bpf: do not save/restore callee-saved registers in function prolog/epilog

bpf: do not save/restore callee-saved registers in function prolog/epilog

BPF considers that every call to a function allocates a fresh set of
registers that are available to the callee, of which the first five
may have bee initialized with the function arguments.  This is
implemented by both interpreter and JIT in the Linux kernel.

This is enforced by the kernel BPF verifier, which will reject any
code in which non-initialized registers are accessed before being
written.  Consequently, the spill instructions generated in function
prologue were causing the verifier to reject our compiled programs.

This patch makes GCC to not save/restore callee-saved registers in
function prologue/epilogue, unless xBPF mode is enabled.

2020-05-19  Jose E. Marchesi  <jose.marchesi@oracle.com>

gcc/
	* config/bpf/bpf.c (bpf_compute_frame_layout): Include space for
	callee saved registers only in xBPF.
	(bpf_expand_prologue): Save callee saved registers only in xBPF.
	(bpf_expand_epilogue): Likewise for restoring.
	* doc/invoke.texi (eBPF Options): Document this is activated by
	-mxbpf.

gcc/testsuite/
	* gcc.target/bpf/xbpf-callee-saved-regs-1.c: New test.
	* gcc.target/bpf/xbpf-callee-saved-regs-2.c: Likewise.