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authorAndrew Burgess <andrew.burgess@embecosm.com>2019-12-30 21:04:51 +0000
committerAndrew Burgess <andrew.burgess@embecosm.com>2020-03-10 22:32:07 +0000
commit8c95582da858ac981f689a6f599acacb8c5c490f (patch)
tree4a48968bceed4a3d14a653155c092fbd8e808b9f /gdb/symtab.c
parentcecf854779049c3f491caf23f80d13eabbb90c86 (diff)
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gdb: Add support for tracking the DWARF line table is-stmt field
This commit brings support for the DWARF line table is_stmt field to GDB. The is_stmt field is used by the compiler when a single source line is split into multiple assembler instructions, especially if the assembler instructions are interleaved with instruction from other source lines. The compiler will set the is_stmt flag false from some instructions from the source lines, these instructions are not a good place to insert a breakpoint in order to stop at the source line. Instructions which are marked with the is_stmt flag true are a good place to insert a breakpoint for that source line. Currently GDB ignores all instructions for which is_stmt is false. This is fine in a lot of cases, however, there are some cases where this means the debug experience is not as good as it could be. Consider stopping at a random instruction, currently this instruction will be attributed to the last line table entry before this point for which is_stmt was true - as these are the only line table entries that GDB tracks. This can easily be incorrect in code with even a low level of optimisation. With is_stmt tracking in place, when stopping at a random instruction we now attribute the instruction back to the real source line, even when is_stmt is false for that instruction in the line table. When inserting breakpoints we still select line table entries for which is_stmt is true, so the breakpoint placing behaviour should not change. When stepping though code (at the line level, not the instruction level) we will still stop at instruction where is_stmt is true, I think this is more likely to be the desired behaviour. Instruction stepping is, of course, unchanged, stepping one instruction at a time, but we should now report more accurate line table information with each instruction step. The original motivation for this work was a patch posted by Bernd here: https://sourceware.org/ml/gdb-patches/2019-11/msg00792.html As part of that thread it was suggested that many issues would be resolved if GDB supported line table views, this isn't something I've attempted in this patch, though reading the spec, it seems like this would be a useful feature to support in GDB in the future. The spec is here: http://dwarfstd.org/ShowIssue.php?issue=170427.1 And Bernd gives a brief description of the benefits here: https://sourceware.org/ml/gdb-patches/2020-01/msg00147.html With that all said, I think that there is benefit to having proper is_stmt support regardless of whether we have views support, so I think we should consider getting this in first, and then building view support on top of this. The gdb.cp/step-and-next-inline.exp test is based off a test proposed by Bernd Edlinger in this message: https://sourceware.org/ml/gdb-patches/2019-12/msg00842.html gdb/ChangeLog: * buildsym-legacy.c (record_line): Pass extra parameter to record_line. * buildsym.c (buildsym_compunit::record_line): Take an extra parameter, reduce duplication in the line table, and record the is_stmt flag in the line table. * buildsym.h (buildsym_compunit::record_line): Add extra parameter. * disasm.c (do_mixed_source_and_assembly_deprecated): Ignore non-statement lines. * dwarf2/read.c (dwarf_record_line_1): Add extra parameter, pass this to the symtab builder. (dwarf_finish_line): Pass extra parameter to dwarf_record_line_1. (lnp_state_machine::record_line): Pass a suitable is_stmt flag through to dwarf_record_line_1. * infrun.c (process_event_stop_test): When stepping, don't stop at a non-statement instruction, and only refresh the step info when we land in the middle of a line's range. Also add an extra comment. * jit.c (jit_symtab_line_mapping_add_impl): Initialise is_stmt field. * record-btrace.c (btrace_find_line_range): Only record lines marked as is-statement. * stack.c (frame_show_address): Show the frame address if we are in a non-statement sal. * symmisc.c (dump_symtab_1): Print the is_stmt flag. (maintenance_print_one_line_table): Print a header for the is_stmt column, and include is_stmt information in the output. * symtab.c (find_pc_sect_line): Find lines marked as statements in preference to non-statements. (find_pcs_for_symtab_line): Prefer is-statement entries. (find_line_common): Likewise. * symtab.h (struct linetable_entry): Add is_stmt field. (struct symtab_and_line): Likewise. * xcoffread.c (arrange_linetable): Initialise is_stmt field when arranging the line table. gdb/testsuite/ChangeLog: * gdb.cp/step-and-next-inline.cc: New file. * gdb.cp/step-and-next-inline.exp: New file. * gdb.cp/step-and-next-inline.h: New file. * gdb.dwarf2/dw2-is-stmt.c: New file. * gdb.dwarf2/dw2-is-stmt.exp: New file. * gdb.dwarf2/dw2-is-stmt-2.c: New file. * gdb.dwarf2/dw2-is-stmt-2.exp: New file. * gdb.dwarf2/dw2-ranges-base.exp: Update line table pattern.
Diffstat (limited to 'gdb/symtab.c')
-rw-r--r--gdb/symtab.c25
1 files changed, 24 insertions, 1 deletions
diff --git a/gdb/symtab.c b/gdb/symtab.c
index a80b80d..44b7113 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -3236,6 +3236,23 @@ find_pc_sect_line (CORE_ADDR pc, struct obj_section *section, int notcurrent)
best = prev;
best_symtab = iter_s;
+ /* If during the binary search we land on a non-statement entry,
+ scan backward through entries at the same address to see if
+ there is an entry marked as is-statement. In theory this
+ duplication should have been removed from the line table
+ during construction, this is just a double check. If the line
+ table has had the duplication removed then this should be
+ pretty cheap. */
+ if (!best->is_stmt)
+ {
+ struct linetable_entry *tmp = best;
+ while (tmp > first && (tmp - 1)->pc == tmp->pc
+ && (tmp - 1)->line != 0 && !tmp->is_stmt)
+ --tmp;
+ if (tmp->is_stmt)
+ best = tmp;
+ }
+
/* Discard BEST_END if it's before the PC of the current BEST. */
if (best_end <= best->pc)
best_end = 0;
@@ -3266,6 +3283,7 @@ find_pc_sect_line (CORE_ADDR pc, struct obj_section *section, int notcurrent)
}
else
{
+ val.is_stmt = best->is_stmt;
val.symtab = best_symtab;
val.line = best->line;
val.pc = best->pc;
@@ -3434,7 +3452,8 @@ find_pcs_for_symtab_line (struct symtab *symtab, int line,
{
struct linetable_entry *item = &SYMTAB_LINETABLE (symtab)->item[idx];
- if (*best_item == NULL || item->line < (*best_item)->line)
+ if (*best_item == NULL
+ || (item->line < (*best_item)->line && item->is_stmt))
*best_item = item;
break;
@@ -3545,6 +3564,10 @@ find_line_common (struct linetable *l, int lineno,
{
struct linetable_entry *item = &(l->item[i]);
+ /* Ignore non-statements. */
+ if (!item->is_stmt)
+ continue;
+
if (item->line == lineno)
{
/* Return the first (lowest address) entry which matches. */