Re: Causes to nvptx bootstrap fail: [PATCH v5] Introduce strub: machine-independent stack scrubbing

[Thomas Schwinge <thomas@codesourcery.com>]

Hi Alexandre!

On 2023-12-06T09:36:33+0100, Tobias Burnus <tobias@codesourcery.com> wrote:
> FYI the newly added file libgcc/strub.c of this patch (aka commit r14-6201-gf0a90c7d7333fc )
> causes that nvptx does not bootstrap, failing with:

('s%bootstrap%build'.)

> ./gcc/as -v -o strub.o strub.s
> Verifying sm_30 code with sm_50 code generation.
>   ptxas -c -o /dev/null strub.o --gpu-name sm_50 -O0
> ptxas strub.o, line 22; error   : Arguments mismatch for instruction 'st'
> ptxas strub.o, line 22; error   : Unknown symbol '%frame'
> [...]

Per commit r14-6201-gf0a90c7d7333fc7f554b906245c84bdf04d716d7
"Introduce strub: machine-independent stack scrubbing", we have:

    A function associated with @code{at-calls} @code{strub} mode
    (@code{strub("at-calls")}, or just @code{strub}) undergoes interface
    changes.  Its callers are adjusted to match the changes, and to scrub
    (overwrite with zeros) the stack space used by the called function after
    it returns.

As I understand things, this cannot be implemented (at the call site) for
nvptx, given that the callee's stack is not visible there: PTX is unusual
in that the concept of a "standard" stack isn't exposed.

Instead of allowing "strub" pieces that can be implemented, should this
whole machinery generally be disabled (forced '-fstrub=disable', or via a
new target hook?)?  The libgcc functions should then not get defined
(thus, linker error upon accidental use), or should just '__builtin_trap'
if that makes more sense?  Need an effective-target for the test cases.

Alternatively, we may also leave the generic middle end handling alive,
and 'sorry' (or similar) in the nvptx back end, as necessary?


Grüße
 Thomas
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