15 January 2007
Lampson: Lazy and Speculative Execution
19
Lazy:
Carry-save adders
nDon’t
propagate carries until need a clean result
oRepresent x as x1 + x2
oFor add or subtract, x + y = x1 + x2
+ y = r1 + r2
▬r1i := x1i Å x2i Å yi ; r2i+1 := maj(x1i, x2i,
yi)
nThe
bet: Another add before a test or multiply