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From: "vries at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libgomp/97384] New: [libgomp, nvptx] Handle
 -msoft-stack-reserve-local=<n> overflow in plugin
Date: Mon, 12 Oct 2020 14:29:30 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D97384

            Bug ID: 97384
           Summary: [libgomp, nvptx] Handle -msoft-stack-reserve-local=3D<n>
                    overflow in plugin
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: libgomp
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
                CC: jakub at gcc dot gnu.org
  Target Milestone: ---

Using the option -msoft-stack-reserve-local=3D<n> results in a:
...
.local .align 8 .b8 %simtstack_ar[n+8];
...

However, the CU_LIMIT_STACK_SIZE is set by default to 1kb for my card/driver
combo, so if I specify say -msoft-stack-reserve-local=3D2048, I run into:
...
libgomp: cuCtxSynchronize error: an illegal memory access was encountered
...
or:
...
libgomp: cuCtxSynchronize error: an illegal instruction was encountered
...
[ The latter at GOMP_NVPTX_JIT=3D-O0. ] Which may look a lot like the behav=
iour
we're trying to fix by adding -msoft-stack-reserve-local.

There's currently no way to make this work.

We could add an env var, say GOMP_NVPTX_LIMIT_STACK_SIZE which is used to s=
et:
...
  r =3D cuCtxSetLimit(CU_LIMIT_STACK_SIZE, gomp_nvptx_limit_stack_size);
...
and then do:
...
$ GOMP_NVPTX_LIMIT_STACK_SIZE=3D3072 ./a.out
...
[ Note that GOMP_NVPTX_LIMIT_STACK_SIZE id chosen to be larger than 2048 to
accommodate for other .local usage. ]

[ It would be nice if we could attempt to accommodate the requested stack s=
ize
in the libgomp plugin automatically.  In the current setup, that would mean
scanning the ptx code for "simtstack_ar[<n>]", which is a bit cumbersome and
probably too slow.  Perhaps emitting an additional additional line before t=
he
pre-amble like this:
...
// SIMTSTACK_AR_SIZE: 2048
...
would be possible to handle quick enough. ]=