cmd/internal/obj/arm64: Add helpers for span7 passes

Adds helper functions for the literal pooling, large branch handling
and code emission stages of the span7 assembler pass. This hides the
implementation of the current assembler from the general workflow in
span7 to make the implementation easier to change in future.

Updates #44734

Change-Id: I8859956b23ad4faebeeff6df28051b098ef90fed
Reviewed-on: https://go-review.googlesource.com/c/go/+/595755
Reviewed-by: Cherry Mui <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: snickolls-arm

src/cmd/internal/obj/arm64/asm7.go

@@ -33,6 +33,7 @@ package arm64
 import (
 	"cmd/internal/obj"
 	"cmd/internal/objabi"
+	"encoding/binary"
 	"fmt"
 	"log"
 	"math"
@@ -1099,133 +1100,57 @@ func span7(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 	c := ctxt7{ctxt: ctxt, newprog: newprog, cursym: cursym, autosize: int32(p.To.Offset & 0xffffffff), extrasize: int32(p.To.Offset >> 32)}
 	p.To.Offset &= 0xffffffff // extrasize is no longer needed
 
-	bflag := 1
+	// Process literal pool and allocate initial program counter for each Prog, before
+	// generating branch veneers.
 	pc := int64(0)
 	p.Pc = pc
-	var m int
-	var o *Optab
 	for p = p.Link; p != nil; p = p.Link {
 		p.Pc = pc
-		o = c.oplook(p)
-		m = o.size(c.ctxt, p)
-		if m == 0 {
-			switch p.As {
-			case obj.APCALIGN, obj.APCALIGNMAX:
-				m = obj.AlignmentPadding(int32(pc), p, ctxt, cursym)
-				break
-			case obj.ANOP, obj.AFUNCDATA, obj.APCDATA:
-				continue
-			default:
-				c.ctxt.Diag("zero-width instruction\n%v", p)
-			}
-		}
-		pc += int64(m)
-
-		if o.flag&LFROM != 0 {
-			c.addpool(p, &p.From)
-		}
-		if o.flag&LTO != 0 {
-			c.addpool(p, &p.To)
-		}
-		if c.blitrl != nil {
-			c.checkpool(p)
-		}
+		c.addLiteralsToPool(p)
+		pc += int64(c.asmsizeBytes(p))
 	}
 
-	c.cursym.Size = pc
-
 	/*
 	 * if any procedure is large enough to
 	 * generate a large SBRA branch, then
 	 * generate extra passes putting branches
 	 * around jmps to fix. this is rare.
 	 */
-	for bflag != 0 {
-		bflag = 0
+	changed := true
+	for changed {
+		changed = false
 		pc = 0
 		for p = c.cursym.Func().Text.Link; p != nil; p = p.Link {
 			p.Pc = pc
-			o = c.oplook(p)
-
-			/* very large branches */
-			if (o.flag&BRANCH14BITS != 0 || o.flag&BRANCH19BITS != 0) && p.To.Target() != nil {
-				otxt := p.To.Target().Pc - pc
-				var toofar bool
-				if o.flag&BRANCH14BITS != 0 { // branch instruction encodes 14 bits
-					toofar = otxt <= -(1<<15)+10 || otxt >= (1<<15)-10
-				} else if o.flag&BRANCH19BITS != 0 { // branch instruction encodes 19 bits
-					toofar = otxt <= -(1<<20)+10 || otxt >= (1<<20)-10
-				}
-				if toofar {
-					q := c.newprog()
-					q.Link = p.Link
-					p.Link = q
-					q.As = AB
-					q.To.Type = obj.TYPE_BRANCH
-					q.To.SetTarget(p.To.Target())
-					p.To.SetTarget(q)
-					q = c.newprog()
-					q.Link = p.Link
-					p.Link = q
-					q.As = AB
-					q.To.Type = obj.TYPE_BRANCH
-					q.To.SetTarget(q.Link.Link)
-					bflag = 1
-				}
-			}
-			m = o.size(c.ctxt, p)
-
-			if m == 0 {
-				switch p.As {
-				case obj.APCALIGN, obj.APCALIGNMAX:
-					m = obj.AlignmentPaddingLength(int32(pc), p, ctxt)
-					break
-				case obj.ANOP, obj.AFUNCDATA, obj.APCDATA:
-					continue
-				default:
-					c.ctxt.Diag("zero-width instruction\n%v", p)
-				}
-			}
-
-			pc += int64(m)
+			changed = changed || c.fixUpLongBranch(p)
+			pc += int64(c.asmsizeBytes(p))
 		}
 	}
 
-	pc += -pc & (funcAlign - 1)
-	c.cursym.Size = pc
-
 	/*
 	 * lay out the code, emitting code and data relocations.
 	 */
-	c.cursym.Grow(c.cursym.Size)
-	bp := c.cursym.P
-	psz := int32(0)
-	var i int
-	var out [6]uint32
+	buf := codeBuffer{&c.cursym.P}
+
 	for p := c.cursym.Func().Text.Link; p != nil; p = p.Link {
 		c.pc = p.Pc
-		o = c.oplook(p)
-		sz := o.size(c.ctxt, p)
-		if sz > 4*len(out) {
-			log.Fatalf("out array in span7 is too small, need at least %d for %v", sz/4, p)
-		}
-		if p.As == obj.APCALIGN || p.As == obj.APCALIGNMAX {
+		switch p.As {
+		case obj.APCALIGN, obj.APCALIGNMAX:
 			v := obj.AlignmentPaddingLength(int32(p.Pc), p, c.ctxt)
-			for i = 0; i < int(v/4); i++ {
+			for i := 0; i < int(v/4); i++ {
 				// emit ANOOP instruction by the padding size
-				c.ctxt.Arch.ByteOrder.PutUint32(bp, OP_NOOP)
-				bp = bp[4:]
-				psz += 4
-			}
-		} else {
-			c.asmout(p, o, out[:])
-			for i = 0; i < sz/4; i++ {
-				c.ctxt.Arch.ByteOrder.PutUint32(bp, out[i])
-				bp = bp[4:]
-				psz += 4
+				buf.emit(OP_NOOP)
 			}
+		case obj.ANOP, obj.AFUNCDATA, obj.APCDATA:
+			continue
+		default:
+			var out [6]uint32
+			count := c.asmout(p, out[:])
+			buf.emit(out[:count]...)
 		}
 	}
+	buf.finish()
+	c.cursym.Size = int64(len(c.cursym.P))
 
 	// Mark nonpreemptible instruction sequences.
 	// We use REGTMP as a scratch register during call injection,
@@ -1244,6 +1169,92 @@ func span7(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 	}
 }
 
+type codeBuffer struct {
+	data *[]byte
+}
+
+func (cb *codeBuffer) pc() int64 {
+	return int64(len(*cb.data))
+}
+
+// Write a sequence of opcodes into the code buffer.
+func (cb *codeBuffer) emit(op ...uint32) {
+	for _, o := range op {
+		*cb.data = binary.LittleEndian.AppendUint32(*cb.data, o)
+	}
+}
+
+// Completes the code buffer for the function by padding the buffer to function alignment
+// with zero values.
+func (cb *codeBuffer) finish() {
+	for len(*cb.data)%funcAlign > 0 {
+		*cb.data = append(*cb.data, 0)
+	}
+}
+
+// Return the size of the assembled Prog, in bytes.
+func (c *ctxt7) asmsizeBytes(p *obj.Prog) int {
+	switch p.As {
+	case obj.APCALIGN, obj.APCALIGNMAX:
+		return obj.AlignmentPadding(int32(p.Pc), p, c.ctxt, c.cursym)
+	case obj.ANOP, obj.AFUNCDATA, obj.APCDATA:
+		return 0
+	default:
+		o := c.oplook(p)
+		return o.size(c.ctxt, p)
+	}
+}
+
+// Modify the Prog list if the Prog is a branch with a large offset that cannot be
+// encoded in the instruction. Return true if a modification was made, false if not.
+func (c *ctxt7) fixUpLongBranch(p *obj.Prog) bool {
+	var toofar bool
+
+	o := c.oplook(p)
+
+	/* very large branches */
+	if (o.flag&BRANCH14BITS != 0 || o.flag&BRANCH19BITS != 0) && p.To.Target() != nil {
+		otxt := p.To.Target().Pc - p.Pc
+		if o.flag&BRANCH14BITS != 0 { // branch instruction encodes 14 bits
+			toofar = otxt <= -(1<<15)+10 || otxt >= (1<<15)-10
+		} else if o.flag&BRANCH19BITS != 0 { // branch instruction encodes 19 bits
+			toofar = otxt <= -(1<<20)+10 || otxt >= (1<<20)-10
+		}
+		if toofar {
+			q := c.newprog()
+			q.Link = p.Link
+			p.Link = q
+			q.As = AB
+			q.To.Type = obj.TYPE_BRANCH
+			q.To.SetTarget(p.To.Target())
+			p.To.SetTarget(q)
+			q = c.newprog()
+			q.Link = p.Link
+			p.Link = q
+			q.As = AB
+			q.To.Type = obj.TYPE_BRANCH
+			q.To.SetTarget(q.Link.Link)
+		}
+	}
+
+	return toofar
+}
+
+// Adds literal values from the Prog into the literal pool if necessary.
+func (c *ctxt7) addLiteralsToPool(p *obj.Prog) {
+	o := c.oplook(p)
+
+	if o.flag&LFROM != 0 {
+		c.addpool(p, &p.From)
+	}
+	if o.flag&LTO != 0 {
+		c.addpool(p, &p.To)
+	}
+	if c.blitrl != nil {
+		c.checkpool(p)
+	}
+}
+
 // isUnsafePoint returns whether p is an unsafe point.
 func (c *ctxt7) isUnsafePoint(p *obj.Prog) bool {
 	// If p explicitly uses REGTMP, it's unsafe to preempt, because the
@@ -3456,7 +3467,9 @@ func (c *ctxt7) checkShiftAmount(p *obj.Prog, a *obj.Addr) {
 	}
 }
 
-func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
+func (c *ctxt7) asmout(p *obj.Prog, out []uint32) (count int) {
+	o := c.oplook(p)
+
 	var os [5]uint32
 	o1 := uint32(0)
 	o2 := uint32(0)
@@ -5896,6 +5909,8 @@ func (c *ctxt7) asmout(p *obj.Prog, o *Optab, out []uint32) {
 	out[2] = o3
 	out[3] = o4
 	out[4] = o5
+
+	return int(o.size(c.ctxt, p) / 4)
 }
 
 func (c *ctxt7) addrRelocType(p *obj.Prog) objabi.RelocType {