[PING][PATCH,v5] Fix reverse stepping multiple contiguous PC ranges over the line table

Carl Love via Gdb-patches
2022-05-13T17:00:44Z

GDB maintainers:

We have addressed the comments from Bruno. Unfortunately, Bruno is not
a maintainer and can't give approval for the patch. We are hoping a
maintainer can review the patch and provide us feedback.

Thank you for your time.

Carl Love
---------------------------------------------------

On Fri, 2022-05-06 at 09:48 -0700, Carl Love via Gdb-patches wrote:
> Bruno, GDB maintainers:
>
> The patch has been updated per the comments on the testcase from
> Bruno.
>
> The patch was retested on Power 10 to ensure the test still works
> correctly.
>
> Please let us know if there are any additional comments or if the
> patch
> is ready to commit. We thank you for your help with this patch.
>
> Carl Love
> --------------------------------------------------------
> Fix reverse stepping multiple contiguous PC ranges over the line
> table
> v5:- Updated test case comments on the purpose of the test.- Add test
> to check system supports record-replay.- Removed now unnecessary
> reord
> test when activating record.
> v4:- Updated testcase to make it a bit longer so it can exercise
> reverse-stepping multiple times.- Cleaned up debugging prints.
> v3:- Updated testcase. The format for writing the DWARF program body
> in the testcase expect file changed. See commit
> gdb/testsuite/dwarf:
> simplify line number program syntax (commit
> d4c4a2298cad06ca71cfef725f5248f68205f0be)
> v2:- Check if both the line and symtab match for a particular line
> table entry.
> --
> When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also
> spotted onthe ppc backend), I noticed some failures in
> gdb.reverse/solib-precsave.expand gdb.reverse/solib-reverse.exp.
> The failure happens around the following code:
> 38 b[1] = shr2(17); /* middle part two */40 b[0] =
> 6; b[1] = 9; /* generic statement, end part two */42 shr1
> ("message
> 1\n"); /* shr1 one */
> Normal execution:
> - step from line 38 will land on line 40.- step from line 40 will
> land
> on line 42.
> Reverse execution:
> - step from line 42 will land on line 40.- step from line 40 will
> land
> on line 40.- step from line 40 will land on line 38.
> The problem here is that line 40 contains two contiguous but
> distinctPC
> ranges in the line table, like so:
> Line 40 - [0x7ec ~ 0x7f4]Line 40 - [0x7f4 ~ 0x7fc]
> The two distinct ranges are generated because GCC started outputting
> sourcecolumn information, which GDB doesn't take into account at the
> moment.
> When stepping forward from line 40, we skip both of these ranges and
> land online 42. When stepping backward from line 42, we stop at the
> start PC of thesecond (or first, going backwards) range of line 40.
> This happens because we have this check in
> infrun.c:process_event_stop_test:
> /* When stepping backward, stop at beginning of line range
> (unless it's the function entry point, in which case keep going
> back to the call point). */ CORE_ADDR stop_pc = ecs-
> > event_thread->stop_pc (); if (stop_pc == ecs->event_thread-
> > control.step_range_start && stop_pc != ecs->stop_func_start
> && execution_direction == EXEC_REVERSE) end_stepping_range
> (ecs); else keep_going (ecs);
> Since we've reached ecs->event_thread->control.step_range_start, we
> stopstepping backwards.
> The right thing to do is to look for adjacent PC ranges for the same
> line,until we notice a line change. Then we take that as the start PC
> of therange.
> Another solution I thought about is to merge the contiguous ranges
> whenwe are reading the line tables. Though I'm not sure if we really
> want to processthat data as opposed to keeping it as the compiler
> created, and then workingaround that.
> In any case, the following patch addresses this problem.
> Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl Love
> hasverified that it does fix a similar issue on ppc.
> Ubuntu 18.04 doesn't actually run into these failures because the
> compilerdoesn't generate distinct PC ranges for the same line.
> I see similar failures on x86_64 in the gdb.reverse
> tests(gdb.reverse/step-reverse.exp and gdb.reverse/step-reverse.exp).
> Those arealso fixed by this patch.
> The included testcase (based on a test Carl wrote) exercises this
> problem forArm, ppc and x86. It shows full passes with the patch
> applied.
> Co-authored-by: Carl Love <cel@us.ibm.com>---
> gdb/infrun.c | 22 ++-
> gdb/symtab.c | 49 ++++++
> gdb/symtab.h | 16 ++
> gdb/testsuite/gdb.reverse/map-to-same-line.c | 55 +++++++
> .../gdb.reverse/map-to-same-line.exp | 141
> ++++++++++++++++++
> 5 files changed, 282 insertions(+), 1 deletion(-) create mode 100644
> gdb/testsuite/gdb.reverse/map-to-same-line.c create mode 100644
> gdb/testsuite/gdb.reverse/map-to-same-line.exp
> diff --git a/gdb/infrun.c b/gdb/infrun.cindex
> 6e5853ef42a..82c28961aeb
> 100644--- a/gdb/infrun.c+++ b/gdb/infrun.c@@ -6955,11 +6955,31 @@ if
> (ecs->event_thread->control.proceed_to_finish have software
> watchpoints). */ ecs->event_thread->control.may_range_step =
> 1;
> + /* When we are stepping inside a particular line range, in
> reverse,+ and we are sitting at the first address of that range,
> we need to+ check if this address also shows up in another line
> range as the+ end address.++ If so, we need to check what line
> such
> a step range points to.+ If it points to the same line as the
> current step range, that+ means we need to keep going in order
> to reach the first address+ of the line range. We repeat this
> until we eventually get to the+ first address of a particular
> line
> we're stepping through. */+ CORE_ADDR range_start = ecs-
> > event_thread->control.step_range_start;+ if
> > (execution_direction
> == EXEC_REVERSE)+ {+ gdb::optional<CORE_ADDR>
> real_range_start+ = find_line_range_start (ecs->event_thread-
> > stop_pc ());++ if (real_range_start.has_value ())+
> > range_start
> = *real_range_start;+ }+ /* When stepping backward, stop at
> beginning of line range (unless it's the function entry point,
> in which case keep going back to the call
> point). */ CORE_ADDR stop_pc = ecs->event_thread->stop_pc
> ();- if (stop_pc == ecs->event_thread-
> > control.step_range_start+ if (stop_pc == range_start &&
> stop_pc != ecs->stop_func_start && execution_direction ==
> EXEC_REVERSE) end_stepping_range (ecs);diff --git
> a/gdb/symtab.c
> b/gdb/symtab.cindex 4b33d6c91af..de4cb5dd0eb 100644---
> a/gdb/symtab.c+++ b/gdb/symtab.c@@ -3433,6 +3433,55 @@ find_pc_line
> (CORE_ADDR pc, int notcurrent) return sal; } +/* Compare two
> symtab_and_line entries. Return true if both have+ the same line
> number and the same symtab pointer. That means we+ are dealing
> with
> two entries from the same line and from the same+ source
> file.++ Return false otherwise. */++static
> bool+sal_line_symtab_matches_p (const symtab_and_line &sal1,+
> const symtab_and_line &sal2)+{+ return (sal1.line ==
> sal2.line && sal1.symtab == sal2.symtab);+}++/* See
> symtah.h. */++gdb::optional<CORE_ADDR>+find_line_range_start
> (CORE_ADDR pc)+{+ struct symtab_and_line current_sal = find_pc_line
> (pc, 0);++ if (current_sal.line == 0)+ return {};++ struct
> symtab_and_line prev_sal = find_pc_line (current_sal.pc - 1,
> 0);++ /*
> If the previous entry is for a different line, that means we are
> already+ at the entry with the start PC for this line. */+ if
> (!sal_line_symtab_matches_p (prev_sal, current_sal))+ return
> current_sal.pc;++ /* Otherwise, keep looking for entries for the
> same
> line but with+ smaller PC's. */+ bool done = false;+ CORE_ADDR
> prev_pc;+ while (!done)+ {+ prev_pc =
> prev_sal.pc;++ prev_sal = find_pc_line (prev_pc - 1,
> 0);++ /*
> Did we notice a line change? If so, we are done with the
> search. */+ if (!sal_line_symtab_matches_p (prev_sal,
> current_sal))+ done = true;+ }++ return prev_pc;+}+ /* See
> symtab.h. */ struct symtab *diff --git a/gdb/symtab.h
> b/gdb/symtab.hindex b1cf84f756f..226fe8803db 100644---
> a/gdb/symtab.h+++ b/gdb/symtab.h@@ -2285,6 +2285,22 @@ extern struct
> symtab_and_line find_pc_line (CORE_ADDR, int); extern struct
> symtab_and_line find_pc_sect_line (CORE_ADDR,
>
> struct obj_section *, int); +/* Given PC, and assuming
> it is part of a range of addresses that is part of a+ line, go back
> through the linetable and find the starting PC of
> that+ line.++ For
> example, suppose we have 3 PC ranges for line X:++ Line X - [0x0 -
> 0x8]+ Line X - [0x8 - 0x10]+ Line X - [0x10 - 0x18]++ If we
> call
> the function with PC == 0x14, we want to return 0x0, as that
> is+ the
> starting PC of line X, and the ranges are contiguous.+*/++extern
> gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR pc);+ /*
> Wrapper around find_pc_line to just return the symtab. */ extern
> struct symtab *find_pc_line_symtab (CORE_ADDR);diff --git
> a/gdb/testsuite/gdb.reverse/map-to-same-line.c
> b/gdb/testsuite/gdb.reverse/map-to-same-line.cnew file mode
> 100644index
> 00000000000..dd9f9f8a400--- /dev/null+++
> b/gdb/testsuite/gdb.reverse/map-to-same-line.c@@ -0,0 +1,55 @@+/*
> Copyright 2022 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/
> >. */++/* The purpose of this test is to create a DWARF line table
> that contains two+ or more entries for the same line. When
> stepping
> (forwards or backwards),+ GDB should step over the entire line and
> not just a particular entry in the+ line table. */++int+main
> ()+{ /* TAG: main prologue */+ asm ("main_label: .globl
> main_label");+ int i = 1, j = 2, k;+ float f1 = 2.0, f2 = 4.1,
> f3;+ const char *str_1 = "foo", *str_2 = "bar", *str_3;++ asm
> ("line1: .globl line1");+ k = i; f3 = f1; str_3 = str_1; /* TAG:
> line 1 */++ asm ("line2: .globl line2");+ k = j; f3 = f2; str_3 =
> str_2; /* TAG: line 2 */++ asm ("line3: .globl line3");+ k = i;
> f3
> = f1; str_3 = str_1; /* TAG: line 3 */++ asm ("line4: .globl
> line4");+ k = j; f3 = f2; str_3 = str_2; /* TAG: line 4 */++ asm
> ("line5: .globl line5");+ k = i; f3 = f1; str_3 = str_1; /* TAG:
> line 5 */++ asm ("line6: .globl line6");+ k = j; f3 = f2; str_3 =
> str_2; /* TAG: line 6 */++ asm ("line7: .globl line7");+ k = i;
> f3
> = f1; str_3 = str_1; /* TAG: line 7 */++ asm ("line8: .globl
> line8");+ k = j; f3 = f2; str_3 = str_2; /* TAG: line 8 */++ asm
> ("main_return: .globl main_return");+ return 0; /* TAG: main return
> */+}diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> b/gdb/testsuite/gdb.reverse/map-to-same-line.expnew file mode
> 100644index 00000000000..c3fb859be55--- /dev/null+++
> b/gdb/testsuite/gdb.reverse/map-to-same-line.exp@@ -0,0 +1,141 @@+#
> Copyright 2022 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/
> >.++# When stepping (forwards or backwards), GDB should step over
> the
> entire line+# and not just a particular entry in the line table. This
> test was added to+# verify the find_line_range_start function
> properly
> sets the step range for a+# line that consists of multiple
> statements,
> i.e. multiple entries in the line+# table. This test creates a DWARF
> line table that contains two entries for+# the same line to do the
> needed testing.++load_lib dwarf.exp++# This test can only be run on
> targets which support DWARF-2 and use gas.+if {![dwarf2_support]}
> {+ unsupported "dwarf2 support required for this test"+ return
> 0+}++if [get_compiler_info] {+ return -1+}++# The DWARF assembler
> requires the gcc compiler.+if {!$gcc_compiled} {+ unsupported "gcc
> is required for this test"+ return 0+}++# This test suitable only
> for process record-replay+if ![supports_process_record]
> {+ return+}++standard_testfile .c .S++if { [prepare_for_testing
> "failed to prepare" ${testfile} ${srcfile}] } {+ return -1+}++set
> asm_file [standard_output_file $srcfile2]+Dwarf::assemble $asm_file
> {+ global srcdir subdir srcfile+ declare_labels integer_label
> L++ # Find start address and length of program+ lassign
> [function_range main [list ${srcdir}/${subdir}/$srcfile]] \+ main_st
> art main_len+ set main_end "$main_start + $main_len"++ cu {} {+
>
> compile_unit {+ {language @DW_LANG_C}+ {name
> map-to-same-
> line.c}+ {stmt_list $L DW_FORM_sec_offset}+ {low_pc 0
> addr}+ } {+ subprogram {+ {external 1
> flag}+
> {name main}+ {low_pc $main_start addr}+
> {high_pc $main_len DW_FORM_data4}+ }+ }+ }++ lines
> {version 2 default_is_stmt 1} L {+ include_dir
> "${srcdir}/${subdir}"+ file_name "$srcfile" 1++ # Generate
> the
> line table program with distinct source lines being+ # mapped to the
> same line entry. Line 1, 5 and 8 contain 1 statement+ # each. Line 2
> contains 2 statements. Line 3 contains 3 statements.+ program
> {+
> DW_LNE_set_address $main_start+ line [gdb_get_line_number
> "TAG: main prologue"]+ DW_LNS_copy+ DW_LNE_set_addres
> s
> line1+ line [gdb_get_line_number "TAG: line 1" ]+ D
> W_LNS_copy
> + DW_LNE_set_address line2+ line [gdb_get_line_number
> "TAG: line 2" ]+ DW_LNS_copy+ DW_LNE_set_address
> line3+ line [gdb_get_line_number "TAG: line 2" ]+ D
> W_LNS_copy
> + DW_LNE_set_address line4+ line [gdb_get_line_number
> "TAG: line 3" ]+ DW_LNS_copy+ DW_LNE_set_address
> line5+ line [gdb_get_line_number "TAG: line 3" ]+ D
> W_LNS_copy
> + DW_LNE_set_address line6+ line [gdb_get_line_number
> "TAG: line 3" ]+ DW_LNS_copy+ DW_LNE_set_address
> line7+ line [gdb_get_line_number "TAG: line 5" ]+ D
> W_LNS_copy
> + DW_LNE_set_address line8+ line [gdb_get_line_number
> "TAG: line 8" ]+ DW_LNS_copy+ DW_LNE_set_address
> main_return+ line [gdb_get_line_number "TAG: main return"]+
> DW_LNS_copy+ DW_LNE_end_sequence+ }+ }+}++if {
> [prepare_for_testing "failed to prepare" ${testfile} \+ [list
> $srcfile
> $asm_file] {nodebug} ] } {+ return -1+}++if ![runto_main]
> {+ return -1+}++# Activate process
> record/replay+gdb_test_no_output
> "record" "turn on process record"++gdb_test "tbreak main_return"
> "Temporary breakpoint .*" "breakpoint at return"+gdb_test "continue"
> "Temporary breakpoint .*" "run to end of main"++# At this point, GDB
> has already recorded the execution up until the return+#
> statement. Reverse-step and test if GDB transitions between lines in
> the+# expected order. It should reverse-step across lines 8, 5, 3, 2
> and 1.+foreach line {8 5 3 2 1} {+ gdb_test "reverse-step" ".*TAG:
> line $line.*" "reverse step to line $line"+}-- 2.31.1
>
>