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go/ssa: opt: avoid an allocation

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In the dom tree traversal, the final child of each node now
inherits the parent's renaming map, reducing garbage.

This reduces allocations by 1.4% and bytes allocated by 2.0% when
building SSA for the entire standard library.

Change-Id: Id19b6d6766b3e0bf32d1db1238eff8a42d11b242
Reviewed-on: https://go-review.googlesource.com/45833
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Alan Donovan 2017-06-15 10:28:26 -04:00
parent f2cd9d3b51
commit 365911de24
1 changed files with 9 additions and 4 deletions
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13
go/ssa/lift.go
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@ -630,10 +630,15 @@ func rename(u *BasicBlock, renaming []Value, newPhis newPhiMap) {
// Continue depth-first recursion over domtree, pushing a
// fresh copy of the renaming map for each subtree.
for _, v := range u.dom.children {
// TODO(adonovan): opt: avoid copy on final iteration; use destructive update.
r := make([]Value, len(renaming))
copy(r, renaming)
for i, v := range u.dom.children {
r := renaming
if i < len(u.dom.children)-1 {
// On all but the final iteration, we must make
// a copy to avoid destructive update.
r = make([]Value, len(renaming))
copy(r, renaming)
}
rename(v, r, newPhis)
}
}