Messages in this thread | | | Date | Sun, 9 Aug 2009 12:40:48 +0300 | From | Sergey Senozhatsky <> | Subject | Re: [PATCH] Make shr to divide by power of 2 |
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On (08/08/09 10:22), Robert Hancock wrote: > Actually, the Intel Architecture Optimization Reference Manual doesn't > say divide may be faster, but it does say that "On processors based on > Intel NetBurst microarchitecture, latencies of some instructions are > relatively significant (including shifts, rotates, integer multiplies, > and moves from memory with sign extension)." and that "The SHIFT and > ROTATE instructions have a longer latency on processor with a CPUID > signature corresponding to family 15 and model encoding of 0, 1, or 2. > The latency of a sequence of adds will be shorter for left shifts of three or less."
Intel Architecture Optimization Reference Manual does say about latency:
Table C-13a. General Purpose Instructions Instruction Latency Throughput IDIV | 11-21 13-23 17-41 22 | 5-13 5-14 12-36 22 SAL/SAR/SHL/SHR | 1 1 1 | 0.33 0.33 0.33
For example, Table 12-2. Intel® Atom™ Microarchitecture Instructions Latency Data Instruction Latency Throughput IDIV r/m8; IDIV r/m16; | 33;42; | 32;41;56;196 IDIV r/m32; IDIV r/m64; | 57;197 | | | ROL; ROR; SAL; | 1 | 1 SAR; SHL; SHR | |
*SHLD/SHRD |4;2-11 |3;1-10
On (08/08/09 09:35), Andi Kleen wrote: > DIV should be always slower than a SHIFT. > > But it has nothing really to do with the CPU. The point is that the compiler > always selects a suitable one by itself. Rewriting x / 2 to x >> 1 is > one of the easiest exercises in compiler optimizations. > > The only case when the compiler cannot do this easily by itself is > when the dividend is not a constant. >
int width = (vc->vc_font.width + 7) >> 3;
> That said -Os sometimes screws us up on this, but it's still not worth > doing this change manually. >
My point is that it should 'look the same'. I mean there are 5 int width = (vc->vc_font.width + 7) >> 3; *not exactly this one, but vc->vc_font.width (+ 7)? >> 3
and _only_ one int width = (vc->vc_font.width + 7) / 8;
P.S. Sorry, hit "reply", not "reply to all".
Sergey [unhandled content-type:application/pgp-signature] | |