By the way, I still don't understand how birth points would work. Can
someone give an example of what the insn stream would look like with
birth points, and what the DU/UD chains would look like?
With a big IIUC, and using a high-level IR for simplicity
if (a < 5) goto BB1; else goto BB2;
BB1: b = 3; goto BB3;
BB2: b = c; goto BB3;
BB3: return b * b;
DF info for b:
insn <b = 3> has def D1
insn <b = c> has def D2
insn <b * b> has use U1 (use-def chain [D1,D2]) and
use U2 (use-def chain [D1,D2])
becomes
if (a < 5) goto BB1; else goto BB2;
BB1: b = 3; goto BB2;
BB2: b = c; goto BB3;
BB3: b = b; return b * b;
DF info for b:
insn <b = 3> has def D1
insn <b = c> has def D2
birthpoint <b = b> has use U1 (use-def chain [D1,D2])
and def D3
insn <b * b> has use U2 (use-def chain [D3])
and use U3 (use-def chain [D3])
Basically the only non-singleton UD chains are for a birthpoint's RHS,
and the UD chains of birthpoints correspond to PHI operands. The
singleton UD chains correspond to subscripted SSA variables. I think
this is isomorphic to FUD chains.
Paolo
