cmd/compile: refactor stackmap dumping code

Also, fix a byte-ordering problem with stack maps for assembly
function signatures on big-endian targets.

Change-Id: I6e8698f5fbb04b31771a65f4a8f3f9c045ff3c98
Reviewed-on: https://go-review.googlesource.com/30816Reviewed-by: Austin Clements <austin@google.com>

src/cmd/compile/internal/gc/obj.go

@@ -271,6 +271,19 @@ func duintptr(s *Sym, off int, v uint64) int {
 	return duintxx(s, off, v, Widthptr)
 }
 
+func dbvec(s *Sym, off int, bv bvec) int {
+	for j := 0; int32(j) < bv.n; j += 32 {
+		word := bv.b[j/32]
+
+		// Runtime reads the bitmaps as byte arrays. Oblige.
+		off = duint8(s, off, uint8(word))
+		off = duint8(s, off, uint8(word>>8))
+		off = duint8(s, off, uint8(word>>16))
+		off = duint8(s, off, uint8(word>>24))
+	}
+	return off
+}
+
 // stringConstantSyms holds the pair of symbols we create for a
 // constant string.
 type stringConstantSyms struct {

src/cmd/compile/internal/gc/pgen.go

@@ -157,15 +157,11 @@ func emitptrargsmap() {
 		onebitwalktype1(Curfn.Type.Params(), &xoffset, bv)
 	}
 
-	for j := 0; int32(j) < bv.n; j += 32 {
-		off = duint32(sym, off, bv.b[j/32])
-	}
+	off = dbvec(sym, off, bv)
 	if Curfn.Type.Results().NumFields() > 0 {
 		xoffset = 0
 		onebitwalktype1(Curfn.Type.Results(), &xoffset, bv)
-		for j := 0; int32(j) < bv.n; j += 32 {
-			off = duint32(sym, off, bv.b[j/32])
-		}
+		off = dbvec(sym, off, bv)
 	}
 
 	ggloblsym(sym, int32(off), obj.RODATA|obj.LOCAL)

src/cmd/compile/internal/gc/plive.go

@@ -1666,7 +1666,7 @@ func livenessprintdebug(lv *Liveness) {
 // Dumps a slice of bitmaps to a symbol as a sequence of uint32 values. The
 // first word dumped is the total number of bitmaps. The second word is the
 // length of the bitmaps. All bitmaps are assumed to be of equal length. The
-// words that are followed are the raw bitmap words.
+// remaining bytes are the raw bitmaps.
 func onebitwritesymbol(arr []bvec, sym *Sym) {
 	off := 4                                  // number of bitmaps, to fill in later
 	off = duint32(sym, off, uint32(arr[0].n)) // number of bits in each bitmap
@@ -1678,16 +1678,7 @@ func onebitwritesymbol(arr []bvec, sym *Sym) {
 		if bv.b == nil {
 			break
 		}
-		for j := 0; int32(j) < bv.n; j += 32 {
-			word := bv.b[j/32]
-
-			// Runtime reads the bitmaps as byte arrays. Oblige.
-			off = duint8(sym, off, uint8(word))
-
-			off = duint8(sym, off, uint8(word>>8))
-			off = duint8(sym, off, uint8(word>>16))
-			off = duint8(sym, off, uint8(word>>24))
-		}
+		off = dbvec(sym, off, bv)
 	}
 
 	duint32(sym, 0, uint32(i)) // number of bitmaps