[PATCH v2 0/1] Fix block filter heuristic in linespec to preserve distinct code paths

[PATCH v2 0/1] Fix block filter heuristic in linespec to preserve distinct code paths

[Klaus Gerlicher]

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From: "Gerlicher, Klaus" <klaus.gerlicher@intel.com>

Hi all, hi Christina,

thanks for notificing the test failures with V1.

This is V2 of the relax block filter patch. The only change to V1 is the 
test for target execution.

Thanks
Klaus

Klaus Gerlicher (1):
  gdb/linespec: relax block filter to preserve distinct code paths

 gdb/NEWS                                      |  11 +
 gdb/amd64-tdep.c                              |  57 ++++
 gdb/arch-utils.c                              |  10 +
 gdb/arch-utils.h                              |   1 +
 gdb/gdbarch-gen.c                             |  22 ++
 gdb/gdbarch-gen.h                             |  10 +
 gdb/gdbarch_components.py                     |  15 ++
 gdb/linespec.c                                |  98 ++++++-
 .../gdb.linespec/block-filter-cases.c         | 126 +++++++++
 .../gdb.linespec/block-filter-cases.exp       | 248 ++++++++++++++++++
 10 files changed, 595 insertions(+), 3 deletions(-)
 create mode 100644 gdb/testsuite/gdb.linespec/block-filter-cases.c
 create mode 100644 gdb/testsuite/gdb.linespec/block-filter-cases.exp

-- 
2.34.1


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[PATCH v2 1/1] gdb/linespec: relax block filter to preserve distinct code paths

[Klaus Gerlicher]

The block-based duplicate filter in create_sals_line_offset keeps only
the first is_stmt line table entry per block.  This heuristic reduces
multiple for-loop header entries (init/test/increment on same line) to
single breakpoint locations.  However, it is too aggressive for
vectorized loops, where the compiler emits separate code regions for
the vectorized body and scalar remainder, both on the same source line.
The filter discards the scalar remainder entry, causing breakpoints to
be silently missed when only the scalar path executes.

The fix introduces range_has_unconditional_flow_control, which analyzes
the instruction stream for two conditions indicating distinct code paths:

1. Unconditional exit: The last instruction before the second PC is an
   unconditional jmp or ret (execution cannot fall through).

2. Backward branch: Any instruction in the range is a relative branch
   targeting before the first PC (loop back-edge indicating separate
   loop bodies).

A new gdbarch method gdbarch_insn_branch_target decodes relative branch
targets. Implemented for amd64; other architectures default to
conservative behavior, preserving backward compatibility.

Testing covers:
- for_loop_empty_body (-O0): 1 location
- for_loop_large_body (-O0): 1 location (exceeds former scan limits)
- for_loop_vectorized (-O2): >= 2 locations with runtime verification
- nested_vectorized_loops (-O2): >= 2 locations
- loop_with_early_break (-O2): >= 2 or 1 location
- ifelse_separate_lines (-O0): 2 locations (sanity check)

Tested with GCC 11 and ICX 2025.2 on x86_64.
---
 gdb/NEWS                                      |  11 +
 gdb/amd64-tdep.c                              |  57 ++++
 gdb/arch-utils.c                              |  10 +
 gdb/arch-utils.h                              |   1 +
 gdb/gdbarch-gen.c                             |  22 ++
 gdb/gdbarch-gen.h                             |  10 +
 gdb/gdbarch_components.py                     |  15 ++
 gdb/linespec.c                                |  98 ++++++-
 .../gdb.linespec/block-filter-cases.c         | 126 +++++++++
 .../gdb.linespec/block-filter-cases.exp       | 248 ++++++++++++++++++
 10 files changed, 595 insertions(+), 3 deletions(-)
 create mode 100644 gdb/testsuite/gdb.linespec/block-filter-cases.c
 create mode 100644 gdb/testsuite/gdb.linespec/block-filter-cases.exp

diff --git a/gdb/NEWS b/gdb/NEWS
index 7c8cf9af4c2..f55b6518344 100644
--- a/gdb/NEWS
+++ b/gdb/NEWS
@@ -64,6 +64,17 @@
   This fixes an issue where GDB could fail to find a type when relying
   on the index.  Any existing indexes should be regenerated.
 
+* Breakpoint location resolution has been improved for vectorized loops.
+  Previously, when setting a breakpoint on a source line with vectorized
+  code (at -O2 with -ftree-vectorize), GDB's block filter heuristic would
+  discard the scalar remainder location, causing breakpoints to be missed
+  when only the scalar path executed. GDB now correctly preserves multiple
+  locations for vectorized and scalar loop paths, improving debuggability
+  of optimized code.
+
+  Use "set linespec-extended-block-filter off" to disable the new
+  heuristic and revert to the original behavior if needed.
+
 * Support for Floating Point Mode Register (FPMR) in AArch64.
 
 * When running on the Windows Terminal console, GDB now supports
diff --git a/gdb/amd64-tdep.c b/gdb/amd64-tdep.c
index 0d23abb3164..76450775d45 100755
--- a/gdb/amd64-tdep.c
+++ b/gdb/amd64-tdep.c
@@ -1822,6 +1822,62 @@ amd64_insn_is_jump (struct gdbarch *gdbarch, CORE_ADDR addr)
   return amd64_classify_insn_at (gdbarch, addr, amd64_jmp_p);
 }
 
+/* Return the statically-known target of a relative branch instruction
+   (conditional Jcc, JMP, LOOP, JCXZ).  Return std::nullopt for non-branch,
+   indirect branches, or when memory cannot be read.  */
+
+static std::optional<CORE_ADDR>
+amd64_insn_branch_target (struct gdbarch *gdbarch, CORE_ADDR addr)
+{
+  gdb_byte buf[16];
+
+  /* Read enough bytes for the longest relative-branch encoding.  */
+  if (target_read_memory (addr, buf, sizeof (buf)) != 0)
+    return std::nullopt;
+
+  /* Skip a single REX prefix byte (0x40-0x4F).  */
+  int off = 0;
+  if ((buf[0] & 0xf0) == 0x40)
+    off = 1;
+
+  const gdb_byte opc = buf[off];
+
+  /* Short conditional Jcc: 7x cc  (2 bytes, signed 8-bit displacement).  */
+  if ((opc & 0xf0) == 0x70)
+    return addr + off + 2 + (LONGEST) (signed char) buf[off + 1];
+
+  /* Short unconditional JMP: EB cc  (2 bytes).  */
+  if (opc == 0xeb)
+    return addr + off + 2 + (LONGEST) (signed char) buf[off + 1];
+
+  /* LOOP/LOOPE/LOOPNE: E2/E1/E0 cc  (2 bytes, signed 8-bit displacement).  */
+  if ((opc & 0xfd) == 0xe0)
+    return addr + off + 2 + (LONGEST) (signed char) buf[off + 1];
+
+  /* JCXZ/JECXZ/JRCXZ: E3 cc  (2 bytes, signed 8-bit displacement).  */
+  if (opc == 0xe3)
+    return addr + off + 2 + (LONGEST) (signed char) buf[off + 1];
+
+  /* Near conditional Jcc: 0F 8x cc cc cc cc  (6 bytes, signed 32-bit).  */
+  if (opc == 0x0f && (buf[off + 1] & 0xf0) == 0x80)
+    {
+      int32_t disp;
+      memcpy (&disp, buf + off + 2, sizeof (disp));
+      return addr + off + 6 + (LONGEST) disp;
+    }
+
+  /* Near unconditional JMP: E9 cc cc cc cc  (5 bytes, signed 32-bit).  */
+  if (opc == 0xe9)
+    {
+      int32_t disp;
+      memcpy (&disp, buf + off + 1, sizeof (disp));
+      return addr + off + 5 + (LONGEST) disp;
+    }
+
+  /* Indirect or far branches: target cannot be determined statically.  */
+  return std::nullopt;
+}
+
 /* Fix up the state of registers and memory after having single-stepped
    a displaced instruction.  */
 
@@ -3643,6 +3699,7 @@ amd64_init_abi (struct gdbarch_info info, struct gdbarch *gdbarch,
   set_gdbarch_insn_is_call (gdbarch, amd64_insn_is_call);
   set_gdbarch_insn_is_ret (gdbarch, amd64_insn_is_ret);
   set_gdbarch_insn_is_jump (gdbarch, amd64_insn_is_jump);
+  set_gdbarch_insn_branch_target (gdbarch, amd64_insn_branch_target);
 
   set_gdbarch_in_indirect_branch_thunk (gdbarch,
 					amd64_in_indirect_branch_thunk);
diff --git a/gdb/arch-utils.c b/gdb/arch-utils.c
index 5e15fa5b434..aaad41ee376 100644
--- a/gdb/arch-utils.c
+++ b/gdb/arch-utils.c
@@ -45,6 +45,8 @@
 
 #include "dis-asm.h"
 
+#include <optional>
+
 bool
 default_displaced_step_hw_singlestep (struct gdbarch *gdbarch)
 {
@@ -923,6 +925,14 @@ default_insn_is_jump (struct gdbarch *gdbarch, CORE_ADDR addr)
 
 /*  See arch-utils.h.  */
 
+std::optional<CORE_ADDR>
+default_insn_branch_target (struct gdbarch *gdbarch, CORE_ADDR addr)
+{
+  return std::nullopt;
+}
+
+/*  See arch-utils.h.  */
+
 bool
 default_program_breakpoint_here_p (struct gdbarch *gdbarch,
 				   CORE_ADDR address)
diff --git a/gdb/arch-utils.h b/gdb/arch-utils.h
index 06902050043..46d9d89e668 100644
--- a/gdb/arch-utils.h
+++ b/gdb/arch-utils.h
@@ -327,6 +327,7 @@ extern bool default_return_in_first_hidden_param_p (struct gdbarch *,
 extern bool default_insn_is_call (struct gdbarch *, CORE_ADDR);
 extern bool default_insn_is_ret (struct gdbarch *, CORE_ADDR);
 extern bool default_insn_is_jump (struct gdbarch *, CORE_ADDR);
+extern std::optional<CORE_ADDR> default_insn_branch_target (struct gdbarch *, CORE_ADDR);
 
 /* Default implementation of gdbarch_program_breakpoint_here_p.  */
 extern bool default_program_breakpoint_here_p (struct gdbarch *gdbarch,
diff --git a/gdb/gdbarch-gen.c b/gdb/gdbarch-gen.c
index f424fa2a86e..ae3b20f04e0 100644
--- a/gdb/gdbarch-gen.c
+++ b/gdb/gdbarch-gen.c
@@ -234,6 +234,7 @@ struct gdbarch
   gdbarch_insn_is_call_ftype *insn_is_call = default_insn_is_call;
   gdbarch_insn_is_ret_ftype *insn_is_ret = default_insn_is_ret;
   gdbarch_insn_is_jump_ftype *insn_is_jump = default_insn_is_jump;
+  gdbarch_insn_branch_target_ftype *insn_branch_target = default_insn_branch_target;
   gdbarch_program_breakpoint_here_p_ftype *program_breakpoint_here_p = default_program_breakpoint_here_p;
   gdbarch_auxv_parse_ftype *auxv_parse = nullptr;
   gdbarch_print_auxv_entry_ftype *print_auxv_entry = default_print_auxv_entry;
@@ -494,6 +495,7 @@ verify_gdbarch (struct gdbarch *gdbarch)
   /* Skip verify of insn_is_call, invalid_p == 0.  */
   /* Skip verify of insn_is_ret, invalid_p == 0.  */
   /* Skip verify of insn_is_jump, invalid_p == 0.  */
+  /* Skip verify of insn_branch_target, invalid_p == 0.  */
   /* Skip verify of program_breakpoint_here_p, invalid_p == 0.  */
   /* Skip verify of auxv_parse, has predicate.  */
   /* Skip verify of print_auxv_entry, invalid_p == 0.  */
@@ -1276,6 +1278,9 @@ gdbarch_dump (struct gdbarch *gdbarch, struct ui_file *file)
   gdb_printf (file,
 	      "gdbarch_dump: insn_is_jump = <%s>\n",
 	      host_address_to_string (gdbarch->insn_is_jump));
+  gdb_printf (file,
+	      "gdbarch_dump: insn_branch_target = <%s>\n",
+	      host_address_to_string (gdbarch->insn_branch_target));
   gdb_printf (file,
 	      "gdbarch_dump: program_breakpoint_here_p = <%s>\n",
 	      host_address_to_string (gdbarch->program_breakpoint_here_p));
@@ -4950,6 +4955,23 @@ set_gdbarch_insn_is_jump (struct gdbarch *gdbarch,
   gdbarch->insn_is_jump = insn_is_jump;
 }
 
+std::optional<CORE_ADDR>
+gdbarch_insn_branch_target (struct gdbarch *gdbarch, CORE_ADDR addr)
+{
+  gdb_assert (gdbarch != nullptr);
+  gdb_assert (gdbarch->insn_branch_target != nullptr);
+  if (gdbarch_debug >= 2)
+    gdb_printf (gdb_stdlog, "gdbarch_insn_branch_target called\n");
+  return gdbarch->insn_branch_target (gdbarch, addr);
+}
+
+void
+set_gdbarch_insn_branch_target (struct gdbarch *gdbarch,
+				gdbarch_insn_branch_target_ftype insn_branch_target)
+{
+  gdbarch->insn_branch_target = insn_branch_target;
+}
+
 bool
 gdbarch_program_breakpoint_here_p (struct gdbarch *gdbarch, CORE_ADDR address)
 {
diff --git a/gdb/gdbarch-gen.h b/gdb/gdbarch-gen.h
index 678b308fba5..98c2837106e 100644
--- a/gdb/gdbarch-gen.h
+++ b/gdb/gdbarch-gen.h
@@ -1592,6 +1592,16 @@ using gdbarch_insn_is_jump_ftype = bool (struct gdbarch *gdbarch, CORE_ADDR addr
 bool gdbarch_insn_is_jump (struct gdbarch *gdbarch, CORE_ADDR addr);
 void set_gdbarch_insn_is_jump (struct gdbarch *gdbarch, gdbarch_insn_is_jump_ftype *insn_is_jump);
 
+/* For a relative branch instruction at ADDR (conditional or
+   unconditional), return the statically-known branch target address.
+   Return std::nullopt for non-branch instructions, indirect branches,
+   or when reading memory fails.  This is used to detect backward
+   loop back-edges during DWARF line-table analysis. */
+
+using gdbarch_insn_branch_target_ftype = std::optional<CORE_ADDR> (struct gdbarch *gdbarch, CORE_ADDR addr);
+std::optional<CORE_ADDR> gdbarch_insn_branch_target (struct gdbarch *gdbarch, CORE_ADDR addr);
+void set_gdbarch_insn_branch_target (struct gdbarch *gdbarch, gdbarch_insn_branch_target_ftype *insn_branch_target);
+
 /* Return true if there's a program/permanent breakpoint planted in
    memory at ADDRESS, return false otherwise. */
 
diff --git a/gdb/gdbarch_components.py b/gdb/gdbarch_components.py
index b9304d3036d..99f5b3fe0bf 100644
--- a/gdb/gdbarch_components.py
+++ b/gdb/gdbarch_components.py
@@ -2524,6 +2524,21 @@ Return true if the instruction at ADDR is a jump; false otherwise.
     invalid=False,
 )
 
+Method(
+    comment="""
+For a relative branch instruction at ADDR (conditional or
+unconditional), return the statically-known branch target address.
+Return std::nullopt for non-branch instructions, indirect branches,
+or when reading memory fails.  This is used to detect backward
+loop back-edges during DWARF line-table analysis.
+""",
+    type="std::optional<CORE_ADDR>",
+    name="insn_branch_target",
+    params=[("CORE_ADDR", "addr")],
+    predefault="default_insn_branch_target",
+    invalid=False,
+)
+
 Method(
     comment="""
 Return true if there's a program/permanent breakpoint planted in
diff --git a/gdb/linespec.c b/gdb/linespec.c
index 5371d8426ae..a9a1f0c7527 100644
--- a/gdb/linespec.c
+++ b/gdb/linespec.c
@@ -44,9 +44,12 @@
 #include "gdbsupport/function-view.h"
 #include "gdbsupport/def-vector.h"
 #include <algorithm>
+#include "disasm.h"
+#include "gdbarch.h"
 #include "inferior.h"
 #include "gdbsupport/unordered_set.h"
 #include "cli/cli-style.h"
+#include "cli/cli-cmds.h"
 
 /* An enumeration of the various things a user might attempt to
    complete for a linespec location.  */
@@ -432,6 +435,11 @@ static bool compare_msymbols (const bound_minimal_symbol &a,
    previous parser.  */
 static const char linespec_quote_characters[] = "\"\'";
 
+/* Control whether the extended block-filter heuristic is enabled.
+   This heuristic improves breakpoint location resolution for vectorized
+   loops but can be disabled if it causes unexpected behavior.  */
+static bool debug_linespec_extended_block_filter = true;
+
 /* Lexer functions.  */
 
 /* Lex a number from the input in PARSER.  This only supports
@@ -1986,6 +1994,48 @@ canonicalize_linespec (struct linespec_state *state, const linespec *ls)
     explicit_loc->set_string (explicit_loc->to_linespec ());
 }
 
+/* Return true if PC_TO is unreachable by fall-through from PC_FROM.
+   Two conditions are sufficient: (1) any ret in [PC_FROM, PC_TO), or
+   (2) unconditional jump/ret immediately before PC_TO.  Only the last
+   instruction before PC_TO matters; mid-range branches are ignored to
+   preserve for-loop deduplication.  */
+
+static bool
+range_has_unconditional_flow_control (struct gdbarch *gdbarch,
+				      CORE_ADDR pc_from, CORE_ADDR pc_to)
+{
+  if (pc_from >= pc_to)
+    return false;
+
+  /* Return true if: last instruction before PC_TO is unconditional
+     jmp/ret, or any instruction in range is a backward branch
+     (indicating separate loop bodies).  */
+
+  CORE_ADDR pc = pc_from;
+
+  while (pc < pc_to)
+    {
+      int len = gdb_insn_length (gdbarch, pc);
+      if (len <= 0)
+	return false;
+
+      /* Check for backward branch (loop back-edge).  */
+      std::optional<CORE_ADDR> target = gdbarch_insn_branch_target (gdbarch,
+								    pc);
+      if (target.has_value() && target.value() < pc_from)
+	return true;
+
+      /* Is this the last instruction before PC_TO?  */
+      if (pc + len >= pc_to)
+	return gdbarch_insn_is_jump (gdbarch, pc)
+	       || gdbarch_insn_is_ret (gdbarch, pc);
+
+      pc += len;
+    }
+
+  return false;
+}
+
 /* Given a line offset in LS, construct the relevant SALs.  */
 
 static std::vector<symtab_and_line>
@@ -2084,13 +2134,38 @@ create_sals_line_offset (struct linespec_state *self,
 
       for (i = 0; i < intermediate_results.size (); ++i)
 	{
-	  if (blocks[i] != NULL)
+	  if (blocks[i] != NULL && filter[i])
 	    for (j = i + 1; j < intermediate_results.size (); ++j)
 	      {
-		if (blocks[j] == blocks[i])
+		if (blocks[j] == blocks[i] && filter[j])
 		  {
+		    const symtab_and_line &sal_i = intermediate_results[i];
+		    const symtab_and_line &sal_j = intermediate_results[j];
+		    
+		    auto [pc_lo, pc_hi] = std::minmax (sal_i.pc, sal_j.pc);
+
+		    /* Without symtab info, don't filter — we can't reliably
+		       determine if these are distinct code paths.  */
+		    if (sal_i.symtab == nullptr || sal_j.symtab == nullptr)
+		      continue;
+
+		    if (sal_i.symtab != sal_j.symtab)
+		      continue;
+
+		    /* Check for unconditional flow control in the range
+		       (extended heuristic; can be disabled).  Skip if the
+		       target is not running — code memory may not be
+		       readable before relocation.  */
+		    objfile *ofile = sal_i.symtab->compunit ()->objfile ();
+		    gdbarch *gdbarch = ofile->arch ();
+
+		    if (debug_linespec_extended_block_filter
+		        && target_has_execution ()
+		        && range_has_unconditional_flow_control (gdbarch,
+							        pc_lo, pc_hi))
+		      continue;
+
 		    filter[j] = 0;
-		    break;
 		  }
 	      }
 	}
@@ -4342,3 +4417,20 @@ get_gdb_linespec_parser_quote_characters (void)
 {
   return linespec_quote_characters;
 }
+
+/* Register linespec debugging commands.  */
+
+INIT_GDB_FILE (linespec)
+{
+  add_setshow_boolean_cmd ("linespec-extended-block-filter",
+			   class_support,
+			   &debug_linespec_extended_block_filter, _("\
+Set whether the extended block-filter heuristic is enabled."), _("\
+Show whether the extended block-filter heuristic is enabled."), _("\
+The extended heuristic improves breakpoint location resolution for\n\
+vectorized loops at -O2. When off, breakpoints use only the original\n\
+heuristic, which may miss locations in vectorized code.\n\
+Default is on."),
+			   nullptr, nullptr,
+			   &setlist, &showlist);
+}
diff --git a/gdb/testsuite/gdb.linespec/block-filter-cases.c b/gdb/testsuite/gdb.linespec/block-filter-cases.c
new file mode 100644
index 00000000000..8cf834e95f4
--- /dev/null
+++ b/gdb/testsuite/gdb.linespec/block-filter-cases.c
@@ -0,0 +1,126 @@
+/* Test cases for the block-filter heuristic in create_sals_line_offset.
+
+   Copyright (C) 2026 Free Software Foundation, Inc.
+
+   This file is part of GDB.
+
+   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/>.
+
+   One function per tested pattern.  The accompanying .exp file verifies
+   the breakpoint location count for each pattern and, where vectorisation
+   is present, confirms that both the vectorized and scalar-remainder paths
+   are reachable at runtime.
+
+   argc > 1  ->  for_loop_vectorized called with n=1024  (vectorized path)
+   argc == 1 ->  for_loop_vectorized called with n=3     (scalar path only)  */
+
+volatile int sink;
+
+/* for-loop with empty body (-O0).  Expected: 1 location.  */
+
+void __attribute__ ((noinline))
+for_loop_empty_body (int n)
+{
+  for (int i = 0; i < n; i++) {}  /* empty-for-line */
+}
+
+/* for-loop with large body (-O0).  Expected: 1 location.  */
+
+void __attribute__ ((noinline))
+for_loop_large_body (volatile int *a, volatile int *b, volatile int *c,
+		     int n)
+{
+  for (int i = 0; i < n; i++) {  /* large-body-for-line */
+    sink = a[i] * 3 + b[i];
+    sink = sink ^ (sink >> 5);
+    sink += c[i] * 7 - a[i];
+    sink = (sink << 3) | (sink >> 29);
+    sink += a[i] + b[i] + c[i];
+    sink = sink * sink + a[i];
+    sink ^= b[i] * c[i];
+    sink += (a[i] | b[i]) & c[i];
+    sink = sink - a[i] * b[i];
+    sink += c[i] ^ (a[i] + b[i]);
+    sink += a[i] * 5 - b[i] * 3;
+    sink ^= (sink << 7) | (sink >> 25);
+    sink += c[i] * c[i] + a[i];
+    sink = sink / (a[i] + 1);
+    sink += b[i] * b[i] - c[i];
+    sink ^= a[i] ^ b[i] ^ c[i];
+    sink += (a[i] + b[i]) * (b[i] + c[i]);
+    sink = (sink >> 3) + a[i];
+    sink += c[i] * (a[i] - b[i]);
+    sink ^= sink >> 1;
+  }
+}
+
+/* Vectorized for-loop (-O2).  Expected: >= 2 locations.  */
+
+void __attribute__ ((noinline))
+for_loop_vectorized (int *arr, int n)
+{
+  for (int i = 0; i < n; i++)
+    arr[i] = i * 3;  /* vec-loop-line */
+}
+
+/* Nested vectorized for-loops (-O2).  Expected: >= 2 locations.  */
+
+void __attribute__ ((noinline))
+nested_vectorized_loops (int *arr, int n)
+{
+  for (int i = 0; i < n; i++)
+    for (int j = 0; j < n; j++)
+      arr[i * n + j] = i * j;  /* nested-vec-line */
+}
+
+/* For-loop with early break (-O2).  Expected: >= 2 or 1 location.  */
+
+void __attribute__ ((noinline))
+loop_with_early_break (int *arr, int n, int target)
+{
+  for (int i = 0; i < n; i++) {
+    arr[i] = i * 2;  /* break-loop-line */
+    if (arr[i] == target)
+      break;
+  }
+}
+
+/* If-else with both branches on same line (-O0).  Expected: 2 locations.  */
+
+void __attribute__ ((noinline))
+ifelse_separate_lines (int x, int *result)
+{
+  if (x > 0)
+    *result = x * 2;  else *result = -x;  /* ifelse-line */
+}
+
+
+int
+main (int argc, char *argv[])
+{
+  int arr[1024];
+  volatile int a[4] = {1, 2, 3, 4};
+  volatile int b[4] = {4, 3, 2, 1};
+  volatile int c[4] = {1, 1, 1, 1};
+  int result = 0;
+
+  for_loop_empty_body (4);
+  for_loop_large_body (a, b, c, 4);
+  for_loop_vectorized (arr, argc > 1 ? 1024 : 3);
+  nested_vectorized_loops (arr, argc > 1 ? 32 : 3);
+  loop_with_early_break (arr, argc > 1 ? 1024 : 3, 42);
+  ifelse_separate_lines (argc > 1 ? 10 : -5, &result);
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.linespec/block-filter-cases.exp b/gdb/testsuite/gdb.linespec/block-filter-cases.exp
new file mode 100644
index 00000000000..fa4ce5315c6
--- /dev/null
+++ b/gdb/testsuite/gdb.linespec/block-filter-cases.exp
@@ -0,0 +1,248 @@
+# Copyright (C) 2026 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/>.
+
+# Verify the block-filter heuristic in create_sals_line_offset against
+# known patterns.  The compiler is selected by the board file; this test
+# uses consistent expectations across compilers (GCC, ICX, etc.).
+
+standard_testfile
+
+if { [build_executable "failed to prepare (O0)" \
+	  "${testfile}-O0" ${srcfile} { debug optimize=-O0 }] } {
+    return -1
+}
+
+if { [build_executable "failed to prepare (O2)" \
+	  "${testfile}-O2" ${srcfile} \
+	  { debug optimize=-O2 additional_flags=-ftree-vectorize }] } {
+    return -1
+}
+
+# Retrieve all target line numbers from the single shared source file.
+set line_empty_for      [gdb_get_line_number "empty-for-line" ${srcfile}]
+set line_large_body_for [gdb_get_line_number "large-body-for-line" ${srcfile}]
+set line_vec_loop       [gdb_get_line_number "vec-loop-line" ${srcfile}]
+set line_nested_vec     [gdb_get_line_number "nested-vec-line" ${srcfile}]
+set line_break_loop     [gdb_get_line_number "break-loop-line" ${srcfile}]
+set line_ifelse         [gdb_get_line_number "ifelse-line" ${srcfile}]
+
+# Set breakpoint and return how many locations it resolves to.
+proc bp_nlocs { srcfile line desc } {
+    global gdb_prompt
+    set nlocs -1
+    gdb_test_multiple "break ${srcfile}:${line}" $desc {
+	-re "Breakpoint \[0-9\]+ at .*\\((\[0-9\]+) locations\\).*$gdb_prompt $" {
+	    set nlocs $expect_out(1,string)
+	    pass $desc
+	}
+	-re "Breakpoint \[0-9\]+ at .*$gdb_prompt $" {
+	    set nlocs 1
+	    pass $desc
+	}
+    }
+    return $nlocs
+}
+
+clean_restart "${testfile}-O0"
+
+# Verify the linespec-extended-block-filter setting exists and works.
+gdb_test "show linespec-extended-block-filter" \
+    "Whether the extended block-filter heuristic is enabled is on\\." \
+    "show default linespec-extended-block-filter (on)"
+
+gdb_test_no_output "set linespec-extended-block-filter off" \
+    "set linespec-extended-block-filter off"
+
+gdb_test "show linespec-extended-block-filter" \
+    "Whether the extended block-filter heuristic is enabled is off\\." \
+    "show linespec-extended-block-filter (off)"
+
+gdb_test_no_output "set linespec-extended-block-filter on" \
+    "set linespec-extended-block-filter on"
+
+gdb_test "show linespec-extended-block-filter" \
+    "Whether the extended block-filter heuristic is enabled is on\\." \
+    "show linespec-extended-block-filter re-enabled"
+
+with_test_prefix "for-loop empty body (-O0)" {
+    set nlocs [bp_nlocs ${srcfile} ${line_empty_for} "set breakpoint"]
+
+    if { $nlocs == 1 } {
+	pass "1 breakpoint location (correct)"
+    } else {
+	fail "expected 1 location, got $nlocs"
+    }
+
+    delete_breakpoints
+}
+
+with_test_prefix "for-loop exceeding scan limit (-O0)" {
+    set nlocs [bp_nlocs ${srcfile} ${line_large_body_for} "set breakpoint"]
+
+    if { $nlocs == 1 } {
+	pass "1 breakpoint location (correct)"
+    } else {
+	fail "expected 1 location, got $nlocs"
+    }
+
+    delete_breakpoints
+}
+
+with_test_prefix "if-else separate lines (-O0)" {
+    set nlocs [bp_nlocs ${srcfile} ${line_ifelse} "set breakpoint"]
+
+    if { $nlocs == 2 } {
+	pass "2 locations (if-else branches on same line, different blocks)"
+    } else {
+	fail "expected 2 locations, got $nlocs"
+    }
+
+    delete_breakpoints
+}
+
+clean_restart "${testfile}-O2"
+
+with_test_prefix "for-loop vectorized (-O2)" {
+    set nlocs [bp_nlocs ${srcfile} ${line_vec_loop} "set breakpoint"]
+
+    if { $nlocs < 2 } {
+	unsupported "compiler did not produce multiple locations;\
+		     vectorization may not have fired"
+    } else {
+	pass "$nlocs locations preserved (vectorized + scalar paths)"
+    }
+
+    delete_breakpoints
+}
+
+with_test_prefix "nested vectorized loops (-O2)" {
+    set nlocs [bp_nlocs ${srcfile} ${line_nested_vec} "set breakpoint"]
+
+    if { $nlocs < 2 } {
+	unsupported "compiler did not produce multiple locations;\
+		     nested vectorization may not have fired"
+    } else {
+	pass "$nlocs locations preserved (nested paths)"
+    }
+
+    delete_breakpoints
+}
+
+with_test_prefix "loop with early break (-O2)" {
+    set nlocs [bp_nlocs ${srcfile} ${line_break_loop} "set breakpoint"]
+
+    if { $nlocs >= 2 } {
+	pass "$nlocs locations preserved (normal and break paths)"
+    } else {
+	pass "$nlocs location (break path not separately compiled)"
+    }
+
+    delete_breakpoints
+}
+
+with_test_prefix "for-loop vectorized runtime (-O2)" {
+    clean_restart "${testfile}-O2"
+
+    set nlocs [bp_nlocs ${srcfile} ${line_vec_loop} "set breakpoint (probe)"]
+    delete_breakpoints
+
+    if { $nlocs < 2 } {
+	unsupported "only 1 location; skip reachability test"
+    } else {
+	with_test_prefix "scalar (n=3)" {
+	    if { ![runto_main] } {
+		return -1
+	    }
+
+	    gdb_test "break ${srcfile}:${line_vec_loop}" \
+		"Breakpoint .* at .*" \
+		"set breakpoint"
+
+	    gdb_test "continue" \
+		"Breakpoint .*, for_loop_vectorized .*" \
+		"hit breakpoint on scalar path"
+
+	    delete_breakpoints
+	    gdb_test "continue" ".*exited normally.*" "program exits cleanly"
+	}
+
+	clean_restart "${testfile}-O2"
+	gdb_test_no_output "set args foo" "set args for vectorized run"
+
+	if { ![runto_main] } {
+	    return -1
+	}
+
+	with_test_prefix "vectorized (n=1024)" {
+	    gdb_test "break ${srcfile}:${line_vec_loop}" \
+		"Breakpoint .* at .*" \
+		"set breakpoint"
+
+	    gdb_test "continue" \
+		"Breakpoint .*, for_loop_vectorized .*" \
+		"hit breakpoint on vectorized path"
+
+	    delete_breakpoints
+	    gdb_test "continue" ".*exited normally.*" "program exits cleanly"
+	}
+    }
+}
+
+# Behavioral test: verify setting actually affects breakpoint resolution.
+with_test_prefix "behavioral: setting toggle affects resolution" {
+    clean_restart "${testfile}-O0"
+
+    # Test with extended filter ON (default).
+    gdb_test_no_output "set linespec-extended-block-filter on" \
+	"enable extended block-filter"
+
+    set nlocs_on [bp_nlocs ${srcfile} ${line_ifelse} \
+	"set breakpoint on if-else line with filter ON"]
+    delete_breakpoints
+
+    if { $nlocs_on == 2 } {
+	pass "if-else with extended filter ON: 2 locations"
+    } else {
+	fail "expected 2 locations with extended filter ON, got $nlocs_on"
+    }
+
+    # Test with extended filter OFF.
+    gdb_test_no_output "set linespec-extended-block-filter off" \
+	"disable extended block-filter"
+
+    set nlocs_off [bp_nlocs ${srcfile} ${line_ifelse} \
+	"set breakpoint on if-else line with filter OFF"]
+    delete_breakpoints
+
+    if { $nlocs_off == 1 } {
+	pass "if-else with extended filter OFF: 1 location"
+    } else {
+	fail "expected 1 location with extended filter OFF, got $nlocs_off"
+    }
+
+    # Verify toggling it back ON restores the two locations.
+    gdb_test_no_output "set linespec-extended-block-filter on" \
+	"re-enable extended block-filter"
+
+    set nlocs_on_again [bp_nlocs ${srcfile} ${line_ifelse} \
+	"set breakpoint on if-else line with filter re-enabled"]
+    delete_breakpoints
+
+    if { $nlocs_on_again == 2 } {
+	pass "if-else with extended filter re-enabled: 2 locations (verified)"
+    } else {
+	fail "expected 2 locations after re-enabling, got $nlocs_on_again"
+    }
+}
-- 
2.34.1



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Re: [PATCH v2 1/1] gdb/linespec: relax block filter to preserve distinct code paths

[Eli Zaretskii]

> From: Klaus Gerlicher <klaus.gerlicher@intel.com>
> Date: Thu,  4 Jun 2026 13:29:38 +0000
> 
>  gdb/NEWS                                      |  11 +
>  gdb/amd64-tdep.c                              |  57 ++++
>  gdb/arch-utils.c                              |  10 +
>  gdb/arch-utils.h                              |   1 +
>  gdb/gdbarch-gen.c                             |  22 ++
>  gdb/gdbarch-gen.h                             |  10 +
>  gdb/gdbarch_components.py                     |  15 ++
>  gdb/linespec.c                                |  98 ++++++-
>  .../gdb.linespec/block-filter-cases.c         | 126 +++++++++
>  .../gdb.linespec/block-filter-cases.exp       | 248 ++++++++++++++++++
>  10 files changed, 595 insertions(+), 3 deletions(-)
>  create mode 100644 gdb/testsuite/gdb.linespec/block-filter-cases.c
>  create mode 100644 gdb/testsuite/gdb.linespec/block-filter-cases.exp

Thanks.

> diff --git a/gdb/NEWS b/gdb/NEWS
> index 7c8cf9af4c2..f55b6518344 100644
> --- a/gdb/NEWS
> +++ b/gdb/NEWS
> @@ -64,6 +64,17 @@
>    This fixes an issue where GDB could fail to find a type when relying
>    on the index.  Any existing indexes should be regenerated.
>  
> +* Breakpoint location resolution has been improved for vectorized loops.
> +  Previously, when setting a breakpoint on a source line with vectorized
> +  code (at -O2 with -ftree-vectorize), GDB's block filter heuristic would
> +  discard the scalar remainder location, causing breakpoints to be missed
> +  when only the scalar path executed. GDB now correctly preserves multiple
                                       ^^
Please leave two spaces between sentences, per our conventions.

Reviewed-By: Eli Zaretskii <eliz@gnu.org>