On Tue, Nov 16, 2021 at 11:49:18AM +0000, Andrew Stubbs wrote:
> This patch is needed for AMD GCN offloading when we use the assembler from
> LLVM 13+.
>
> The GCN runtime (libgomp+ROCm) requires that the location of all variables
> in the offloaded variables table are discoverable at runtime (using the
> "hsa_executable_symbol_get_info" API), and this only works when the symbols
> are exported from the binary. Previously we solved this by having mkoffload
> insert ".global" directives into the assembler text, but newer LLVM
> assemblers emit an error if we do this when then variable was previously
> declared ".local" (which happens when a variable is zero-initialized and
> placed in the BSS).
>
> Since we can no longer easily fix them up after the fact, this patch fixes
> them up during OMP lowering.
I'm confused, how can that ever work reliably?
The !TREE_PUBLIC offload_vars can be static locals or static globals
or static anon namespace vars, but their names can very easily clash with
either static or non-static variables from other TUs.
Consider in one TU
static int a = 5;
static int baz (void) { static int b;
#pragma omp declare target to (b)
return ++b; }
int foo (void) { return ++a + baz (); }
#pragma omp declare target to (a, foo)
and
static int a = 5;
static int baz (void) { static int b;
#pragma omp declare target to (b)
return ++b; }
int bar (void) { return ++a + baz (); }
#pragma omp declare target to (a, bar)
int
main ()
{
int v;
#pragma omp target (from: v)
v = foo () + bar ();
}
in another one. This has
.quad a
.quad 4
.quad b.0
.quad 4
in .offload_var_table. I'd guess this must fail to link or load
with GCN if it makes them forcibly TREE_PUBLIC.
Why does the GCN plugin or runtime need to know those vars?
It needs to know the single array that contains their addresses of course...
Jakub
