riscv64: exclude S2 from callee-saved registers, reduce frame size.

S2 is a callee-saved GPR by the RISC-V calling convention, but it's used
for shadow stack by the compiler/bionic (https://r.android.com/2427910).
This may change later if RISC-V gets hardware support for shadow stack,
but at the moment S2 should be treated as yet another unallocatable
platform-specific GPR like TP and GP.

Removing S2 from callee-saved stack frames allows us to reduce frame
size by 16 bytes (the other 8 bytes come from padding, as the stack
frames are 16-bytes aligned).

Bug: https://github.com/google/android-riscv64/issues/55
Test: lunch aosp_riscv64-userdebug \
  && art/tools/buildbot-build.sh --target

Change-Id: I727d2e8faac01fb0e8c96ee7494a8a5ef486f685

0 files changed, 0 insertions, 0 deletions