Re: [PATCH #2v2/2] strub: sparc64: unbias the stack address [PR112917]

[Richard Biener <richard.guenther@gmail.com>]

On Wed, Dec 20, 2023 at 12:51 AM Alexandre Oliva <oliva@adacore.com> wrote:
>
> On Dec 15, 2023, Richard Biener <richard.guenther@gmail.com> wrote:
>
> > It might be worth amending the documentation in case this
> > is unexpected to users?
>
> Oh, yes indeed, thanks!
>
> Here's a patch that brings relevant parts of the implementation comment
> to the user-facing documentation, so that it reflects the change in
> implementation.
>
> Regstrapped on x86_64-linux-gnu.  Ok to install?

OK.

>
> strub: sparc64: unbias the stack address [PR112917]
>
> The stack pointer is biased by 2047 bytes on sparc64, so the range it
> delimits is way off.  Unbias the addresses returned by
> __builtin_stack_address (), so that the strub builtins, inlined or
> not, can function correctly.  I've considered introducing a new target
> macro, but using STACK_POINTER_OFFSET seems safe, and it enables the
> register save areas to be scrubbed as well.
>
> Because of the large fixed-size outgoing args area next to the
> register save area on sparc, we still need __strub_leave to not
> allocate its own frame, otherwise it won't be able to clear part of
> the frame it should.
>
>
> for  gcc/ChangeLog
>
>         PR middle-end/112917
>         * builtins.cc (expand_bultin_stack_address): Add
>         STACK_POINTER_OFFSET.
>         * doc/extend.texi (__builtin_stack_address): Adjust.
> ---
>  gcc/builtins.cc     |   34 ++++++++++++++++++++++++++++++++--
>  gcc/doc/extend.texi |   23 ++++++++++++++++++++++-
>  2 files changed, 54 insertions(+), 3 deletions(-)
>
> diff --git a/gcc/builtins.cc b/gcc/builtins.cc
> index 7c2732ab79e6f..4c8c514fe8618 100644
> --- a/gcc/builtins.cc
> +++ b/gcc/builtins.cc
> @@ -5443,8 +5443,38 @@ expand_builtin_frame_address (tree fndecl, tree exp)
>  static rtx
>  expand_builtin_stack_address ()
>  {
> -  return convert_to_mode (ptr_mode, copy_to_reg (stack_pointer_rtx),
> -                         STACK_UNSIGNED);
> +  rtx ret = convert_to_mode (ptr_mode, copy_to_reg (stack_pointer_rtx),
> +                            STACK_UNSIGNED);
> +
> +  /* Unbias the stack pointer, bringing it to the boundary between the
> +     stack area claimed by the active function calling this builtin,
> +     and stack ranges that could get clobbered if it called another
> +     function.  It should NOT encompass any stack red zone, that is
> +     used in leaf functions.
> +
> +     On SPARC, the register save area is *not* considered active or
> +     used by the active function, but rather as akin to the area in
> +     which call-preserved registers are saved by callees.  This
> +     enables __strub_leave to clear what would otherwise overlap with
> +     its own register save area.
> +
> +     If the address is computed too high or too low, parts of a stack
> +     range that should be scrubbed may be left unscrubbed, scrubbing
> +     may corrupt active portions of the stack frame, and stack ranges
> +     may be doubly-scrubbed by caller and callee.
> +
> +     In order for it to be just right, the area delimited by
> +     @code{__builtin_stack_address} and @code{__builtin_frame_address
> +     (0)} should encompass caller's registers saved by the function,
> +     local on-stack variables and @code{alloca} stack areas.
> +     Accumulated outgoing on-stack arguments, preallocated as part of
> +     a function's own prologue, are to be regarded as part of the
> +     (caller) function's active area as well, whereas those pushed or
> +     allocated temporarily for a call are regarded as part of the
> +     callee's stack range, rather than the caller's.  */
> +  ret = plus_constant (ptr_mode, ret, STACK_POINTER_OFFSET);
> +
> +  return force_reg (ptr_mode, ret);
>  }
>
>  /* Expand a call to builtin function __builtin_strub_enter.  */
> diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi
> index b585e2d810230..5ac6a820e2a03 100644
> --- a/gcc/doc/extend.texi
> +++ b/gcc/doc/extend.texi
> @@ -12706,7 +12706,28 @@ situations.
>  @enddefbuiltin
>
>  @deftypefn {Built-in Function} {void *} __builtin_stack_address ()
> -This function returns the value of the stack pointer register.
> +This function returns the stack pointer register, offset by
> +@code{STACK_POINTER_OFFSET}.
> +
> +Conceptually, the returned address returned by this built-in function is
> +the boundary between the stack area allocated for use by its caller, and
> +the area that could be modified by a function call, that the caller
> +could safely zero-out before or after (but not during) the call
> +sequence.
> +
> +Arguments for a callee may be preallocated as part of the caller's stack
> +frame, or allocated on a per-call basis, depending on the target, so
> +they may be on either side of this boundary.
> +
> +Even if the stack pointer is biased, the result is not.  The register
> +save area on SPARC is regarded as modifiable by calls, rather than as
> +allocated for use by the caller function, since it is never in use while
> +the caller function itself is running.
> +
> +Red zones that only leaf functions could use are also regarded as
> +modifiable by calls, rather than as allocated for use by the caller.
> +This is only theoretical, since leaf functions do not issue calls, but a
> +constant offset makes this built-in function more predictable.
>  @end deftypefn
>
>  @node Stack Scrubbing
>
>
> --
> Alexandre Oliva, happy hacker            https://FSFLA.org/blogs/lxo/
>    Free Software Activist                   GNU Toolchain Engineer
> More tolerance and less prejudice are key for inclusion and diversity
> Excluding neuro-others for not behaving ""normal"" is *not* inclusive