Re: [PATCH 2/2] nvptx: Prevent emitting duplicate declarations for '__nvptx_stacks', '__nvptx_uni'

[Thomas Schwinge <thomas@codesourcery.com>]

Hi!

On 2022-12-19T21:40:07+0100, Thomas Schwinge <thomas@codesourcery.com> wrote:
> As I have reported to Nvidia in 2022-12-01 'NVIDIA Incident Report (3891704):
> ptxas: Duplicate declaration error: "cannot be resolved by a '.static'"',
> 'ptxas' has an inscrutable error mode for duplicate declarations:
>
>     ptxas softstack-decl-1.o, line 11; error   : '.extern' variable '__nvptx_stacks' cannot be resolved by a '.static'
>     ptxas fatal   : Ptx assembly aborted due to errors
>     nvptx-as: ptxas returned 255 exit status
>
>     ptxas uniform-simt-decl-1.o, line 12; error   : '.extern' variable '__nvptx_uni' cannot be resolved by a '.static'
>     ptxas fatal   : Ptx assembly aborted due to errors
>     nvptx-as: ptxas returned 255 exit status
>
> This is inscrutable, because (a) what is "cannot be resolved by a '.static'"
> supposed to tell me (there is no '.static' in PTX?), and (b) why arent't
> repeated declaration just verified to match the first, but otherwise a no-op
> (like in other programming languages)?

Since my report, this had its 'Status changed [...] to "Closed - Fixed"'
(2023-01-28), with comment:

| [...] fix should be available in a later release.
| The compiler was modified to allow duplicate declaration of extern symbol. You will not see an error for this case.
| The documentation is also being changed to reflect this new change.

I've not yet verified the CUDA/'ptxas'-level fix, but I suggest to
retract my GCC-level proposed change:

> --- a/gcc/config/nvptx/nvptx.cc
> +++ b/gcc/config/nvptx/nvptx.cc

> +static bool have_softstack_decl;

> +static bool have_unisimt_decl;

> @@ -2571,6 +2573,13 @@ nvptx_assemble_undefined_decl (FILE *file, const char *name, const_tree decl)
>                              TREE_TYPE (decl), size ? tree_to_shwi (size) : 0,
>                              DECL_ALIGN (decl), true);
>    nvptx_assemble_decl_end ();
> +
> +  static tree softstack_id = get_identifier ("__nvptx_stacks");
> +  static tree unisimt_id = get_identifier ("__nvptx_uni");
> +  if (DECL_NAME (decl) == softstack_id)
> +    have_softstack_decl = true;
> +  else if (DECL_NAME (decl) == unisimt_id)
> +    have_unisimt_decl = true;
>  }

> @@ -6002,7 +6011,7 @@ nvptx_file_end (void)
>      write_shared_buffer (asm_out_file, gang_private_shared_sym,
>                          gang_private_shared_align, gang_private_shared_size);
>
> -  if (need_softstack_decl)
> +  if (need_softstack_decl && !have_softstack_decl)
>      {
>        write_var_marker (asm_out_file, false, true, "__nvptx_stacks");
>        /* 32 is the maximum number of warps in a block.  Even though it's an
> @@ -6011,7 +6020,8 @@ nvptx_file_end (void)
>        fprintf (asm_out_file, ".extern .shared .u%d __nvptx_stacks[32];\n",
>                POINTER_SIZE);
>      }
> -  if (need_unisimt_decl)
> +
> +  if (need_unisimt_decl && !have_unisimt_decl)
>      {
>        write_var_marker (asm_out_file, false, true, "__nvptx_uni");
>        fprintf (asm_out_file, ".extern .shared .u32 __nvptx_uni[32];\n");

..., and suggest that we instead fix up duplicate declarations in the
nvptx-tools 'as', and once GCC depends on a nvptx-tools version with that
addressed, we still change the test cases from "compile" to "assemble" as
proposed:

> --- a/gcc/testsuite/gcc.target/nvptx/softstack-decl-1.c
> +++ b/gcc/testsuite/gcc.target/nvptx/softstack-decl-1.c
> @@ -1,4 +1,4 @@
> -/* { dg-do compile } */
> +/* { dg-do assemble } */
>  /* { dg-options {-save-temps -O0 -msoft-stack} } */
>
>  extern void *__nvptx_stacks[32] __attribute__((shared,nocommon));

> --- a/gcc/testsuite/gcc.target/nvptx/uniform-simt-decl-1.c
> +++ b/gcc/testsuite/gcc.target/nvptx/uniform-simt-decl-1.c
> @@ -1,4 +1,4 @@
> -/* { dg-do compile } */
> +/* { dg-do assemble } */
>  /* { dg-options {-save-temps -O0 -muniform-simt} } */
>
>  extern unsigned __nvptx_uni[32] __attribute__((shared,nocommon));

..., but (obviously) without the following changes:

> --- a/gcc/testsuite/gcc.target/nvptx/softstack-decl-1.c
> +++ b/gcc/testsuite/gcc.target/nvptx/softstack-decl-1.c

> -/* The implicit (via 'need_softstack_decl') and explicit declarations of
> -   '__nvptx_stacks' are both emitted:
> -   { dg-final { scan-assembler-times {(?n)\.extern .* __nvptx_stacks\[32\];} 2 } }
> +/* Of the implicit (via 'need_softstack_decl') and explicit declarations of
> +   '__nvptx_stacks', only one is emitted:
> +   { dg-final { scan-assembler-times {(?n)\.extern .* __nvptx_stacks\[32\];} 1 } }

> --- a/gcc/testsuite/gcc.target/nvptx/uniform-simt-decl-1.c
> +++ b/gcc/testsuite/gcc.target/nvptx/uniform-simt-decl-1.c

> -/* The implicit (via 'need_unisimt_decl') and explicit declarations of
> -   '__nvptx_uni' are both emitted:
> -   { dg-final { scan-assembler-times {(?n)\.extern .* __nvptx_uni\[32\];} 2 } }
> +/* Of the implicit (via 'need_unisimt_decl') and explicit declarations of
> +   '__nvptx_uni', only one is emitted:
> +   { dg-final { scan-assembler-times {(?n)\.extern .* __nvptx_uni\[32\];} 1 } }


Grüße
 Thomas
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