[dev.cc] doc/go1.5.txt: assembler changes

Change-Id: Id544d435620efffaf5757dd9d9ebbc6e969a052c
Reviewed-on: https://go-review.googlesource.com/4823Reviewed-by: Rob Pike <r@golang.org>

doc/go1.5.txt

@@ -31,3 +31,22 @@ sort: number of Sort performance optimizations (https://golang.org/cl/2100, http
 strconv: optimize decimal to string conversion (https://golang.org/cl/2105)
 math/big: faster assembly kernels for amd64 and 386 (https://golang.org/cl/2503, https://golang.org/cl/2560)
 math/big: faster "pure Go" kernels for platforms w/o assembly kernels (https://golang.org/cl/2480)
+
+Assembler:
+
+ARM assembly syntax has had some features removed.
+
+	- mentioning SP or PC as a hardware register
+		These are always pseudo-registers except that in some contexts
+		they're not, and it's confusing because the context should not affect
+		which register you mean. Change the references to the hardware
+		registers to be explicit: R13 for SP, R15 for PC.
+	- constant creation using assignment
+		The files say a=b when they could instead say #define a b.
+		There is no reason to have both mechanisms.
+	- R(0) to refer to R0.
+		Some macros use this to a great extent. Again, it's easy just to
+		use a #define to rename a register.
+	
+Also expression evaluation now uses uint64s instead of signed integers and the
+precedence of operators is now Go-like rather than C-like.