Hi!
On 2025-03-21T08:37:16-0400, Jason Merrill <ja...@redhat.com> wrote:
> On 3/20/25 12:48 PM, Thomas Schwinge wrote:
>> On 2025-03-20T11:59:07-0400, Jason Merrill <ja...@redhat.com> wrote:
>>> On 3/20/25 11:35 AM, Thomas Schwinge wrote:
>>>> 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?
>>>
>>> Hmm, seems weird to use aliases in that one case but not others.
>>
>> Yeah... The problem is the mismatch between host and offload target:
>> the host of course typically is 'TARGET_SUPPORTS_ALIASES', but nvptx
>> offloading typically is '!TARGET_SUPPORTS_ALIASES', because:
>>
>>> If
>>> you're going to use aliases, why not just turn on alias support?
>>
>> We can't unconditionally enable GCC/nvptx' '-malias', because the support
>> is incomplete, and if enabled, will cause GCC target library build
>> failures, for example.
>>
>>> If aliases aren't supported, I think you want to fix can_alias_cdtor to
>>> return false.
>>
>> We can't: we don't want to pessimize host code generation. Therefore, we
>> have to handle this in the nvptx offloading flow.
>
> Ah.
>
> Can you use DECL_CXX_{CON,DE}STRUCTOR_P instead of a new attribute?
Yes, thanks! After re-testing, I've pushed to trunk branch
"nvptx: In offloading compilation, special-case certain host-setup symbol
aliases [PR101544]",
see attached.
Grüße
Thomas
>From 65b31b3fff2fced015ded1026733605f34053796 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/config/nvptx/nvptx.cc | 28 +++++++++++++++++++++++++++-
gcc/varasm.cc | 7 ++++++-
2 files changed, 33 insertions(+), 2 deletions(-)
diff --git a/gcc/config/nvptx/nvptx.cc b/gcc/config/nvptx/nvptx.cc
index 5cd4b2427714..022037f13627 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 +7713,30 @@ 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 (DECL_CXX_CONSTRUCTOR_P (name)
+ || DECL_CXX_DESTRUCTOR_P (name))
+ {
+ /* ..., but symbol aliases 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', if available, even for
+ (default) '-mno-alias'. */
+ if (TARGET_PTX_6_3)
+ {
+ DECL_ATTRIBUTES (name)
+ = tree_cons (get_identifier ("symbol 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 +7768,8 @@ 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/varasm.cc b/gcc/varasm.cc
index eddfb6a3524d..10c1d2e31371 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 ("symbol 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
