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The Go programming language
be302e6d43
- 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:
---8<------
genssa
00000 (t.go:8) TEXT "".Search(SB), $0
00001 (t.go:8) FUNCDATA $0, "".gcargs·0(SB)
00002 (t.go:8) FUNCDATA $1, "".gclocals·1(SB)
00003 (t.go:8) TYPE "".r(FP)type.*"".Node, $8
00004 (t.go:8) TYPE "".k+8(FP)type.int, $8
00005 (t.go:8) TYPE "".~r2+16(FP)type.*"".Node, $8
v40 00006 (t.go:9) MOVQ "".k+8(FP), AX
v34 00007 (t.go:9) MOVQ "".r(FP), CX
v33 00008 (t.go:9) TESTQ CX, CX
b2 00009 (t.go:9) JEQ $0, 22
v16 00010 (t.go:11) MOVQ (CX), DX
v21 00011 (t.go:11) CMPQ DX, AX
b9 00012 (t.go:11) JEQ $0, 19
v64 00013 (t.go:13) CMPQ AX, DX
b13 00014 (t.go:13) JGE 17
v36 00015 (t.go:14) MOVQ 8(CX), CX
b4 00016 (t.go:9) JMP 8 <---+
v42 00017 (t.go:16) MOVQ 16(CX), CX |
b21 00018 (t.go:10) JMP 16 ----+
v28 00019 (t.go:12) VARDEF "".~r2+16(FP)
v29 00020 (t.go:12) MOVQ CX, "".~r2+16(FP)
b10 00021 (t.go:12) RET
v44 00022 (t.go:19) VARDEF "".~r2+16(FP)
v45 00023 (t.go:19) MOVQ $0, "".~r2+16(FP)
b5 00024 (t.go:19) RET
00025 (<unknown line number>) END
---8<------
Note the jump at 18 jumps to another jump at 16.
Looking at the function after trimming:
--8<------
after trim [199 ns]
b1:
v1 = InitMem <mem>
v2 = SP <uintptr> : SP
v67 = Arg <*Node> {r} : r[*Node]
v59 = Arg <int> {k} : k[int]
v40 = LoadReg <int> v59 : AX
v34 = LoadReg <*Node> v67 : CX
Plain → b2
b2: ← b1 b4
v8 = Phi <*Node> v34 v68 : CX
v33 = TESTQ <flags> v8 v8
NE v33 → b9 b5 (likely)
b9: ← b2
v16 = MOVQload <int> v8 v1 : DX
v21 = CMPQ <flags> v16 v40
EQ v21 → b10 b13 (unlikely)
b13: ← b9
v64 = CMPQ <flags> v40 v16
LT v64 → b19 b21
b19: ← b13
v36 = MOVQload <*Node> [8] v8 v1 : CX
Plain → b4
b4: ← b21 b19 <
v68 = Phi <*Node> v42 v36 : CX <- no actual code
Plain → b2 <
b21: ← b13
v42 = MOVQload <*Node> [16] v8 v1 : CX
Plain → b4
b10: ← b9
v28 = VarDef <mem> {~r2} v1
v29 = MOVQstore <mem> {~r2} v2 v8 v28
v30 = Copy <mem> v29
Ret v30
b5: ← b2
v44 = VarDef <mem> {~r2} v1
v45 = MOVQstoreconst <mem> {~r2} [val=0,off=0] v2 v44
v47 = Copy <mem> v45
Ret v47
--8<------
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
Change-Id: I0ab91779f931f404d11008f5c45606d985d7fbaa
Reviewed-on: https://go-review.googlesource.com/28812
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
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The Go Programming Language
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--
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