Module A03 · concept player
Divergence, one branch split at a time
A wavefront hits an if / else. Some lanes take one side, the rest take the other.
The hardware runs both sides in turn — so drag the split and watch which lanes sit idle move,
while the total time refuses to.
if branch: 32
if body length: 8 cycles
else body length: 8 cycles
What the hardware actually runs, in order
Phase 1 · if body
Phase 2 · else body
lane executing this phase lane idle, waiting for the other side
These are counts, not a measurement. Both phases always run to completion whenever at
least one lane needs them — that is what "the sum of both paths" means. Real hardware can
also predict a branch and skip a phase entirely if it turns out uniform after all, which this
model does not represent.
A03's
deep dive traces this on AMD's 64-bit EXEC mask; a uniform branch (split at 0
or at the full width) skips the untaken phase entirely, which is the "cycles if every lane
agreed" reading above.
