signed nibble

[Patrick Schluter]

On Monday, 7 January 2019 at 20:28:21 UTC, H. S. Teoh wrote:
> On Mon, Jan 07, 2019 at 08:06:17PM +0000, Patrick Schluter via 
> Digitalmars-d-learn wrote:
>> On Monday, 7 January 2019 at 18:56:17 UTC, H. S. Teoh wrote:
>> > On Mon, Jan 07, 2019 at 06:42:13PM +0000, Patrick Schluter 
>> > via Digitalmars-d-learn wrote:
> [...]
>> > > byte b = nibble | ((nibble & 0x40)?0xF0:0);
>> > 
>> > This is equivalent to doing a bit comparison (implied by the 
>> > ? operator).  You can do it without a branch:
>> > 
>> > 	cast(byte)(nibble << 4) >> 4
>> > 
>> > will use the natural sign extension of a (signed) byte to 
>> > "stretch" the upper bit.  It just takes 2-3 CPU instructions.
>> > 
>> 
>> Yeah, my bit-fiddle-fu goes back to pre-barrel-shifter days. 
>> Up to 32 bit processors, shifting was more expensive than 
>> branching.
>
> Really?  Haha, never knew that, even though I date all the way 
> back to writing assembly on 8-bit processors. :-D
>
Most of my career was programming for 80186. Shifting by one was 
2 cycles in register and 15 in memory. Shifting by 4, 9 cycles 
for regs/21 for mem. And 80186 was a fast shifter compared to 
8088/86 or 68000 (8+2n cycles).