[gdb/symtab] Trust epilogue unwind info for unknown producer (-g0 case)

For a -g0 -fasynchronous-unwind-tables exec (without .debug_info but with
.eh_frame section), start using the dwarf2 unwinder instead of the
"amd64 epilogue override" unwinder, by returning true in
compunit_epilogue_unwind_valid for cust == nullptr.

This has effect both on the amd64 and i386 targets, but only add amd64
test-case gdb.base/unwind-on-each-insn-amd64-2.exp.

gdb/symtab.h

@@ -1924,9 +1924,9 @@ static inline bool
 compunit_epilogue_unwind_valid (struct compunit_symtab *cust)
 {
   /* In absence of producer information, assume epilogue unwind info is
-     invalid.  */
+     valid.  */
   if (cust == nullptr)
-    return false;
+    return true;
 
   return cust->epilogue_unwind_valid ();
 }

gdb/testsuite/gdb.base/unwind-on-each-insn-amd64-2.exp

@@ -0,0 +1,52 @@
+# 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/>.  */
+
+# Check that epilogue unwind info is used, even if no debug info is available.
+
+require is_x86_64_m64_target
+
+set srcfile_flags {debug}
+set srcfile2_flags {nodebug}
+
+if [info exists COMPILE] {
+    # Make sure that we use .eh_frame info, by generating it
+    # using -fasynchronous-unwind-tables.
+    if { [gdb_can_simple_compile fasynchronous-unwind-tables \
+	      { void foo () { } } object -fasynchronous-unwind-tables] } {
+	lappend srcfile2_flags additional_flags=-fasynchronous-unwind-tables
+    } else {
+	unsupported "required: .eh_frame"
+	return
+    }
+    standard_testfile unwind-on-each-insn.c unwind-on-each-insn-foo.c
+    # When updating the .s file, use these flags to generate the file:
+    #lappend srcfile2_flags additional_flags=-save-temps
+    #lappend srcfile2_flags additional_flags=-dA
+    # and do the following:
+    # - copy it in place, run the test-case and verify that all tests pass.
+    # - confuse the amd64 epilogue unwinder by inserting the following
+    #   in foo:
+    #          nop
+    #     +    pushq $.L1
+    #     +    ret
+    #     +  .L1:
+    #     +    nop
+    #          popq %rbp
+    # - verify that the test-case passes.
+} else {
+    standard_testfile unwind-on-each-insn.c .s
+}
+
+source $srcdir/$subdir/unwind-on-each-insn.exp.tcl

gdb/testsuite/gdb.base/unwind-on-each-insn-amd64-2.s

@@ -0,0 +1,54 @@
+	.file	"unwind-on-each-insn-foo.c"
+	.text
+	.globl	foo
+	.type	foo, @function
+foo:
+.LFB0:
+	.cfi_startproc
+# BLOCK 2 seq:0
+# PRED: ENTRY (FALLTHRU)
+	pushq	%rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset 6, -16
+	movq	%rsp, %rbp
+	.cfi_def_cfa_register 6
+	movq	%rdi, -8(%rbp)
+	nop
+        pushq    $.L1
+        ret
+.L1:
+        nop
+	popq	%rbp
+	.cfi_def_cfa 7, 8
+# SUCC: EXIT [100.0%] 
+	ret
+	.cfi_endproc
+.LFE0:
+	.size	foo, .-foo
+	.globl	bar
+	.type	bar, @function
+bar:
+.LFB1:
+	.cfi_startproc
+# BLOCK 2 seq:0
+# PRED: ENTRY (FALLTHRU)
+	pushq	%rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset 6, -16
+	movq	%rsp, %rbp
+	.cfi_def_cfa_register 6
+	subq	$8, %rsp
+	movq	%rdi, -8(%rbp)
+	movq	-8(%rbp), %rax
+	movq	%rax, %rdi
+	call	foo
+	nop
+	leave
+	.cfi_def_cfa 7, 8
+# SUCC: EXIT [100.0%] 
+	ret
+	.cfi_endproc
+.LFE1:
+	.size	bar, .-bar
+	.ident	"GCC: (SUSE Linux) 7.5.0"
+	.section	.note.GNU-stack,"",@progbits