Hi!
Appears that I'm too dumb to implement
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101544#c22>:
| We "simply" need to transform any symbol aliases into thunks calling the
aliasee (or duplicate the bodies, if we must).
..., and implementing proper support for symbol aliases via the nvptx
'ld': <https://gcc.gnu.org/PR105018> "[nvptx] Need better alias support",
won't be complete any time soon, but fortunately there's a way to handle
the cases that we're concerned about here in an even simpler way, for
reasonably modern configurations; any comments before I push the attached
"nvptx: In offloading compilation, special-case certain host-setup symbol
aliases [PR101544]"?
In particular, is the use of a new 'c++ cdtor alias' attribute OK?
Grüße
Thomas
>From 9bc9f515ed26602e97ebbcac453f73cddb8afcc0 Mon Sep 17 00:00:00 2001
From: Thomas Schwinge <tschwi...@baylibre.com>
Date: Thu, 20 Mar 2025 14:21:26 +0100
Subject: [PATCH] nvptx: In offloading compilation, special-case certain
host-setup symbol aliases [PR101544]
Namely, use PTX '.alias' even for (default) '-mno-alias' if the host made the
C++ "base and complete [cd]tor aliases".
PR target/101544
gcc/
* config/nvptx/nvptx.cc (nvptx_asm_output_def_from_decls)
[ACCEL_COMPILER]: Special-case certain host-setup symbol aliases.
* varasm.cc (do_assemble_alias) [ACCEL_COMPILER]: Adjust.
gcc/cp/
* optimize.cc (maybe_clone_body): Tag 'clone's with
'c++ cdtor alias' attribute.
---
gcc/config/nvptx/nvptx.cc | 25 ++++++++++++++++++++++++-
gcc/cp/optimize.cc | 6 ++++++
gcc/varasm.cc | 7 ++++++-
3 files changed, 36 insertions(+), 2 deletions(-)
diff --git a/gcc/config/nvptx/nvptx.cc b/gcc/config/nvptx/nvptx.cc
index 95cc3da7876..418b9cca7e6 100644
--- a/gcc/config/nvptx/nvptx.cc
+++ b/gcc/config/nvptx/nvptx.cc
@@ -245,7 +245,7 @@ default_ptx_version_option (void)
warp convergence. */
res = MAX (res, PTX_VERSION_6_0);
- /* Pick at least 6.3. */
+ /* Pick at least 6.3, to enable PTX '.alias'. */
res = MAX (res, PTX_VERSION_6_3);
/* For sm_52+, pick at least 7.3, to enable PTX 'alloca'. */
@@ -7713,6 +7713,27 @@ nvptx_asm_output_def_from_decls (FILE *stream, tree name,
{
if (nvptx_alias == 0 || !TARGET_PTX_6_3)
{
+ /* Symbol aliases are not supported here. */
+#ifdef ACCEL_COMPILER
+ if (lookup_attribute ("c++ cdtor alias", DECL_ATTRIBUTES (name)))
+ {
+ /* ..., but are supported and used in the host system, via
+ 'gcc/cp/optimize.cc:can_alias_cdtor'. */
+ gcc_assert (!lookup_attribute ("weak", DECL_ATTRIBUTES (name)));
+ gcc_assert (TREE_CODE (name) == FUNCTION_DECL);
+
+ /* In this specific case, use PTX '.alias' even for (default)
+ '-mno-alias'. */
+ if (TARGET_PTX_6_3)
+ {
+ DECL_ATTRIBUTES (name)
+ = tree_cons (get_identifier ("c++ cdtor alias handled"),
+ NULL_TREE, DECL_ATTRIBUTES (name));
+ goto emit_ptx_alias;
+ }
+ }
+#endif
+
/* Copied from assemble_alias. */
error_at (DECL_SOURCE_LOCATION (name),
"alias definitions not supported in this configuration");
@@ -7744,6 +7765,8 @@ nvptx_asm_output_def_from_decls (FILE *stream, tree name,
return;
}
+ emit_ptx_alias:
+
cgraph_node *cnode = cgraph_node::get (name);
if (!cnode->referred_to_p ())
/* Prevent "Internal error: reference to deleted section". */
diff --git a/gcc/cp/optimize.cc b/gcc/cp/optimize.cc
index 6f9a77f407a..0e1a07c4223 100644
--- a/gcc/cp/optimize.cc
+++ b/gcc/cp/optimize.cc
@@ -717,6 +717,12 @@ maybe_clone_body (tree fn)
emit_associated_thunks (clone);
/* We didn't generate a body, so remove the empty one. */
DECL_SAVED_TREE (clone) = void_node;
+
+ /* Help '!TARGET_SUPPORTS_ALIASES' offload targets. */
+ if (flag_openacc || flag_openmp)
+ DECL_ATTRIBUTES (clone)
+ = tree_cons (get_identifier ("c++ cdtor alias"),
+ NULL_TREE, DECL_ATTRIBUTES (clone));
}
else
expand_or_defer_fn (clone);
diff --git a/gcc/varasm.cc b/gcc/varasm.cc
index eddfb6a3524..1198ca009aa 100644
--- a/gcc/varasm.cc
+++ b/gcc/varasm.cc
@@ -6525,7 +6525,12 @@ do_assemble_alias (tree decl, tree target)
IDENTIFIER_POINTER (target));
# endif
/* If symbol aliases aren't actually supported... */
- if (!TARGET_SUPPORTS_ALIASES)
+ if (!TARGET_SUPPORTS_ALIASES
+# ifdef ACCEL_COMPILER
+ /* ..., and unless special-cased... */
+ && !lookup_attribute ("c++ cdtor alias handled", DECL_ATTRIBUTES (decl))
+# endif
+ )
/* ..., 'ASM_OUTPUT_DEF{,_FROM_DECLS}' better have raised an error. */
gcc_checking_assert (seen_error ());
#elif defined (ASM_OUTPUT_WEAK_ALIAS) || defined (ASM_WEAKEN_DECL)
--
2.34.1
