[RFC][debug] Add -fadd-debug-nops

[Tom de Vries <tdevries@suse.de>]

Hi,

this is an idea that I'm currently playing around with: adding nops in
an optimized application with debug info can improve the debug info.


Consider f.i. this gdb session in foo of pr54200-2.c (using -Os):
...
(gdb) n
26            return a; /* { dg-final { gdb-test . "(int)a" "6" } } */
(gdb) p a
'a' has unknown type; cast it to its declared type
(gdb) n
main () at pr54200-2.c:34
34        return 0;
...

The problem is that the scope in which a is declared ends at .LBE7, and the
statement .loc for line 26 ends up attached to the ret insn:
...
        .loc 1 24 11
        addl    %edx, %eax
.LVL1:
        # DEBUG a => ax
        .loc 1 26 7 is_stmt 1
.LBE7:
        .loc 1 28 1 is_stmt 0
        ret
        .cfi_endproc
...

This patch fixes the problem (for Og and Os, the 'DEBUG a => ax' is missing
for O1 and higher) by adding a nop before the ret insn:
...
        .loc 1 24 11
        addl    %edx, %eax
 .LVL1:
        # DEBUG a => ax
        .loc 1 26 7 is_stmt 1
+       nop
 .LBE7:
        .loc 1 28 1 is_stmt 0
        ret
        .cfi_endproc
...

and instead we have:
...
(gdb) n
26            return a; /* { dg-final { gdb-test . "(int)a" "6" } } */
(gdb) p a
$1 = 6
(gdb) n
main () at pr54200-2.c:34
34        return 0;
...

Any comments?

Thanks,
- Tom

[debug] Add -fadd-debug-nops

---
 gcc/common.opt                           |  4 ++++
 gcc/testsuite/gcc.dg/guality/pr54200-2.c | 37 ++++++++++++++++++++++++++++++++
 gcc/var-tracking.c                       | 16 ++++++++++++++
 3 files changed, 57 insertions(+)

diff --git a/gcc/common.opt b/gcc/common.opt
index c29abdb5cb1..8e2876d2b8e 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -1504,6 +1504,10 @@ Common Report Var(flag_hoist_adjacent_loads) Optimization
 Enable hoisting adjacent loads to encourage generating conditional move
 instructions.
 
+fadd-debug-nops
+Common Report Var(flag_add_debug_nops) Optimization
+Add nops if that improves debugging of optimized code
+
 fkeep-gc-roots-live
 Common Undocumented Report Var(flag_keep_gc_roots_live) Optimization
 ; Always keep a pointer to a live memory block
diff --git a/gcc/testsuite/gcc.dg/guality/pr54200-2.c b/gcc/testsuite/gcc.dg/guality/pr54200-2.c
new file mode 100644
index 00000000000..2694f7401e2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/guality/pr54200-2.c
@@ -0,0 +1,37 @@
+/* { dg-do run } */
+/* { dg-skip-if "" { *-*-* }  { "*" } { "-Og" "-Os" "-O0" } } */
+/* { dg-options "-g -fadd-debug-nops -DMAIN" } */
+
+#include "prevent-optimization.h"
+
+int o ATTRIBUTE_USED;
+
+void
+bar (void)
+{
+  o = 2;
+}
+
+int __attribute__((noinline,noclone))
+foo (int z, int x, int b)
+{
+  if (x == 1)
+    {
+      bar ();
+      return z;
+    }
+  else
+    {
+      int a = (x + z) + b;
+      /* Add cast to prevent unsupported.  */
+      return a; /* { dg-final { gdb-test . "(int)a" "6" } } */
+    }
+}
+
+#ifdef MAIN
+int main ()
+{
+  foo (3, 2, 1);
+  return 0;
+}
+#endif
diff --git a/gcc/var-tracking.c b/gcc/var-tracking.c
index 5537fa64085..230845a38a0 100644
--- a/gcc/var-tracking.c
+++ b/gcc/var-tracking.c
@@ -9994,6 +9994,15 @@ reemit_marker_as_note (rtx_insn *insn)
     }
 }
 
+static void
+emit_nop_before (rtx_insn *insn)
+{
+  rtx_insn *insert_after = PREV_INSN (insn);
+  while (INSN_LOCATION (insert_after) == INSN_LOCATION (insn))
+    insert_after = PREV_INSN (insert_after);
+  emit_insn_after (gen_nop (), insert_after);
+}
+
 /* Allocate and initialize the data structures for variable tracking
    and parse the RTL to get the micro operations.  */
 
@@ -10224,6 +10233,13 @@ vt_initialize (void)
 		      continue;
 		    }
 
+		  /* Add debug nops before ret insn.  */
+		  if (optimize
+		      && flag_add_debug_nops
+		      && cfun->debug_nonbind_markers
+		      && returnjump_p (insn))
+		    emit_nop_before (insn);
+
 		  if (MAY_HAVE_DEBUG_BIND_INSNS)
 		    {
 		      if (CALL_P (insn))