[gdb/symtab] Ignore zero line table entries

[gdb/symtab] Ignore zero line table entries

The DWARF standard states for the line register in the line number information
state machine the following:
...
An unsigned integer indicating a source line number.  Lines are numbered
beginning at 1.  The compiler may emit the value 0 in cases where an
instruction cannot be attributed to any source line.
...

So, it's possible to have a zero line number in the DWARF line table.

This is currently not handled by GDB.  The zero value is read in as any other
line number, but internally the zero value has a special meaning:
end-of-sequence, so the line table entry ends up having a different
interpretation than intended in some situations.

I've created a test-case where various aspects are tested, which has these 4
interesting tests.

1. Next-step through a zero-line instruction, is_stmt == 1
gdb.dwarf2/dw2-line-number-zero.exp: bar1, 2nd next

2. Next-step through a zero-line instruction, is_stmt == 0
gdb.dwarf2/dw2-line-number-zero.exp: bar2, 2nd next

3. Show source location at zero-line instruction, is_stmt == 1
gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar1_label_3

4. Show source location at zero-line instruction, is_stmt == 0
gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar2_label_3

And we have the following results:

8.3.1, 9.2:
...
FAIL: gdb.dwarf2/dw2-line-number-zero.exp: bar1, 2nd next
PASS: gdb.dwarf2/dw2-line-number-zero.exp: bar2, 2nd next
PASS: gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar1_label_3
FAIL: gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar2_label_3
...

commit 8c95582da8 "gdb: Add support for tracking the DWARF line table is-stmt
field":
...
PASS: gdb.dwarf2/dw2-line-number-zero.exp: bar1, 2nd next
PASS: gdb.dwarf2/dw2-line-number-zero.exp: bar2, 2nd next
FAIL: gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar1_label_3
FAIL: gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar2_label_3
...

commit d8cc8af6a1 "[gdb/symtab] Fix line-table end-of-sequence sorting",
master:
FAIL: gdb.dwarf2/dw2-line-number-zero.exp: bar1, 2nd next
FAIL: gdb.dwarf2/dw2-line-number-zero.exp: bar2, 2nd next
PASS: gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar1_label_3
PASS: gdb.dwarf2/dw2-line-number-zero.exp: continue to breakpoint: bar2_label_3
...

The regression in test 2 at commit d8cc8af6a1 was filed as PR symtab/26243,
where clang emits zero line numbers.

The way to fix all tests is to make sure line number zero internally doesn't
clash with special meaning values, and by handling it appropriately
everywhere.  That however looks too intrusive for the GDB 10 release.

Instead, we decide to ensure defined behaviour for line number zero by
ignoring it.  This gives us back the test results from before commit
d8cc8af6a1, fixing PR26243.

We mark the FAILs for tests 3 and 4 as KFAILs.  Test 4 was already failing for
the 9.2 release, and we consider the regression of test 3 from gdb 9.2 to gdb
10 the cost for having defined behaviour.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-07-25  Tom de Vries  <tdevries@suse.de>

	PR symtab/26243
	* dwarf2/read.c (lnp_state_machine::record_line): Ignore zero line
	entries.

gdb/testsuite/ChangeLog:

2020-07-25  Tom de Vries  <tdevries@suse.de>

	PR symtab/26243
	* gdb.dwarf2/dw2-line-number-zero.c: New test.
	* gdb.dwarf2/dw2-line-number-zero.exp: New file.