gdb: Support stepping out from signal handler on riscv*-linux

Currently, gdb cannot step outside of a signal handler on RISC-V
platforms.  This causes multiple failures in gdb.base/sigstep.exp:

	FAIL: gdb.base/sigstep.exp: continue to handler, nothing in handler, step from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: continue to handler, si+advance in handler, step from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: continue to handler, nothing in handler, next from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: continue to handler, si+advance in handler, next from handler: leave handler (timeout)
	FAIL: gdb.base/sigstep.exp: stepi from handleri: leave signal trampoline
	FAIL: gdb.base/sigstep.exp: nexti from handleri: leave signal trampoline

	                === gdb Summary ===

	# of expected passes            587
	# of unexpected failures        6

This patch adds support for stepping outside of a signal handler on
riscv*-*-linux*.

Implementation is heavily inspired from mips_linux_syscall_next_pc and
surroundings as advised by Pedro Alves.

After this patch, all tests in gdb.base/sigstep.exp pass.

Build and tested on riscv64-linux-gnu.

1 files changed, 10 insertions, 0 deletions

diff --git a/gdb/riscv-tdep.c b/gdb/riscv-tdep.c
index 19e2616..b5b0d2d 100644
--- a/gdb/riscv-tdep.c
+++ b/gdb/riscv-tdep.c
@@ -1421,6 +1421,8 @@ public:
       /* These are needed for stepping over atomic sequences.  */
       LR,
       SC,
+      /* This instruction is used to do a syscall.  */
+      ECALL,
 
       /* Other instructions are not interesting during the prologue scan, and
 	 are ignored.  */
@@ -1711,6 +1713,8 @@ riscv_insn::decode (struct gdbarch *gdbarch, CORE_ADDR pc)
 	decode_r_type_insn (SC, ival);
       else if (is_sc_d_insn (ival))
 	decode_r_type_insn (SC, ival);
+      else if (is_ecall_insn (ival))
+	decode_i_type_insn (ECALL, ival);
       else
 	/* None of the other fields are valid in this case.  */
 	m_opcode = OTHER;
@@ -3764,6 +3768,7 @@ static CORE_ADDR
 riscv_next_pc (struct regcache *regcache, CORE_ADDR pc)
 {
   struct gdbarch *gdbarch = regcache->arch ();
+  const struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
   struct riscv_insn insn;
   CORE_ADDR next_pc;
 
@@ -3826,6 +3831,11 @@ riscv_next_pc (struct regcache *regcache, CORE_ADDR pc)
       if (src1 >= src2)
 	next_pc = pc + insn.imm_signed ();
     }
+  else if (insn.opcode () == riscv_insn::ECALL)
+    {
+      if (tdep->syscall_next_pc != nullptr)
+	next_pc = tdep->syscall_next_pc (get_current_frame ());
+    }
 
   return next_pc;
 }