[PATCH] Keep track of the FUNCTION_BEG note

[PATCH] Keep track of the FUNCTION_BEG note

[Richard Sandiford]

function.cc emits a NOTE_FUNCTION_BEG after all arguments have
been copied to pseudos.  It then records this note in parm_birth_insn.
Various other pieces of code use this insn as a convenient place to
insert things at the start of the function.

However, cfgexpand later changes parm_birth_insn as follows:

  /* If we emitted any instructions for setting up the variables,
     emit them before the FUNCTION_START note.  */
  if (var_seq)
    {
      emit_insn_before (var_seq, parm_birth_insn);

      /* In expand_function_end we'll insert the alloca save/restore
	 before parm_birth_insn.  We've just insertted an alloca call.
	 Adjust the pointer to match.  */
      parm_birth_insn = var_seq;
    }

But the FUNCTION_BEG note is still useful for things that aren't
sensitive to stack allocation, and it has the advantage that
(unlike the var_seq above) it is never deleted or combined.
This patch adds a separate variable to track it.

Tested on aarch64-linux-gnu, where it's needed for fixing PR113196.
OK to install?

Richard


gcc/
	* emit-rtl.h (rtl_data::x_function_beg_note): New member variable.
	(function_beg_insn): New macro.
	* function.cc (expand_function_start): Initialize function_beg_insn.
---
 gcc/emit-rtl.h  | 4 ++++
 gcc/function.cc | 2 +-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/emit-rtl.h b/gcc/emit-rtl.h
index f749ca9f2a0..34f44cb2990 100644
--- a/gcc/emit-rtl.h
+++ b/gcc/emit-rtl.h
@@ -141,6 +141,9 @@ struct GTY(()) rtl_data {
      If stack grows up, this is the address for the next slot.  */
   poly_int64 x_frame_offset;
 
+  /* The function's FUNCTION_BEG note.  */
+  rtx_insn *x_function_beg_insn;
+
   /* Insn after which register parms and SAVE_EXPRs are born, if nonopt.  */
   rtx_insn *x_parm_birth_insn;
 
@@ -323,6 +326,7 @@ struct GTY(()) rtl_data {
 #define return_label (crtl->x_return_label)
 #define naked_return_label (crtl->x_naked_return_label)
 #define stack_slot_list (crtl->x_stack_slot_list)
+#define function_beg_insn (crtl->x_function_beg_insn)
 #define parm_birth_insn (crtl->x_parm_birth_insn)
 #define frame_offset (crtl->x_frame_offset)
 #define stack_check_probe_note (crtl->x_stack_check_probe_note)
diff --git a/gcc/function.cc b/gcc/function.cc
index de356f7fba3..5ffd438475e 100644
--- a/gcc/function.cc
+++ b/gcc/function.cc
@@ -5202,7 +5202,7 @@ expand_function_start (tree subr)
 
   gcc_assert (NOTE_P (get_last_insn ()));
 
-  parm_birth_insn = get_last_insn ();
+  function_beg_insn = parm_birth_insn = get_last_insn ();
 
   /* If the function receives a non-local goto, then store the
      bits we need to restore the frame pointer.  */
-- 
2.25.1

Re: [PATCH] Keep track of the FUNCTION_BEG note

[Jeff Law]

On 1/5/24 09:28, Richard Sandiford wrote:
> function.cc emits a NOTE_FUNCTION_BEG after all arguments have
> been copied to pseudos.  It then records this note in parm_birth_insn.
> Various other pieces of code use this insn as a convenient place to
> insert things at the start of the function.
> 
> However, cfgexpand later changes parm_birth_insn as follows:
> 
>    /* If we emitted any instructions for setting up the variables,
>       emit them before the FUNCTION_START note.  */
>    if (var_seq)
>      {
>        emit_insn_before (var_seq, parm_birth_insn);
> 
>        /* In expand_function_end we'll insert the alloca save/restore
> 	 before parm_birth_insn.  We've just insertted an alloca call.
> 	 Adjust the pointer to match.  */
>        parm_birth_insn = var_seq;
>      }
> 
> But the FUNCTION_BEG note is still useful for things that aren't
> sensitive to stack allocation, and it has the advantage that
> (unlike the var_seq above) it is never deleted or combined.
> This patch adds a separate variable to track it.
> 
> Tested on aarch64-linux-gnu, where it's needed for fixing PR113196.
> OK to install?
> 
> Richard
> 
> 
> gcc/
> 	* emit-rtl.h (rtl_data::x_function_beg_note): New member variable.
> 	(function_beg_insn): New macro.
> 	* function.cc (expand_function_start): Initialize function_beg_insn.
OK
jeff