cmd/internal/obj/mips: load/store even float registers first

There is a bug in Octeon III processors where storing an odd floating
point register after it has recently been written to by a double
floating point operation will store the old value from before the double
operation (there are some extra details - the operation and store
must be a certain number of cycles apart). However, this bug does not
occur if the even register is stored first. Currently the bug only
happens on big endian because go always loads the even register first on
little endian.

Workaround the bug by always loading / storing the even floating point
register first. Since this is just an instruction reordering, it should
have no performance penalty. This follows other compilers like GCC which
will always store the even register first (although you do have to set
the ISA level to MIPS I to prevent it from using SDC1).

Change-Id: I5e73daa4d724ca1df7bf5228aab19f53f26a4976
Reviewed-on: https://go-review.googlesource.com/97735Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/internal/obj/mips/obj0.go

@@ -558,20 +558,22 @@ func preprocess(ctxt *obj.Link, cursym *obj.LSym, newprog obj.ProgAlloc) {
 			p.Link = q
 			p1 = q.Link
 
-			var regOff int16
+			var addrOff int64
 			if c.ctxt.Arch.ByteOrder == binary.BigEndian {
-				regOff = 1 // load odd register first
+				addrOff = 4 // swap load/save order
 			}
 			if p.From.Type == obj.TYPE_MEM {
 				reg := REG_F0 + (p.To.Reg-REG_F0)&^1
-				p.To.Reg = reg + regOff
-				q.To.Reg = reg + 1 - regOff
-				q.From.Offset += 4
+				p.To.Reg = reg
+				q.To.Reg = reg + 1
+				p.From.Offset += addrOff
+				q.From.Offset += 4 - addrOff
 			} else if p.To.Type == obj.TYPE_MEM {
 				reg := REG_F0 + (p.From.Reg-REG_F0)&^1
-				p.From.Reg = reg + regOff
-				q.From.Reg = reg + 1 - regOff
-				q.To.Offset += 4
+				p.From.Reg = reg
+				q.From.Reg = reg + 1
+				p.To.Offset += addrOff
+				q.To.Offset += 4 - addrOff
 			}
 		}
 	}