cmd/compile: use depth first topological sort algorithm for layout

The current layout algorithm tries to put consecutive blocks together,
so the priority of the successor block is higher than the priority of
the zero indegree block. This algorithm is beneficial for subsequent
register allocation, but will result in more branch instructions.
The depth-first topological sorting algorithm is a well-known layout
algorithm, which has applications in many languages, and it helps to
reduce branch instructions. This CL applies it to the layout pass.
The test results show that it helps to reduce the code size.

This CL also includes the following changes:
1, Removed the primary predecessor mechanism. The new layout algorithm is
  not very friendly to register allocator in some cases, in order to adapt
  to the new layout algorithm, a new primary predecessor selection strategy
  is introduced.
2, Since the new layout implementation may place non-loop blocks between
  loop blocks, some adaptive modifications have also been made to looprotate
  pass.
3, The layout also affects the results of codegen, so this CL also adjusted
  several codegen tests accordingly.

It is inevitable that this CL will cause the code size or performance of a
few functions to decrease, but the number of cases it improves is much larger
than the number of cases it drops.

Statistical data from compilecmp on linux/amd64 is as follow:
name                      old time/op       new time/op       delta
Template                        382ms ± 4%        382ms ± 4%    ~     (p=0.497 n=49+50)
Unicode                         170ms ± 9%        169ms ± 8%    ~     (p=0.344 n=48+50)
GoTypes                         2.01s ± 4%        2.01s ± 4%    ~     (p=0.628 n=50+48)
Compiler                        190ms ±10%        189ms ± 9%    ~     (p=0.734 n=50+50)
SSA                             11.8s ± 2%        11.8s ± 3%    ~     (p=0.877 n=50+50)
Flate                           241ms ± 9%        241ms ± 8%    ~     (p=0.897 n=50+49)
GoParser                        366ms ± 3%        361ms ± 4%  -1.21%  (p=0.004 n=47+50)
Reflect                         835ms ± 3%        838ms ± 3%    ~     (p=0.275 n=50+49)
Tar                             336ms ± 4%        335ms ± 3%    ~     (p=0.454 n=48+48)
XML                             433ms ± 4%        431ms ± 3%    ~     (p=0.071 n=49+48)
LinkCompiler                    706ms ± 4%        705ms ± 4%    ~     (p=0.608 n=50+49)
ExternalLinkCompiler            1.85s ± 3%        1.83s ± 2%  -1.47%  (p=0.000 n=49+48)
LinkWithoutDebugCompiler        437ms ± 5%        437ms ± 6%    ~     (p=0.953 n=49+50)
[Geo mean]                      615ms             613ms       -0.37%

name                      old alloc/op      new alloc/op      delta
Template                       38.7MB ± 1%       38.7MB ± 1%    ~     (p=0.834 n=50+50)
Unicode                        28.1MB ± 0%       28.1MB ± 0%  -0.22%  (p=0.000 n=49+50)
GoTypes                         168MB ± 1%        168MB ± 1%    ~     (p=0.054 n=47+47)
Compiler                       23.0MB ± 1%       23.0MB ± 1%    ~     (p=0.432 n=50+50)
SSA                            1.54GB ± 0%       1.54GB ± 0%  +0.21%  (p=0.000 n=50+50)
Flate                          23.6MB ± 1%       23.6MB ± 1%    ~     (p=0.153 n=43+46)
GoParser                       35.1MB ± 1%       35.1MB ± 2%    ~     (p=0.202 n=50+50)
Reflect                        84.7MB ± 1%       84.7MB ± 1%    ~     (p=0.333 n=48+49)
Tar                            34.5MB ± 1%       34.5MB ± 1%    ~     (p=0.406 n=46+49)
XML                            44.3MB ± 2%       44.2MB ± 3%    ~     (p=0.981 n=50+50)
LinkCompiler                    131MB ± 0%        128MB ± 0%  -2.74%  (p=0.000 n=50+50)
ExternalLinkCompiler            120MB ± 0%        120MB ± 0%  +0.01%  (p=0.007 n=50+50)
LinkWithoutDebugCompiler       77.3MB ± 0%       77.3MB ± 0%  -0.02%  (p=0.000 n=50+50)
[Geo mean]                     69.3MB            69.1MB       -0.22%

file      before    after     Δ        %
addr2line 4104220   4043684   -60536   -1.475%
api       5342502   5249678   -92824   -1.737%
asm       4973785   4858257   -115528  -2.323%
buildid   2667844   2625660   -42184   -1.581%
cgo       4686849   4616313   -70536   -1.505%
compile   23667431  23268406  -399025  -1.686%
cover     4959676   4874108   -85568   -1.725%
dist      3515934   3450422   -65512   -1.863%
doc       3995581   3925469   -70112   -1.755%
fix       3379202   3318522   -60680   -1.796%
link      6743249   6629913   -113336  -1.681%
nm        4047529   3991777   -55752   -1.377%
objdump   4456151   4388151   -68000   -1.526%
pack      2435040   2398072   -36968   -1.518%
pprof     13804080  13565808  -238272  -1.726%
test2json 2690043   2645987   -44056   -1.638%
trace     10418492  10232716  -185776  -1.783%
vet       7258259   7121259   -137000  -1.888%
total     113145867 111204202 -1941665 -1.716%

The situation on linux/arm64 is as follow:
name                      old time/op       new time/op       delta
Template                        280ms ± 1%        282ms ± 1%  +0.75%  (p=0.000 n=46+48)
Unicode                         124ms ± 2%        124ms ± 2%  +0.37%  (p=0.045 n=50+50)
GoTypes                         1.69s ± 1%        1.70s ± 1%  +0.56%  (p=0.000 n=49+50)
Compiler                        122ms ± 1%        123ms ± 1%  +0.93%  (p=0.000 n=50+50)
SSA                             12.6s ± 1%        12.7s ± 0%  +0.72%  (p=0.000 n=50+50)
Flate                           170ms ± 1%        172ms ± 1%  +0.97%  (p=0.000 n=49+49)
GoParser                        262ms ± 1%        263ms ± 1%  +0.39%  (p=0.000 n=49+48)
Reflect                         639ms ± 1%        650ms ± 1%  +1.63%  (p=0.000 n=49+49)
Tar                             243ms ± 1%        245ms ± 1%  +0.82%  (p=0.000 n=50+50)
XML                             324ms ± 1%        327ms ± 1%  +0.72%  (p=0.000 n=50+49)
LinkCompiler                    597ms ± 1%        596ms ± 1%  -0.27%  (p=0.001 n=48+47)
ExternalLinkCompiler            1.90s ± 1%        1.88s ± 1%  -1.00%  (p=0.000 n=50+50)
LinkWithoutDebugCompiler        364ms ± 1%        363ms ± 1%    ~     (p=0.220 n=49+50)
[Geo mean]                      485ms             488ms       +0.49%

name                      old alloc/op      new alloc/op      delta
Template                       38.7MB ± 0%       38.8MB ± 1%    ~     (p=0.093 n=43+49)
Unicode                        28.4MB ± 0%       28.4MB ± 0%  +0.03%  (p=0.000 n=49+45)
GoTypes                         169MB ± 1%        169MB ± 1%  +0.23%  (p=0.010 n=50+50)
Compiler                       23.2MB ± 1%       23.2MB ± 1%  +0.11%  (p=0.000 n=40+44)
SSA                            1.54GB ± 0%       1.55GB ± 0%  +0.45%  (p=0.000 n=47+49)
Flate                          23.8MB ± 2%       23.8MB ± 1%    ~     (p=0.543 n=50+50)
GoParser                       35.3MB ± 1%       35.4MB ± 1%    ~     (p=0.792 n=50+50)
Reflect                        85.2MB ± 1%       85.2MB ± 0%    ~     (p=0.055 n=50+47)
Tar                            34.5MB ± 1%       34.5MB ± 1%  +0.06%  (p=0.015 n=50+50)
XML                            43.8MB ± 2%       43.9MB ± 2%  +0.19%  (p=0.000 n=48+48)
LinkCompiler                    137MB ± 0%        136MB ± 0%  -0.92%  (p=0.000 n=50+50)
ExternalLinkCompiler            127MB ± 0%        127MB ± 0%    ~     (p=0.516 n=50+50)
LinkWithoutDebugCompiler       84.0MB ± 0%       84.0MB ± 0%    ~     (p=0.057 n=50+50)
[Geo mean]                     70.4MB            70.4MB       +0.01%

file      before    after     Δ        %
addr2line 4021557   4002933   -18624   -0.463%
api       5127847   5028503   -99344   -1.937%
asm       5034716   4936836   -97880   -1.944%
buildid   2608118   2594094   -14024   -0.538%
cgo       4488592   4398320   -90272   -2.011%
compile   22501129  22213592  -287537  -1.278%
cover     4742301   4713573   -28728   -0.606%
dist      3388071   3365311   -22760   -0.672%
doc       3802250   3776082   -26168   -0.688%
fix       3306147   3216939   -89208   -2.698%
link      6404483   6363699   -40784   -0.637%
nm        3941026   3921930   -19096   -0.485%
objdump   4383330   4295122   -88208   -2.012%
pack      2404547   2389515   -15032   -0.625%
pprof     12996234  12856818  -139416  -1.073%
test2json 2668500   2586788   -81712   -3.062%
trace     9816276   9609580   -206696  -2.106%
vet       6900682   6787338   -113344  -1.643%
total     108535806 107056973 -1478833 -1.363%

Change-Id: Iaec1cdcaacca8025e9babb0fb8a532fddb70c87d
Reviewed-on: https://go-review.googlesource.com/c/go/+/255239
Reviewed-by: eric fang <eric.fang@arm.com>
Reviewed-by: Keith Randall <khr@golang.org>
Trust: eric fang <eric.fang@arm.com>

src/cmd/compile/internal/ssa/block.go

@@ -358,6 +358,22 @@ func (b *Block) AuxIntString() string {
 	}
 }
 
+// likelyBranch reports whether block b is the likely branch of all of its predecessors.
+func (b *Block) likelyBranch() bool {
+	if len(b.Preds) == 0 {
+		return false
+	}
+	for _, e := range b.Preds {
+		p := e.b
+		if len(p.Succs) == 1 || len(p.Succs) == 2 && (p.Likely == BranchLikely && p.Succs[0].b == b ||
+			p.Likely == BranchUnlikely && p.Succs[1].b == b) {
+			continue
+		}
+		return false
+	}
+	return true
+}
+
 func (b *Block) Logf(msg string, args ...interface{})   { b.Func.Logf(msg, args...) }
 func (b *Block) Log() bool                              { return b.Func.Log() }
 func (b *Block) Fatalf(msg string, args ...interface{}) { b.Func.Fatalf(msg, args...) }

src/cmd/compile/internal/ssa/layout.go

@@ -41,8 +41,13 @@ func layoutOrder(f *Func) []*Block {
 	indegree := make([]int, f.NumBlocks())
 	posdegree := f.newSparseSet(f.NumBlocks()) // blocks with positive remaining degree
 	defer f.retSparseSet(posdegree)
-	zerodegree := f.newSparseSet(f.NumBlocks()) // blocks with zero remaining degree
+	// blocks with zero remaining degree. Use slice to simulate a LIFO queue to implement
-	defer f.retSparseSet(zerodegree)
+	// the depth-first topology sorting algorithm.
+	var zerodegree []ID
+	// LIFO queue. Track the successor blocks of the scheduled block so that when we
+	// encounter loops, we choose to schedule the successor block of the most recently
+	// scheduled block.
+	var succs []ID
 	exit := f.newSparseSet(f.NumBlocks()) // exit blocks
 	defer f.retSparseSet(exit)
 
@@ -88,7 +93,8 @@ func layoutOrder(f *Func) []*Block {
 		}
 		indegree[b.ID] = len(b.Preds)
 		if len(b.Preds) == 0 {
-			zerodegree.add(b.ID)
+			// Push an element to the tail of the queue.
+			zerodegree = append(zerodegree, b.ID)
 		} else {
 			posdegree.add(b.ID)
 		}
@@ -105,12 +111,24 @@ blockloop:
 			break
 		}
 
-		for _, e := range b.Succs {
+		// Here, the order of traversing the b.Succs affects the direction in which the topological
-			c := e.b
+		// sort advances in depth. Take the following cfg as an example, regardless of other factors.
+		//           b1
+		//         0/ \1
+		//        b2   b3
+		// Traverse b.Succs in order, the right child node b3 will be scheduled immediately after
+		// b1, traverse b.Succs in reverse order, the left child node b2 will be scheduled
+		// immediately after b1. The test results show that reverse traversal performs a little
+		// better.
+		// Note: You need to consider both layout and register allocation when testing performance.
+		for i := len(b.Succs) - 1; i >= 0; i-- {
+			c := b.Succs[i].b
 			indegree[c.ID]--
 			if indegree[c.ID] == 0 {
 				posdegree.remove(c.ID)
-				zerodegree.add(c.ID)
+				zerodegree = append(zerodegree, c.ID)
+			} else {
+				succs = append(succs, c.ID)
 			}
 		}
 
@@ -132,30 +150,30 @@ blockloop:
 
 		// Use degree for now.
 		bid = 0
-		mindegree := f.NumBlocks()
-		for _, e := range order[len(order)-1].Succs {
-			c := e.b
-			if scheduled[c.ID] || c.Kind == BlockExit {
-				continue
-			}
-			if indegree[c.ID] < mindegree {
-				mindegree = indegree[c.ID]
-				bid = c.ID
-			}
-		}
-		if bid != 0 {
-			continue
-		}
 		// TODO: improve this part
 		// No successor of the previously scheduled block works.
 		// Pick a zero-degree block if we can.
-		for zerodegree.size() > 0 {
+		for len(zerodegree) > 0 {
-			cid := zerodegree.pop()
+			// Pop an element from the tail of the queue.
+			cid := zerodegree[len(zerodegree)-1]
+			zerodegree = zerodegree[:len(zerodegree)-1]
 			if !scheduled[cid] {
 				bid = cid
 				continue blockloop
 			}
 		}
+
+		// Still nothing, pick the unscheduled successor block encountered most recently.
+		for len(succs) > 0 {
+			// Pop an element from the tail of the queue.
+			cid := succs[len(succs)-1]
+			succs = succs[:len(succs)-1]
+			if !scheduled[cid] {
+				bid = cid
+				continue blockloop
+			}
+		}
+
 		// Still nothing, pick any non-exit block.
 		for posdegree.size() > 0 {
 			cid := posdegree.pop()

src/cmd/compile/internal/ssa/looprotate.go

@@ -68,12 +68,15 @@ func loopRotate(f *Func) {
 			if nextb == p { // original loop predecessor is next
 				break
 			}
-			if loopnest.b2l[nextb.ID] != loop { // about to leave loop
+			if loopnest.b2l[nextb.ID] == loop {
-				break
+				after[p.ID] = append(after[p.ID], nextb)
 			}
-			after[p.ID] = append(after[p.ID], nextb)
 			b = nextb
 		}
+		// Swap b and p so that we'll handle p before b when moving blocks.
+		f.Blocks[idToIdx[loop.header.ID]] = p
+		f.Blocks[idToIdx[p.ID]] = loop.header
+		idToIdx[loop.header.ID], idToIdx[p.ID] = idToIdx[p.ID], idToIdx[loop.header.ID]
 
 		// Place b after p.
 		for _, b := range after[p.ID] {
@@ -86,21 +89,23 @@ func loopRotate(f *Func) {
 	// before the rest of the loop.  And that relies on the
 	// fact that we only identify reducible loops.
 	j := 0
-	for i, b := range f.Blocks {
+	// Some blocks that are not part of a loop may be placed
+	// between loop blocks. In order to avoid these blocks from
+	// being overwritten, use a temporary slice.
+	newOrder := make([]*Block, 0, f.NumBlocks())
+	for _, b := range f.Blocks {
 		if _, ok := move[b.ID]; ok {
 			continue
 		}
-		f.Blocks[j] = b
+		newOrder = append(newOrder, b)
 		j++
 		for _, a := range after[b.ID] {
-			if j > i {
+			newOrder = append(newOrder, a)
-				f.Fatalf("head before tail in loop %s", b)
-			}
-			f.Blocks[j] = a
 			j++
 		}
 	}
 	if j != len(f.Blocks) {
 		f.Fatalf("bad reordering in looprotate")
 	}
+	f.Blocks = newOrder
 }

src/cmd/compile/internal/ssa/regalloc.go

@@ -241,12 +241,6 @@ type regAllocState struct {
 	GReg        register
 	allocatable regMask
 
-	// for each block, its primary predecessor.
-	// A predecessor of b is primary if it is the closest
-	// predecessor that appears before b in the layout order.
-	// We record the index in the Preds list where the primary predecessor sits.
-	primary []int32
-
 	// live values at the end of each block.  live[b.ID] is a list of value IDs
 	// which are live at the end of b, together with a count of how many instructions
 	// forward to the next use.
@@ -304,6 +298,9 @@ type regAllocState struct {
 
 	// choose a good order in which to visit blocks for allocation purposes.
 	visitOrder []*Block
+
+	// blockOrder[b.ID] corresponds to the index of block b in visitOrder.
+	blockOrder []int32
 }
 
 type endReg struct {
@@ -636,9 +633,9 @@ func (s *regAllocState) init(f *Func) {
 
 	// Compute block order. This array allows us to distinguish forward edges
 	// from backward edges and compute how far they go.
-	blockOrder := make([]int32, f.NumBlocks())
+	s.blockOrder = make([]int32, f.NumBlocks())
 	for i, b := range s.visitOrder {
-		blockOrder[b.ID] = int32(i)
+		s.blockOrder[b.ID] = int32(i)
 	}
 
 	s.regs = make([]regState, s.numRegs)
@@ -664,22 +661,6 @@ func (s *regAllocState) init(f *Func) {
 	}
 	s.computeLive()
 
-	// Compute primary predecessors.
-	s.primary = make([]int32, f.NumBlocks())
-	for _, b := range s.visitOrder {
-		best := -1
-		for i, e := range b.Preds {
-			p := e.b
-			if blockOrder[p.ID] >= blockOrder[b.ID] {
-				continue // backward edge
-			}
-			if best == -1 || blockOrder[p.ID] > blockOrder[b.Preds[best].b.ID] {
-				best = i
-			}
-		}
-		s.primary[b.ID] = int32(best)
-	}
-
 	s.endRegs = make([][]endReg, f.NumBlocks())
 	s.startRegs = make([][]startReg, f.NumBlocks())
 	s.spillLive = make([][]ID, f.NumBlocks())
@@ -957,10 +938,49 @@ func (s *regAllocState) regalloc(f *Func) {
 			// This is the complicated case. We have more than one predecessor,
 			// which means we may have Phi ops.
 
-			// Start with the final register state of the primary predecessor
+			// Start with the final register state of the predecessor with least spill values.
-			idx := s.primary[b.ID]
+			// This is based on the following points:
+			// 1, The less spill value indicates that the register pressure of this path is smaller,
+			//    so the values of this block are more likely to be allocated to registers.
+			// 2, Avoid the predecessor that contains the function call, because the predecessor that
+			//    contains the function call usually generates a lot of spills and lose the previous
+			//    allocation state.
+			// TODO: Improve this part. At least the size of endRegs of the predecessor also has
+			// an impact on the code size and compiler speed. But it is not easy to find a simple
+			// and efficient method that combines multiple factors.
+			idx := -1
+			for i, p := range b.Preds {
+				// If the predecessor has not been visited yet, skip it because its end state
+				// (redRegs and spillLive) has not been computed yet.
+				pb := p.b
+				if s.blockOrder[pb.ID] >= s.blockOrder[b.ID] {
+					continue
+				}
+				if idx == -1 {
+					idx = i
+					continue
+				}
+				pSel := b.Preds[idx].b
+				if len(s.spillLive[pb.ID]) < len(s.spillLive[pSel.ID]) {
+					idx = i
+				} else if len(s.spillLive[pb.ID]) == len(s.spillLive[pSel.ID]) {
+					// Use a bit of likely information. After critical pass, pb and pSel must
+					// be plain blocks, so check edge pb->pb.Preds instead of edge pb->b.
+					// TODO: improve the prediction of the likely predecessor. The following
+					// method is only suitable for the simplest cases. For complex cases,
+					// the prediction may be inaccurate, but this does not affect the
+					// correctness of the program.
+					// According to the layout algorithm, the predecessor with the
+					// smaller blockOrder is the true branch, and the test results show
+					// that it is better to choose the predecessor with a smaller
+					// blockOrder than no choice.
+					if pb.likelyBranch() && !pSel.likelyBranch() || s.blockOrder[pb.ID] < s.blockOrder[pSel.ID] {
+						idx = i
+					}
+				}
+			}
 			if idx < 0 {
-				f.Fatalf("block with no primary predecessor %s", b)
+				f.Fatalf("bad visitOrder, no predecessor of %s has been visited before it", b)
 			}
 			p := b.Preds[idx].b
 			s.setState(s.endRegs[p.ID])
@@ -1048,7 +1068,7 @@ func (s *regAllocState) regalloc(f *Func) {
 				// If one of the other inputs of v is in a register, and the register is available,
 				// select this register, which can save some unnecessary copies.
 				for i, pe := range b.Preds {
-					if int32(i) == idx {
+					if i == idx {
 						continue
 					}
 					ri := noRegister

test/codegen/arithmetic.go

@@ -322,6 +322,9 @@ func NoFix64A(divr int64) (int64, int64) {
 	if divr > 5 {
 		d /= divr // amd64:-"JMP"
 		e %= divr // amd64:-"JMP"
+		// The following statement is to avoid conflict between the above check
+		// and the normal JMP generated at the end of the block.
+		d += e
 	}
 	return d, e
 }
@@ -333,6 +336,7 @@ func NoFix64B(divd int64) (int64, int64) {
 	if divd > -9223372036854775808 {
 		d = divd / divr // amd64:-"JMP"
 		e = divd % divr // amd64:-"JMP"
+		d += e
 	}
 	return d, e
 }
@@ -347,6 +351,7 @@ func NoFix32A(divr int32) (int32, int32) {
 		// amd64:-"JMP"
 		// 386:-"JMP"
 		e %= divr
+		d += e
 	}
 	return d, e
 }
@@ -362,6 +367,7 @@ func NoFix32B(divd int32) (int32, int32) {
 		// amd64:-"JMP"
 		// 386:-"JMP"
 		e = divd % divr
+		d += e
 	}
 	return d, e
 }
@@ -376,6 +382,7 @@ func NoFix16A(divr int16) (int16, int16) {
 		// amd64:-"JMP"
 		// 386:-"JMP"
 		e %= divr
+		d += e
 	}
 	return d, e
 }
@@ -391,6 +398,7 @@ func NoFix16B(divd int16) (int16, int16) {
 		// amd64:-"JMP"
 		// 386:-"JMP"
 		e = divd % divr
+		d += e
 	}
 	return d, e
 }

test/codegen/comparisons.go

@@ -426,7 +426,7 @@ func UintGeqZero(a uint8, b uint16, c uint32, d uint64) int {
 }
 
 func UintGtZero(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `CBZW`, `CBNZW`, `CBNZ`, -`(CMPW|CMP|BLS|BHI)`
+	// arm64: `(CBN?ZW)`, `(CBN?Z[^W])`, -`(CMPW|CMP|BLS|BHI)`
 	if a > 0 || b > 0 || c > 0 || d > 0 {
 		return 1
 	}
@@ -434,7 +434,7 @@ func UintGtZero(a uint8, b uint16, c uint32, d uint64) int {
 }
 
 func UintLeqZero(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `CBNZW`, `CBZW`, `CBZ`, -`(CMPW|CMP|BHI|BLS)`
+	// arm64: `(CBN?ZW)`, `(CBN?Z[^W])`, -`(CMPW|CMP|BHI|BLS)`
 	if a <= 0 || b <= 0 || c <= 0 || d <= 0 {
 		return 1
 	}
@@ -442,7 +442,7 @@ func UintLeqZero(a uint8, b uint16, c uint32, d uint64) int {
 }
 
 func UintLtOne(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `CBNZW`, `CBZW`, `CBZW`, `CBZ`, -`(CMPW|CMP|BHS|BLO)`
+	// arm64: `(CBN?ZW)`, `(CBN?Z[^W])`, -`(CMPW|CMP|BHS|BLO)`
 	if a < 1 || b < 1 || c < 1 || d < 1 {
 		return 1
 	}
@@ -450,7 +450,7 @@ func UintLtOne(a uint8, b uint16, c uint32, d uint64) int {
 }
 
 func UintGeqOne(a uint8, b uint16, c uint32, d uint64) int {
-	// arm64: `CBZW`, `CBNZW`, `CBNZ`, -`(CMPW|CMP|BLO|BHS)`
+	// arm64: `(CBN?ZW)`, `(CBN?Z[^W])`, -`(CMPW|CMP|BLO|BHS)`
 	if a >= 1 || b >= 1 || c >= 1 || d >= 1 {
 		return 1
 	}