008, Peter Wang <novalazy at gmail.com> wrote:
> I was looking at why the asm_fast.gc implementation of string.hash is
> about 85% times slower than the C version.  The loop looks like this,
> after some cleanup:
> 
>     MR_def_static(shash__unchecked_hash_2_5_0)
> 	    if (MR_r2 >= MR_r3) {
> 		MR_GOTO_LAB(shash__unchecked_hash_2_5_0_i2);
> 	    }
> 	    {
> 		MR_String Str = (MR_String) MR_r1;
> 		MR_Word MR_tempr1 = Str[MR_r2];
> 
> 		MR_r2 = MR_r2 + 1;
> 		MR_r4 = (MR_r4 ^ (MR_r4 << 5)) ^ MR_tempr1;
> 		MR_np_localtailcall(shash__unchecked_hash_2_5_0);
> 	    }

Your cleanup shows the C << operator, whereas the Mercury source in string.m
uses the Mercury << operator, which does bounds checks. Did you replace
<< with unchecked_left_shift in your copy of string.m?

Zoltan.
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