Note that r14/lr can be used.

armv7/README

@@ -4,6 +4,8 @@ For efficient loads and stores, use ldmia, stmia and friends. Can do
 two loads or stores per cycle with 8-byte aligned addresses, or three
 loads or stores in two cycles, regardless of alignment.
 
+12 usable registers (if we exclude r9).
+
 ABI gnueabi(hf) (not depending on the floating point conventions)
 
 Registers	May be		Argument
@@ -23,7 +25,7 @@ r10		N
 r11		N
 r12 (ip)	Y
 r13 (sp)
-r14 (lr)
+r14 (lr)        N
 r15 (pc)
 
 q0 (d0, d1)	Y		1 (for "hf" abi)