Because GC barriers are used in method entry points for the scalarized calling convention, we have to save all registers before calling into the runtime (see JDK-8242210 and JDK-8251398):
https://github.com/openjdk/valhalla/blob/8efeb68ef0594c2c7c20bc2e45c112d6ef623b2b/src/hotspot/cpu/x86/gc/g1/g1BarrierSetAssembler_x86.cpp#L206
https://github.com/openjdk/valhalla/blob/8efeb68ef0594c2c7c20bc2e45c112d6ef623b2b/src/hotspot/cpu/x86/gc/g1/g1BarrierSetAssembler_x86.cpp#L332

We should investigate if it is sufficient to only save the caller-save argument registers and only do so if the barrier is used in an entry point.

We should also check if increasing the buffer size (JDK-8271324) is still required.

See also JDK-8232094.