Hi!
Honza (or others, of course), there's a question about
'ultimate_alias_target'.
On 2024-08-26T10:50:36+0000, Prathamesh Kulkarni <prathame...@nvidia.com> wrote:
> For the following test (adapted from pr96390.c):
>
> __attribute__((noipa)) int foo () { return 42; }
> int bar () __attribute__((alias ("foo")));
> int baz () __attribute__((alias ("bar")));
> Compiling [for nvptx] results in:
>
> ptxas fatal : Internal error: alias to unknown symbol
> nvptx-as: ptxas returned 255 exit status
Prathamesh: thanks for looking into this, and ACK: one of the many
limitations of PTX '.alias'. :-|
> This happens because ptx code-gen shows:
>
> // BEGIN GLOBAL FUNCTION DEF: foo
> .visible .func (.param.u32 %value_out) foo
> {
> [...]
> }
> .visible .func (.param.u32 %value_out) bar;
> .alias bar,foo;
> .visible .func (.param.u32 %value_out) baz;
> .alias baz,bar;
> .alias baz, bar is invalid since PTX requires aliasee to be a defined
> function:
> https://sw-docs-dgx-station.nvidia.com/cuda-latest/parallel-thread-execution/latest-internal/#kernel-and-function-directives-alias
(Us ordinary mortals need to look at
<https://docs.nvidia.com/cuda/parallel-thread-execution/#kernel-and-function-directives-alias>;
please update the Git commit log.)
> The patch uses cgraph_node::get(name)->ultimate_alias_target () instead of
> the provided value in nvptx_asm_output_def_from_decls.
I confirm that resolving to 'ultimate_alias_target' does work for this
case:
> For the above case, it now generates the following ptx:
>
> .alias baz,foo;
> instead of:
> .alias baz,bar;
>
> which fixes the issue.
..., but I'm not sure if that's conceptually correct; I'm not familiar
with 'ultimate_alias_target' semantics. (Honza?)
Also, I wonder whether 'gcc/varasm.cc:do_assemble_alias' is prepared for
'ASM_OUTPUT_DEF_FROM_DECLS' to disregard the specified 'target'/'value'
and instead do its own thing (here, the proposed resolving to
'ultimate_alias_target')? (No other GCC back end appears to be doing
such a thing; from a quick look, all appear to faithfully use the
specified 'target'/'value'.)
Now, consider the case that the source code is changed as follows:
__attribute__((noipa)) int foo () { return 42; }
-int bar () __attribute__((alias ("foo")));
+int bar () __attribute__((weak, alias ("foo")));
int baz () __attribute__((alias ("bar")));
With 'ultimate_alias_target', I've checked, you'd then still emit
'.alias baz,foo;', losing the ability to override the weak alias with a
strong 'bar' definition in another compilation unit?
Now, that said: GCC/nvptx for such code currently diagnoses
"error: weak alias definitions not supported [...]" ;-| -- so we may be
safe, after all? ..., or is there any other way that the resolving to
'ultimate_alias_target' might cause issues? If not, then at least your
proposed patch shouldn't be causing any harm (doesn't affect
'--target=nvptx-none' test results at all...), and does address one
user-visible issue ('libgomp.c-c++-common/pr96390.c'), and thus makes
sense to install.
> [nvptx] Fix code-gen for alias attribute.
I'd rather suggest something like:
"[nvptx] (Some) support for aliases to aliases" (or similar).
Also, please add "PR target/104957" to the Git commit log, as your change
directly alters this one aspect of PR104957
"[nvptx] Use .alias directive (available starting ptx isa version 6.3)"'s
commit r12-7766-gf8b15e177155960017ac0c5daef8780d1127f91c
"[nvptx] Use .alias directive for mptx >= 6.3":
| Aliases to aliases are not supported (see libgomp.c-c++-common/pr96390.c).
| This is currently not prohibited by the compiler, but with the driver link we
| run into: "Internal error: alias to unknown symbol" .
... which we then have (some) support for with the proposed code changes:
> --- a/gcc/config/nvptx/nvptx.cc
> +++ b/gcc/config/nvptx/nvptx.cc
> @@ -7583,7 +7583,8 @@ nvptx_mem_local_p (rtx mem)
> while (0)
>
> void
> -nvptx_asm_output_def_from_decls (FILE *stream, tree name, tree value)
> +nvptx_asm_output_def_from_decls (FILE *stream, tree name,
> + tree value ATTRIBUTE_UNUSED)
> {
> if (nvptx_alias == 0 || !TARGET_PTX_6_3)
> {
> @@ -7618,7 +7619,8 @@ nvptx_asm_output_def_from_decls (FILE *stream, tree
> name, tree value)
> return;
> }
>
> - if (!cgraph_node::get (name)->referred_to_p ())
> + cgraph_node *cnode = cgraph_node::get (name);
> + if (!cnode->referred_to_p ())
> /* Prevent "Internal error: reference to deleted section". */
> return;
>
> @@ -7627,8 +7629,10 @@ nvptx_asm_output_def_from_decls (FILE *stream, tree
> name, tree value)
> fputs (s.str ().c_str (), stream);
>
> tree id = DECL_ASSEMBLER_NAME (name);
> + symtab_node *alias_target_node = cnode->ultimate_alias_target ();
> + tree alias_target_id = DECL_ASSEMBLER_NAME (alias_target_node->decl);
> NVPTX_ASM_OUTPUT_DEF (stream, IDENTIFIER_POINTER (id),
> - IDENTIFIER_POINTER (value));
> + IDENTIFIER_POINTER (alias_target_id));
> }
>
> #undef NVPTX_ASM_OUTPUT_DEF
Please put some rationale comment before the 'ultimate_alias_target'.
All that said, I'm currently working on <https://gcc.gnu.org/PR105018>
"[nvptx] Need better alias support", via
<https://github.com/SourceryTools/nvptx-tools/issues/32>
"[LD] Handle alias in nvptx-ld as nvptx's .alias does not handle it fully".
Grüße
Thomas
