gdb/riscv: Support c.li in prologue unwinder

I was seeing some failures in gdb.threads/omp-par-scope.exp when run
on a riscv64 target.  It turns out the cause of the problem is that I
didn't have debug information installed for libgomp.so, which this
test makes use of.  The test requires GDB to backtrace through a
libgomp function, and the riscv prologue unwinder was failing to
unwind this particular stack frame.

The reason for the failure to unwind was that the function prologue
includes a c.li (compressed load immediate) instruction, and the riscv
prologue scanning unwinder doesn't know what to do with this
instruction, though the unwinder does understand c.lui (compressed
load unsigned immediate).

This commit adds support for c.li.  After this GDB is able to unwind
through libgomp, and I no longer see any unexpected failures in
gdb.threads/omp-par-scope.exp.

I've also included a new test in gdb.arch/ which specifically checks
for our c.li support.

gdb/riscv-tdep.c

@@ -1562,6 +1562,7 @@ public:
       ADDW,
       AUIPC,
       LUI,
+      LI,
       SD,
       SW,
       LD,
@@ -1943,6 +1944,8 @@ riscv_insn::decode (struct gdbarch *gdbarch, CORE_ADDR pc)
 	  m_rd = decode_register_index (ival, OP_SH_CRS1S);
 	  m_imm.s = EXTRACT_CITYPE_LUI_IMM (ival);
 	}
+      else if (is_c_li_insn (ival))
+	decode_ci_type_insn (LI, ival);
       /* C_SD and C_FSW have the same opcode.  C_SD is RV64 and RV128 only,
 	 and C_FSW is RV32 only.  */
       else if (xlen != 4 && is_c_sd_insn (ival))
@@ -2302,10 +2305,11 @@ riscv_scan_prologue (struct gdbarch *gdbarch,
 	  gdb_assert (insn.rd () < RISCV_NUM_INTEGER_REGS);
 	  regs[insn.rd ()] = pv_constant (cur_pc + insn.imm_signed ());
 	}
-      else if (insn.opcode () == riscv_insn::LUI)
+      else if (insn.opcode () == riscv_insn::LUI
+	       || insn.opcode () == riscv_insn::LI)
 	{
 	  /* Handle: lui REG, n
-	     Where REG is not gp register.  */
+	     or:     li  REG, n  */
 	  gdb_assert (insn.rd () < RISCV_NUM_INTEGER_REGS);
 	  regs[insn.rd ()] = pv_constant (insn.imm_signed ());
 	}

gdb/testsuite/gdb.arch/riscv64-unwind-prologue-with-c_li-foo.s

@@ -0,0 +1,47 @@
+/* Copyright 2023 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+
+/* Simple asm function that makes use of c.li and c.lui in the
+   function prologue before the return address and frame pointers are
+   written the stack.  This ensures that GDB's prologue unwinder can
+   understand these instructions.  */
+
+	.option pic
+	.text
+
+	.globl	foo
+	.type	foo, @function
+foo:
+	addi	sp,sp,-32
+	sd	s1,8(sp)
+
+	c.li	s1,-4
+	c.lui	s1,4
+
+	sd	fp,24(sp)
+	sd	ra,16(sp)
+	addi	fp,sp,32
+
+	call	bar@plt
+
+	ld	s1,8(sp)
+	ld	ra,16(sp)
+	ld	fp,24(sp)
+	addi	sp,sp,32
+	jr	ra
+
+	.size	foo, .-foo
+	.section	.note.GNU-stack,"",@progbits

gdb/testsuite/gdb.arch/riscv64-unwind-prologue-with-c_li.c

@@ -0,0 +1,29 @@
+/* Copyright 2023 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* See riscv64-unwind-prologue-with-c_li-asm.s.  */
+extern int foo (void);
+
+int
+bar (void)
+{
+  return 0;
+}
+
+int
+main (void)
+{
+  return foo ();
+}

gdb/testsuite/gdb.arch/riscv64-unwind-prologue-with-c_li.exp

@@ -0,0 +1,42 @@
+# Copyright 2023 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# This tests GDB's ability to use the RISC-V prologue scanner in order
+# to unwind through a function that uses the compressed 'c.li'
+# instruction in its prologue.
+
+require {istarget "riscv64-*-*"}
+
+standard_testfile .c -foo.s
+
+if {[prepare_for_testing "failed to prepare" $testfile \
+			 "$srcfile $srcfile2"  nodebug]} {
+    return -1
+}
+
+if {![runto_main]} {
+    return 0
+}
+
+gdb_breakpoint "bar"
+gdb_continue_to_breakpoint "bar"
+gdb_test "bt" \
+    [multi_line \
+	 "#0\[ \t\]*$hex in bar \\\(\\\)" \
+	 "#1\[ \t\]*$hex in foo \\\(\\\)" \
+	 "#2\[ \t\]*$hex in main \\\(\\\)"] \
+    "Backtrace to the main frame"
+gdb_test "finish" "foo \\\(\\\)" "finish bar"
+gdb_test "finish" "main \\\(\\\)" "finish foo"