3 ms·

It bothers me that they say implementing SSA eliminates the need for "complicated" data-flow analysis. Data-flow has a really nice theory behind it and enables

• pbiggar 10y ago
  Except they are exactly right!
  • pbiggar 10y ago
    I should add more context here. A data-flow analysis framework has significant overlap with an SSA-based analysis framework, except that the SSA-based one leads to simpler analyses which are more powerful. A good comparison is CCP (conditional copy propagation) vs SCCP (sparse CCP) - the SCCP version is naturally flow-sensitive (that's a property of SSA form), so it can eliminate entire branches, leading to more dead code being eliminated.
    • rayiner 10y ago
      Aren't SSA analyses generally only "kinda flow-sensitive" since values are only renamed where control flow merges, not where it splits? E.g. int x = ... if(x > 0) { use(x) else { use(x) } In a DFA where you track information about each variable at each program point, you could push the constraint implied by the condition into each branch. If you do a sparse analysis on SSA form, that doesn't come naturally, right?
      • pcwalton 10y ago
        That's correct; it doesn't come for free.
      • nikic 10y ago
        To handle this case some SSA implementations add a concept of "pi" nodes, which are used to artificially split variables on branches that establish some useful data flow property. x1 = ... if (x1 > 0) { x2 = pi(x1 & RANGE[1..]) use(x2) } else { x3 = pi(x1 & RANGE[..0]) use(x3) } x4 = phi(x2, x3) // if used I have placed the pi nodes in the blocks, but semantically they are placed along the control edge. Ref e.g. e-SSA in the ABCD value range inference algorithm.