- trim blocks with multiple predecessors
- trim blocks, which contain only phi-functions
- trim blocks, which can be merged into the successor block
As an example, compiling the following source:
---8<------
package p
type Node struct {
Key int
Left, Right *Node
}
func Search(r *Node, k int) *Node {
for r != nil {
switch {
case k == r.Key:
return r
case k < r.Key:
r = r.Left
default:
r = r.Right
}
}
return nil
}
---8<------
with `GOSSAFUNC=Search" go tool compile t.go`, results in the following
code:
The jump at 16 corresponds to the edge b21 -> b4. The block b4 contains
only phi-ops, i.e. no actual code besides the jump to b2. However b4 is
not trimmed, because it a) has more than one predecessor, and b) it is
not empty.
This change enhances trim.go to remove more blocks, subject to the
following criteria:
- block has predecessors (i.e. not the start block)
- block is BlockPlain
- block does not loop back to itself
- block is the single predecessor of its successor; the instructions of
the block are merged into the successor
- block does no emit actual code, besides a possible unconditional
jump.
Currently only OpPhi are considered to not be actual code,
perhaps OpKeepAlive/others should be considered too?
As an example, after the change, the block b4 is trimmed and the jump at
18 jumps directly to 8.
Revision 1: Adjust phi-ops arguments after merge
Ensure the number of phi-ops arguments matches the new number of
predecessors in the merged block.
When moving values, make them refer to the merged block.
Revision 2:
- Make clear the intent that we do not want to trim the entry block
- Double check that we are merging a phi operation
- Minor code style fix
- Fix a potentially dangerous situation when a blocks refers to the
inline value space in another block