PR symtab/18392

Initially there is some chain (let's say the longest one
but that doe snot matter).  Consequently its elements from the middle are
being removed and there remains only some few unambiguous top and bottom ones.

The original idea why the comparison should be sharp ("<") was that if there
are multiple chains like (0xaddr show jmp instruction address):
        main(0x100) -> a(0x200) -> d(0x400)
        main(0x100) -> a(0x200) -> c(0x300) -> d(0x400)
then - such situation cannot exist - if two jmp instructions in "a" have the
same address they must also jump to the same address (*).

(*) jump to a computed address would be never considered for the DWARF
    tail-call records.

So there could be:
        main(0x100) -> a(0x200) -> d(0x400)
        main(0x100) -> a(0x270) -> c(0x300) -> d(0x400)
But then "a" frame itself is ambiguous and it must not be displayed.

I did not realize that there can be self-tail-call:
        main(0x100) -> a(0x200) -> d(0x400)
        main(0x100) -> a(0x280) -> a(0x200) -> d(0x400)
which intersects to:
        main(0x100) -> <???>? -> a(0x200) -> d(0x400)
And so if the first chain was chosen the
        main(0x100) -> a(0x200) -> d(0x400)
then the final intersection has callers+callees==length.

> for example, if CALLERS is 3 and
> CALLEES is 2, what does the chain look like?

main(0x100) -> x(0x150) -> y(0x200) -> <???>? -> a(0x200) -> d(0x400)

And if LENGTH is 7 then:
        call_site[0] = main(0x100)
        call_site[1] = x(0x150)
        call_site[2] = y(0x200)
        call_site[3] = garbage
        call_site[4] = garbage
        call_site[5] = a(0x200)
        call_site[6] = d(0x400)

gdb/ChangeLog
2015-06-01  Andreas Schwab  <schwab@linux-m68k.org>
	    Jan Kratochvil  <jan.kratochvil@redhat.com>

	PR symtab/18392
	* dwarf2-frame-tailcall.c (pretended_chain_levels): Correct
	assertion.
	* dwarf2loc.c (chain_candidate): Likewise.

gdb/testsuite/ChangeLog
2015-06-01  Jan Kratochvil  <jan.kratochvil@redhat.com>

	PR symtab/18392
	* gdb.arch/amd64-tailcall-self.S: New file.
	* gdb.arch/amd64-tailcall-self.c: New file.
	* gdb.arch/amd64-tailcall-self.exp: New file.

1 files changed, 31 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.arch/amd64-tailcall-self.exp b/gdb/testsuite/gdb.arch/amd64-tailcall-self.exp
new file mode 100644
index 00000000000..0803ff27887
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/amd64-tailcall-self.exp
@@ -0,0 +1,31 @@
+# Copyright (C) 2015 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/>.
+
+standard_testfile .S
+
+if { ![istarget x86_64-*-* ] || ![is_lp64_target] } {
+    verbose "Skipping ${testfile}."
+    return
+}
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} {}] } {
+    return -1
+}
+
+if ![runto b] {
+    return -1
+}
+
+gdb_test "bt" "#0 +b \\(\\) at \[^\r\n\]*\r\n#1 +0x\[0-9a-f\]+ in a \\(q=<optimized out>\\) at \[^\r\n\]*\r\n#2 +0x\[0-9a-f\]+ in main \\(\[^\r\n\]*\\) at .*"