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diff --git a/chapter10a/README.md b/chapter10a/README.md index 47fd5ad6..80a1f46c 100644 --- a/chapter10a/README.md +++ b/chapter10a/README.md @@ -98,6 +98,25 @@ A load can also move through a memory Phi when doing so exposes a useful load-after-store fold. On a loop, the backedge must fold, preventing this rewrite from repeatedly moving the load around the cycle. +The reverse direction can save an operation: a Phi of matching Loads or Stores +can become one operation after the join. For example, these two writes: + +```java +if (arg) s.x = arg+1; +else s.x = arg+2; +``` + +can become `s.x = Phi(arg+1,arg+2)`. The general rewrite creates a separate, +correctly typed Phi for each differing operand, including memory and pointers; +identical operands need no Phi. An exact-class copy preserves the operation's +attributes. The proposed result must not widen the original Phi's type. + +Every arm must match the operation and field and have no control input. A Store +must have no other users, so moving it removes the original effect. A Load must +have no possible conflicting write or memory merge among its memory's users; +otherwise delaying the read could change its value. The check happens during +optimization, before any scheduler can supply explicit anti-dependency edges. + The single chain is conservative. A store to `y` can prevent forwarding an earlier store to `x`, even when the objects are identical. This is a missed optimization, not a correctness problem; it motivates the next chapter. |