pub const SIG_BASE_REG: u8 = 3;Expand description
Scratch base register for the signature stores. x3 is spilled — it is not in
the captured set (slots 0..=12) and is invocation-local (dropped at exit), so
clobbering it is invisible to the differential, and both engines agree on
x3/x4 spill semantics (the x3_x4_differential net). Using it as the store
base leaves every captured register untouched, so the stored values are the
program’s exact post-body register file.