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.

gdb/riscv-linux-tdep.c

@@ -27,6 +27,11 @@
 #include "trad-frame.h"
 #include "gdbarch.h"
 
+/* The following value is derived from __NR_rt_sigreturn in
+   <include/uapi/asm-generic/unistd.h> from the Linux source tree.  */
+
+#define RISCV_NR_rt_sigreturn 139
+
 /* Define the general register mapping.  The kernel puts the PC at offset 0,
    gdb puts it at offset 32.  Register x0 is always 0 and can be ignored.
    Registers x1 to x31 are in the same place.  */
@@ -154,11 +159,28 @@ riscv_linux_sigframe_init (const struct tramp_frame *self,
   trad_frame_set_id (this_cache, frame_id_build (frame_sp, func));
 }
 
+/* When FRAME is at a syscall instruction (ECALL), return the PC of the next
+   instruction to be executed.  */
+
+static CORE_ADDR
+riscv_linux_syscall_next_pc (struct frame_info *frame)
+{
+  const CORE_ADDR pc = get_frame_pc (frame);
+  const ULONGEST a7 = get_frame_register_unsigned (frame, RISCV_A7_REGNUM);
+
+  if (a7 == RISCV_NR_rt_sigreturn)
+    return frame_unwind_caller_pc (frame);
+
+  return pc + 4 /* Length of the ECALL insn.  */;
+}
+
 /* Initialize RISC-V Linux ABI info.  */
 
 static void
 riscv_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
 {
+  struct gdbarch_tdep *tdep = gdbarch_tdep (gdbarch);
+
   linux_init_abi (info, gdbarch, 0);
 
   set_gdbarch_software_single_step (gdbarch, riscv_software_single_step);
@@ -182,6 +204,8 @@ riscv_linux_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch)
     (gdbarch, riscv_linux_iterate_over_regset_sections);
 
   tramp_frame_prepend_unwinder (gdbarch, &riscv_linux_sigframe);
+
+  tdep->syscall_next_pc = riscv_linux_syscall_next_pc;
 }
 
 /* Initialize RISC-V Linux target support.  */

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;
 }

gdb/riscv-tdep.h

@@ -34,6 +34,7 @@ enum
   RISCV_FP_REGNUM = 8,		/* Frame Pointer.  */
   RISCV_A0_REGNUM = 10,		/* First argument.  */
   RISCV_A1_REGNUM = 11,		/* Second argument.  */
+  RISCV_A7_REGNUM = 17,		/* Seventh argument.  */
   RISCV_PC_REGNUM = 32,		/* Program Counter.  */
 
   RISCV_NUM_INTEGER_REGS = 32,
@@ -102,6 +103,9 @@ struct gdbarch_tdep
   int duplicate_frm_regnum = -1;
   int duplicate_fcsr_regnum = -1;
 
+  /* Return the expected next PC assuming FRAME is stopped at a syscall
+     instruction.  */
+  CORE_ADDR (*syscall_next_pc) (struct frame_info *frame);
 };