LLVM has a CostPerUse attribute on register definitions that both x86-64 and ARM's Thumb2 mode use. See http://llvm.org/viewvc/llvm-project/llvm/trunk/include/l

LLVM has a CostPerUse attribute on register definitions that both x86-64 and ARM's Thumb2 mode use. See http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Target/Target.td?view=markup http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/... The greedy register allocator uses the CostPerUse hints to prefer the lower registers, but it has to balance many optimization goals at once, so it is not always obvious what is going on. For example, it will: - Defer using a callee-saved register for the first time until it is worth the cost of spilling it in the prologue. It won't create a prolog spill just to use a better register. - Try to reuse function argument and return value registers to minimize register copies around function calls. IIRC, the CostPerUse attribute gave a code size savings of around 1% on x86-64 and Thumb2. On both architectures, the effect is reduced by the fact that all register operands must be from the low register set before you can use the smaller encoding.