18 Aug
2014
18 Aug
'14
7:19 p.m.
On 08/18/2014 07:11 PM, Will Godfrey wrote:
A long time ago ... in a land far away :)
I did some assembler programming on the Acorn Archimedes (ARM 2/3) and worked
out a series of additions and subtractions that would perform very fast
multiplication of awkward numbers by known amounts.
Is there any point in doing this for C programs, or are modern compilers
sophisticated enough to do such things themselves?
If anyone is interested *7 is:
RSB R1, R0, R0, LSL #3
gcc 4.8 for ARM seems to give that instruction too with -O2 here for
a simple function that returns *7 of its input argument.
With no optimization:
00000000 <mul7>:
0: e52db004 push {fp} ; (str fp, [sp, #-4]!)
4: e28db000 add fp, sp, #0
8: e24dd00c sub sp, sp, #12
c: e50b0008 str r0, [fp, #-8]
10: e51b2008 ldr r2, [fp, #-8]
14: e1a03002 mov r3, r2
18: e1a03183 lsl r3, r3, #3
1c: e0623003 rsb r3, r2, r3
20: e1a00003 mov r0, r3
24: e24bd000 sub sp, fp, #0
28: e49db004 pop {fp} ; (ldr fp, [sp], #4)
2c: e12fff1e bx lr