587549 - TM/JM: recognize rotate-left operation

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, defect)

Description

[Nicholas Nethercote [inactive]]

crypto-md5 has this function which is called 31744 times:

function bit_rol(num, cnt)
{
  return (num << cnt) | (num >>> (32 - cnt));
}

crypto-sha1 has the same function, but called "rol", which is called 35392 times.

It might be worthwhile detecting this pattern and generating a rotate instruction, but I don't know how many ms it would save.

In TM this could be done entirely within Nanojit -- it would be easy to add LIR_rol and recognize the above sequence in ExprFilter.  I don't know enough about JM to know how easy it would be to do there.

An alternative would be to have a JSOP for it but that doesn't feel right since it's not a JS primitive operation.

Comment 1

[Nicholas Nethercote [inactive]]

Attachment: patch (against TM 49714:13e72485d286)

This patch adds LIR_lroti to Nanojit and recognizes the pattern.  Instruction count improvements for the two benchmarks:

---------------------------------------------------------------
| millions of instructions executed                           |
| total                        | on-trace (may overestimate)  |
---------------------------------------------------------------
|    46.812    46.694 (1.003x) |     5.660     5.597 (1.011x) | crypto-md5
|    31.224    31.111 (1.004x) |     7.109     7.001 (1.015x) | crypto-sha1

which is not enough to be worthwhile.