Re: [PATCH] c++: End lifetime of objects in constexpr after destructor call [PR71093]

[Jason Merrill <jason@redhat.com>]

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On 12/11/23 03:02, Richard Biener wrote:
> On Sun, 10 Dec 2023, Jason Merrill wrote:
> 
>> On 12/10/23 05:22, Richard Biener wrote:
>>>> Am 09.12.2023 um 21:13 schrieb Jason Merrill <jason@redhat.com>:
>>>>
>>>> On 11/2/23 21:18, Nathaniel Shead wrote:
>>>>> Bootstrapped and regtested on x86-64_pc_linux_gnu.
>>>>> I'm not entirely sure if the change I made to have destructors clobber
>>>>> with
>>>>> CLOBBER_EOL instead of CLOBBER_UNDEF is appropriate, but nothing seemed to
>>>>> have
>>>>> broken by doing this and I wasn't able to find anything else that really
>>>>> depended on this distinction other than a warning pass. Otherwise I could
>>>>> experiment with a new clobber kind for destructor calls.
>>>>
>>>> It seems wrong to me: CLOBBER_EOL is documented to mean that the storage is
>>>> expiring at that point as well, which a (pseudo-)destructor does not imply;
>>>> it's perfectly valid to destroy an object and then create another in the
>>>> same storage.
>>>>
>>>> We probably do want another clobber kind for end of object lifetime. And/or
>>>> one for beginning of object lifetime.
>>>
>>> There?s not much semantically different between UNDEF and end of object but
>>> not storage lifetime?  At least for what middle-end optimizations do.
>>
>> That's fine for the middle-end, but Nathaniel's patch wants to distinguish
>> between the clobbers at beginning and end of object lifetime in order to
>> diagnose stores to an out-of-lifetime object in constexpr evaluation.
> 
> Ah, I see.  I did want to add CLOBBER_SOL (start-of-life) when working
> on PR90348, but I always fail to finish working on that stack-slot sharing
> issue.  But it would be for the storage life, not object life, also
> added by gimplification.
> 
>> One option might be to remove the clobber at the beginning of the constructor;
>> are there any useful optimizations enabled by that, or is it just pedantically
>> breaking people's code?
> 
> It's allowing DSE to the object that was live before the new one.  Not
> all objects require explicit destruction (which would get you a clobber)
> before storage can be re-used.
> 
>>> EOL is used by stack slot sharing and that operates on the underlying
>>> storage, not individual objects live in it.
>>
>> I wonder about changing the name to EOS (end of storage [duration]) to avoid
>> similar confusion with object lifetime?
> 
> EOS{L,D}?  But sure, better names (and documentation) are appreciated.

Maybe something like this?  Or shall we write out the names like 
CLOBBER_OBJECT_START, CLOBBER_STORAGE_END, etc?



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From 14f71a9479bd0cf4249c8c9e917a9caf3eac8c82 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Mon, 11 Dec 2023 11:35:31 -0500
Subject: [PATCH] tree: add to clobber_kind
To: gcc-patches@gcc.gnu.org

In discussion of PR71093 it came up that more clobber_kind options would be
useful within the C++ front-end.

gcc/ChangeLog:

	* tree-core.h (enum clobber_kind): Rename CLOBBER_EOL to
	CLOBBER_EOSD.  Add CLOBBER_BOSD, CLOBBER_BOBL, CLOBBER_EOBL.
	* gimple-lower-bitint.cc
	* gimple-ssa-warn-access.cc
	* gimplify.cc
	* tree-inline.cc
	* tree-pretty-print.cc
	* tree-ssa-ccp.cc: Adjust for rename.
---
 gcc/tree-core.h               | 13 ++++++++++---
 gcc/gimple-lower-bitint.cc    |  8 ++++----
 gcc/gimple-ssa-warn-access.cc |  2 +-
 gcc/gimplify.cc               |  8 ++++----
 gcc/tree-inline.cc            |  4 ++--
 gcc/tree-pretty-print.cc      |  2 +-
 gcc/tree-ssa-ccp.cc           |  2 +-
 7 files changed, 23 insertions(+), 16 deletions(-)

diff --git a/gcc/tree-core.h b/gcc/tree-core.h
index 04c04cf2f37..bdf14605c91 100644
--- a/gcc/tree-core.h
+++ b/gcc/tree-core.h
@@ -986,12 +986,19 @@ enum annot_expr_kind {
   annot_expr_kind_last
 };
 
-/* The kind of a TREE_CLOBBER_P CONSTRUCTOR node.  */
+/* The kind of a TREE_CLOBBER_P CONSTRUCTOR node.  Other than _UNDEF, these are
+   in roughly sequential order.  */
 enum clobber_kind {
   /* Unspecified, this clobber acts as a store of an undefined value.  */
   CLOBBER_UNDEF,
-  /* This clobber ends the lifetime of the storage.  */
-  CLOBBER_EOL,
+  /* Beginning of storage duration, e.g. malloc.  */
+  CLOBBER_BOSD,
+  /* Beginning of object lifetime, e.g. C++ constructor.  */
+  CLOBBER_BOBL,
+  /* End of object lifetime, e.g. C++ destructor.  */
+  CLOBBER_EOBL,
+  /* End of storage duration, e.g. free.  */
+  CLOBBER_EOSD,
   CLOBBER_LAST
 };
 
diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index c55c32fb40d..00c3a5b20a8 100644
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -806,7 +806,7 @@ bitint_large_huge::handle_operand (tree op, tree idx)
 	  && m_after_stmt
 	  && bitmap_bit_p (m_single_use_names, SSA_NAME_VERSION (op)))
 	{
-	  tree clobber = build_clobber (TREE_TYPE (m_vars[p]), CLOBBER_EOL);
+	  tree clobber = build_clobber (TREE_TYPE (m_vars[p]), CLOBBER_EOSD);
 	  g = gimple_build_assign (m_vars[p], clobber);
 	  gimple_stmt_iterator gsi = gsi_for_stmt (m_after_stmt);
 	  gsi_insert_after (&gsi, g, GSI_SAME_STMT);
@@ -2063,7 +2063,7 @@ bitint_large_huge::handle_operand_addr (tree op, gimple *stmt,
       tree ret = build_fold_addr_expr (var);
       if (!stmt_ends_bb_p (gsi_stmt (m_gsi)))
 	{
-	  tree clobber = build_clobber (atype, CLOBBER_EOL);
+	  tree clobber = build_clobber (atype, CLOBBER_EOSD);
 	  g = gimple_build_assign (var, clobber);
 	  gsi_insert_after (&m_gsi, g, GSI_SAME_STMT);
 	}
@@ -2100,7 +2100,7 @@ bitint_large_huge::handle_operand_addr (tree op, gimple *stmt,
 	      ret = build_fold_addr_expr (var);
 	      if (!stmt_ends_bb_p (gsi_stmt (m_gsi)))
 		{
-		  tree clobber = build_clobber (m_limb_type, CLOBBER_EOL);
+		  tree clobber = build_clobber (m_limb_type, CLOBBER_EOSD);
 		  g = gimple_build_assign (var, clobber);
 		  gsi_insert_after (&m_gsi, g, GSI_SAME_STMT);
 		}
@@ -3707,7 +3707,7 @@ bitint_large_huge::finish_arith_overflow (tree var, tree obj, tree type,
     }
   if (var)
     {
-      tree clobber = build_clobber (TREE_TYPE (var), CLOBBER_EOL);
+      tree clobber = build_clobber (TREE_TYPE (var), CLOBBER_EOSD);
       g = gimple_build_assign (var, clobber);
       gsi_insert_after (&m_gsi, g, GSI_SAME_STMT);
     }
diff --git a/gcc/gimple-ssa-warn-access.cc b/gcc/gimple-ssa-warn-access.cc
index 1646bd1be14..e71e37214c5 100644
--- a/gcc/gimple-ssa-warn-access.cc
+++ b/gcc/gimple-ssa-warn-access.cc
@@ -4364,7 +4364,7 @@ void
 pass_waccess::check_stmt (gimple *stmt)
 {
   if (m_check_dangling_p
-      && gimple_clobber_p (stmt, CLOBBER_EOL))
+      && gimple_clobber_p (stmt, CLOBBER_EOSD))
     {
       /* Ignore clobber statements in blocks with exceptional edges.  */
       basic_block bb = gimple_bb (stmt);
diff --git a/gcc/gimplify.cc b/gcc/gimplify.cc
index 342e43a7f25..f7a2a4d472c 100644
--- a/gcc/gimplify.cc
+++ b/gcc/gimplify.cc
@@ -1518,7 +1518,7 @@ gimplify_bind_expr (tree *expr_p, gimple_seq *pre_p)
 		      tmp = build_call_expr_loc (EXPR_LOCATION (*e), tmp, 2, v,
 						 build_zero_cst (ptr_type_node));
 		      tsi_link_after (&e, tmp, TSI_SAME_STMT);
-		      tmp = build_clobber (TREE_TYPE (v), CLOBBER_EOL);
+		      tmp = build_clobber (TREE_TYPE (v), CLOBBER_EOSD);
 		      tmp = fold_build2_loc (loc, MODIFY_EXPR, TREE_TYPE (v), v,
 					     fold_convert (TREE_TYPE (v), tmp));
 		      ++e;
@@ -1651,7 +1651,7 @@ gimplify_bind_expr (tree *expr_p, gimple_seq *pre_p)
 					 build_zero_cst (ptr_type_node));
 	      gimplify_and_add (tmp, &cleanup);
 	      gimple *clobber_stmt;
-	      tmp = build_clobber (TREE_TYPE (v), CLOBBER_EOL);
+	      tmp = build_clobber (TREE_TYPE (v), CLOBBER_EOSD);
 	      clobber_stmt = gimple_build_assign (v, tmp);
 	      gimple_set_location (clobber_stmt, end_locus);
 	      gimplify_seq_add_stmt (&cleanup, clobber_stmt);
@@ -1665,7 +1665,7 @@ gimplify_bind_expr (tree *expr_p, gimple_seq *pre_p)
 	      && !is_gimple_reg (t)
 	      && flag_stack_reuse != SR_NONE)
 	    {
-	      tree clobber = build_clobber (TREE_TYPE (t), CLOBBER_EOL);
+	      tree clobber = build_clobber (TREE_TYPE (t), CLOBBER_EOSD);
 	      gimple *clobber_stmt;
 	      clobber_stmt = gimple_build_assign (t, clobber);
 	      gimple_set_location (clobber_stmt, end_locus);
@@ -7417,7 +7417,7 @@ gimplify_target_expr (tree *expr_p, gimple_seq *pre_p, gimple_seq *post_p)
 	{
 	  if (flag_stack_reuse == SR_ALL)
 	    {
-	      tree clobber = build_clobber (TREE_TYPE (temp), CLOBBER_EOL);
+	      tree clobber = build_clobber (TREE_TYPE (temp), CLOBBER_EOSD);
 	      clobber = build2 (MODIFY_EXPR, TREE_TYPE (temp), temp, clobber);
 	      gimple_push_cleanup (temp, clobber, false, pre_p, true);
 	    }
diff --git a/gcc/tree-inline.cc b/gcc/tree-inline.cc
index a4fc839a22d..e59342b1458 100644
--- a/gcc/tree-inline.cc
+++ b/gcc/tree-inline.cc
@@ -5136,7 +5136,7 @@ expand_call_inline (basic_block bb, gimple *stmt, copy_body_data *id,
 	      && !is_gimple_reg (*varp)
 	      && !(id->debug_map && id->debug_map->get (p)))
 	    {
-	      tree clobber = build_clobber (TREE_TYPE (*varp), CLOBBER_EOL);
+	      tree clobber = build_clobber (TREE_TYPE (*varp), CLOBBER_EOSD);
 	      gimple *clobber_stmt;
 	      clobber_stmt = gimple_build_assign (*varp, clobber);
 	      gimple_set_location (clobber_stmt, gimple_location (stmt));
@@ -5208,7 +5208,7 @@ expand_call_inline (basic_block bb, gimple *stmt, copy_body_data *id,
 	  && !is_gimple_reg (id->retvar)
 	  && !stmt_ends_bb_p (stmt))
 	{
-	  tree clobber = build_clobber (TREE_TYPE (id->retvar), CLOBBER_EOL);
+	  tree clobber = build_clobber (TREE_TYPE (id->retvar), CLOBBER_EOSD);
 	  gimple *clobber_stmt;
 	  clobber_stmt = gimple_build_assign (id->retvar, clobber);
 	  gimple_set_location (clobber_stmt, gimple_location (old_stmt));
diff --git a/gcc/tree-pretty-print.cc b/gcc/tree-pretty-print.cc
index 0dabb6d1580..f05ef7a44a3 100644
--- a/gcc/tree-pretty-print.cc
+++ b/gcc/tree-pretty-print.cc
@@ -2624,7 +2624,7 @@ dump_generic_node (pretty_printer *pp, tree node, int spc, dump_flags_t flags,
 	if (TREE_CLOBBER_P (node))
 	  {
 	    pp_string (pp, "CLOBBER");
-	    if (CLOBBER_KIND (node) == CLOBBER_EOL)
+	    if (CLOBBER_KIND (node) == CLOBBER_EOSD)
 	      pp_string (pp, "(eol)");
 	  }
 	else if (TREE_CODE (TREE_TYPE (node)) == RECORD_TYPE
diff --git a/gcc/tree-ssa-ccp.cc b/gcc/tree-ssa-ccp.cc
index ddcbaaaa417..245abaaf1cf 100644
--- a/gcc/tree-ssa-ccp.cc
+++ b/gcc/tree-ssa-ccp.cc
@@ -2525,7 +2525,7 @@ insert_clobber_before_stack_restore (tree saved_val, tree var,
   FOR_EACH_IMM_USE_STMT (stmt, iter, saved_val)
     if (gimple_call_builtin_p (stmt, BUILT_IN_STACK_RESTORE))
       {
-	clobber = build_clobber (TREE_TYPE (var), CLOBBER_EOL);
+	clobber = build_clobber (TREE_TYPE (var), CLOBBER_EOSD);
 	clobber_stmt = gimple_build_assign (var, clobber);
 
 	i = gsi_for_stmt (stmt);
-- 
2.39.3