Shrink is a rewrite pass for the SSA and SSA2 IntermediateLanguages, invoked from every optimization pass (see SSASimplify and SSA2Simplify).
Description
This pass implements a whole family of compile-time reductions, like:
-
#1(a, b)
⇒a
-
case C x of C y => e
⇒let y = x in e
-
constant folding, copy propagation
-
eta blocks
-
tuple reconstruction elimination
Implementation
Details and Notes
The Shrink implementation also includes functions to eliminate
unreachable blocks from a SSA or SSA2 program or function.
The Shrink pass does not guarantee to eliminate all unreachable
blocks. Doing so would unduly complicate the implementation, and it
is almost always the case that all unreachable blocks are eliminated.
However, a small number of optimization passes require that the input
have no unreachable blocks (essentially, when the analysis works on
the control flow graph and the rewrite iterates on the vector of
blocks). These passes explicitly call eliminateDeadBlocks
.
The Shrink pass has a special case to turn a non-tail call where
the continuation and handler only do Profile
statements into a tail
call where the Profile
statements precede the tail call.