rs6000: Adjust -fpatchable-function-entry* support for dual entry [PR112980]

  Hi,

As the discussion in PR112980, although the current
implementation for -fpatchable-function-entry* conforms
with the documentation (making N NOPs be consecutive),
it's inefficient for both kernel and userspace livepatching
(see comments in PR for the details).

So this patch is to change the current implementation by
emitting the "before" NOPs before global entry point and
the "after" NOPs after local entry point.  The new behavior
would not keep NOPs to be consecutive, so the documentation
is updated to emphasize this.

Bootstrapped and regress-tested on powerpc64-linux-gnu
P8/P9 and powerpc64le-linux-gnu P9 and P10.

Is it ok for trunk?  And backporting to active branches
after burn-in time?  I guess we should also mention this
change in changes.html?

BR,
Kewen
-----
	PR target/112980

gcc/ChangeLog:

	* config/rs6000/rs6000-logue.cc (rs6000_output_function_prologue):
	Adjust the handling on patch area emitting with dual entry, remove
	the restriction on "before" NOPs count, not emit "before" NOPs any
	more but only emit "after" NOPs.
	* config/rs6000/rs6000.cc (rs6000_print_patchable_function_entry):
	Adjust by respecting cfun->machine->stop_patch_area_print.
	(rs6000_elf_declare_function_name): For ELFv2 with dual entry, set
	cfun->machine->stop_patch_area_print as true.
	* config/rs6000/rs6000.h (struct machine_function): Remove member
	global_entry_emitted, add new member stop_patch_area_print.
	* doc/invoke.texi (option -fpatchable-function-entry): Adjust the
	documentation for PowerPC ELFv2 dual entry.

gcc/testsuite/ChangeLog:

	* c-c++-common/patchable_function_entry-default.c: Adjust.
	* gcc.target/powerpc/pr99888-4.c: Likewise.
	* gcc.target/powerpc/pr99888-5.c: Likewise.
	* gcc.target/powerpc/pr99888-6.c: Likewise.
---
 gcc/config/rs6000/rs6000-logue.cc             | 40 +++++--------------
 gcc/config/rs6000/rs6000.cc                   | 15 +++++--
 gcc/config/rs6000/rs6000.h                    | 10 +++--
 gcc/doc/invoke.texi                           |  8 ++--
 .../patchable_function_entry-default.c        |  3 --
 gcc/testsuite/gcc.target/powerpc/pr99888-4.c  |  4 +-
 gcc/testsuite/gcc.target/powerpc/pr99888-5.c  |  4 +-
 gcc/testsuite/gcc.target/powerpc/pr99888-6.c  |  4 +-
 8 files changed, 33 insertions(+), 55 deletions(-)

--
2.39.1
  

On Wed, May 8, 2024 at 7:50 AM Kewen.Lin <linkw@linux.ibm.com> wrote:
>
> Hi,
>
> As the discussion in PR112980, although the current
> implementation for -fpatchable-function-entry* conforms
> with the documentation (making N NOPs be consecutive),
> it's inefficient for both kernel and userspace livepatching
> (see comments in PR for the details).
>
> So this patch is to change the current implementation by
> emitting the "before" NOPs before global entry point and
> the "after" NOPs after local entry point.  The new behavior
> would not keep NOPs to be consecutive, so the documentation
> is updated to emphasize this.
>
> Bootstrapped and regress-tested on powerpc64-linux-gnu
> P8/P9 and powerpc64le-linux-gnu P9 and P10.
>
> Is it ok for trunk?  And backporting to active branches
> after burn-in time?  I guess we should also mention this
> change in changes.html?
>
> BR,
> Kewen
> -----
>         PR target/112980
>
> gcc/ChangeLog:
>
>         * config/rs6000/rs6000-logue.cc (rs6000_output_function_prologue):
>         Adjust the handling on patch area emitting with dual entry, remove
>         the restriction on "before" NOPs count, not emit "before" NOPs any
>         more but only emit "after" NOPs.
>         * config/rs6000/rs6000.cc (rs6000_print_patchable_function_entry):
>         Adjust by respecting cfun->machine->stop_patch_area_print.
>         (rs6000_elf_declare_function_name): For ELFv2 with dual entry, set
>         cfun->machine->stop_patch_area_print as true.
>         * config/rs6000/rs6000.h (struct machine_function): Remove member
>         global_entry_emitted, add new member stop_patch_area_print.
>         * doc/invoke.texi (option -fpatchable-function-entry): Adjust the
>         documentation for PowerPC ELFv2 dual entry.
>
> gcc/testsuite/ChangeLog:
>
>         * c-c++-common/patchable_function_entry-default.c: Adjust.
>         * gcc.target/powerpc/pr99888-4.c: Likewise.
>         * gcc.target/powerpc/pr99888-5.c: Likewise.
>         * gcc.target/powerpc/pr99888-6.c: Likewise.
> ---
>  gcc/config/rs6000/rs6000-logue.cc             | 40 +++++--------------
>  gcc/config/rs6000/rs6000.cc                   | 15 +++++--
>  gcc/config/rs6000/rs6000.h                    | 10 +++--
>  gcc/doc/invoke.texi                           |  8 ++--
>  .../patchable_function_entry-default.c        |  3 --
>  gcc/testsuite/gcc.target/powerpc/pr99888-4.c  |  4 +-
>  gcc/testsuite/gcc.target/powerpc/pr99888-5.c  |  4 +-
>  gcc/testsuite/gcc.target/powerpc/pr99888-6.c  |  4 +-
>  8 files changed, 33 insertions(+), 55 deletions(-)
>
> diff --git a/gcc/config/rs6000/rs6000-logue.cc b/gcc/config/rs6000/rs6000-logue.cc
> index 60ba15a8bc3..0eb019b44b3 100644
> --- a/gcc/config/rs6000/rs6000-logue.cc
> +++ b/gcc/config/rs6000/rs6000-logue.cc
> @@ -4006,43 +4006,21 @@ rs6000_output_function_prologue (FILE *file)
>           fprintf (file, "\tadd 2,2,12\n");
>         }
>
> -      unsigned short patch_area_size = crtl->patch_area_size;
> -      unsigned short patch_area_entry = crtl->patch_area_entry;
> -      /* Need to emit the patching area.  */
> -      if (patch_area_size > 0)
> -       {
> -         cfun->machine->global_entry_emitted = true;
> -         /* As ELFv2 ABI shows, the allowable bytes between the global
> -            and local entry points are 0, 4, 8, 16, 32 and 64 when
> -            there is a local entry point.  Considering there are two
> -            non-prefixed instructions for global entry point prologue
> -            (8 bytes), the count for patchable nops before local entry
> -            point would be 2, 6 and 14.  It's possible to support those
> -            other counts of nops by not making a local entry point, but
> -            we don't have clear use cases for them, so leave them
> -            unsupported for now.  */
> -         if (patch_area_entry > 0)
> -           {
> -             if (patch_area_entry != 2
> -                 && patch_area_entry != 6
> -                 && patch_area_entry != 14)
> -               error ("unsupported number of nops before function entry (%u)",
> -                      patch_area_entry);
> -             rs6000_print_patchable_function_entry (file, patch_area_entry,
> -                                                    true);
> -             patch_area_size -= patch_area_entry;
> -           }
> -       }
> -
>        fputs ("\t.localentry\t", file);
>        assemble_name (file, name);
>        fputs (",.-", file);
>        assemble_name (file, name);
>        fputs ("\n", file);
>        /* Emit the nops after local entry.  */
> -      if (patch_area_size > 0)
> -       rs6000_print_patchable_function_entry (file, patch_area_size,
> -                                              patch_area_entry == 0);
> +      unsigned short patch_area_size = crtl->patch_area_size;
> +      unsigned short patch_area_entry = crtl->patch_area_entry;
> +      if (patch_area_size > patch_area_entry)
> +       {
> +         cfun->machine->stop_patch_area_print = false;
> +         patch_area_size -= patch_area_entry;
> +         rs6000_print_patchable_function_entry (file, patch_area_size,
> +                                                patch_area_entry == 0);
> +       }
>      }
>
>    else if (rs6000_pcrel_p ())
> diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
> index 6ba9df4f02e..7e70741d59f 100644
> --- a/gcc/config/rs6000/rs6000.cc
> +++ b/gcc/config/rs6000/rs6000.cc
> @@ -15190,12 +15190,14 @@ rs6000_print_patchable_function_entry (FILE *file,
>                                        bool record_p)
>  {
>    bool global_entry_needed_p = rs6000_global_entry_point_prologue_needed_p ();
> -  /* For a function which needs global entry point, we will emit the
> -     patchable area before and after local entry point under the control of
> -     cfun->machine->global_entry_emitted, see the handling in function
> +  /* For a function which needs global entry point, we will only emit the
> +     patchable area after local entry point under the control of
> +     !cfun->machine->stop_patch_area_print, see the handling in functions
>       rs6000_output_function_prologue.  */
> -  if (!global_entry_needed_p || cfun->machine->global_entry_emitted)
> +  if (!cfun->machine->stop_patch_area_print)
>      default_print_patchable_function_entry (file, patch_area_size, record_p);
> +  else
> +    gcc_assert (global_entry_needed_p);
>  }
>
>
>  enum rtx_code
> @@ -21381,6 +21383,11 @@ rs6000_elf_declare_function_name (FILE *file, const char *name, tree decl)
>        fprintf (file, "\t.previous\n");
>      }
>    ASM_OUTPUT_FUNCTION_LABEL (file, name, decl);
> +  /* At this time, the "before" NOPs have been already emitted,
> +     let's stop generic code from printing the "after" NOPs and
> +     emit just after local entry later.  */
> +  if (rs6000_global_entry_point_prologue_needed_p ())
> +    cfun->machine->stop_patch_area_print = true;
>  }
>
>  static void rs6000_elf_file_end (void) ATTRIBUTE_UNUSED;
> diff --git a/gcc/config/rs6000/rs6000.h b/gcc/config/rs6000/rs6000.h
> index 68bc45d65ba..73767fdae06 100644
> --- a/gcc/config/rs6000/rs6000.h
> +++ b/gcc/config/rs6000/rs6000.h
> @@ -2434,10 +2434,12 @@ typedef struct GTY(()) machine_function
>    bool lr_is_wrapped_separately;
>    bool toc_is_wrapped_separately;
>    bool mma_return_type_error;
> -  /* Indicate global entry is emitted, only useful when the function requires
> -     global entry.  It helps to control the patchable area before and after
> -     local entry.  */
> -  bool global_entry_emitted;
> +  /* With ELFv2 ABI dual entry points being adopted, generic framework
> +     targetm.asm_out.print_patchable_function_entry would generate "after"
> +     NOPs before local entry, it is wrong.  This flag is to stop it from
> +     printing patch area before local entry, it is only useful when the
> +     function requires dual entry points.  */
> +  bool stop_patch_area_print;
>  } machine_function;
>  #endif
>
> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> index c584664e168..58e48f7dc55 100644
> --- a/gcc/doc/invoke.texi
> +++ b/gcc/doc/invoke.texi
> @@ -18363,11 +18363,11 @@ If @code{N=0}, no pad location is recorded.
>  The NOP instructions are inserted at---and maybe before, depending on
>  @var{M}---the function entry address, even before the prologue.  On
>  PowerPC with the ELFv2 ABI, for a function with dual entry points,
> -the local entry point is this function entry address.
> +@var{M} NOP instructions are inserted before the global entry point and
> +@var{N} - @var{M} NOP instructions are inserted after the local entry
> +point, which means the NOP instructions may not be consecutive.

Isn't it @var{M-1} NOP instructions before the global entry?  I suppose
the existing

"... with the function entry point before the @var{M}th NOP.
If @var{M} is omitted, it defaults to @code{0} so the
function entry points to the address just at the first NOP."

wording is self-contradicting in a way since before the 0th NOP (default)
to me is the same as before the 1st NOP (M == 1).  So maybe that should
be _after_ the @var{M}th NOP instead which would be consistent with your
ELFv2 docs?  Maybe the sentence should be re-worded similar to your
ELVv2 one, specifying the number of NOPs before and after the entry point.

> -The maximum value of @var{N} and @var{M} is 65535.  On PowerPC with the
> -ELFv2 ABI, for a function with dual entry points, the supported values
> -for @var{M} are 0, 2, 6 and 14.
> +The maximum value of @var{N} and @var{M} is 65535.
>  @end table
>
>
> diff --git a/gcc/testsuite/c-c++-common/patchable_function_entry-default.c b/gcc/testsuite/c-c++-common/patchable_function_entry-default.c
> index 3ccbafc87db..899938b4aa3 100644
> --- a/gcc/testsuite/c-c++-common/patchable_function_entry-default.c
> +++ b/gcc/testsuite/c-c++-common/patchable_function_entry-default.c
> @@ -1,9 +1,6 @@
>  /* { dg-do compile { target { ! { nvptx*-*-* visium-*-* } } } } */
>  /* { dg-options "-O2 -fpatchable-function-entry=3,1" } */
>  /* { dg-additional-options "-fno-pie" { target sparc*-*-* } } */
> -/* See PR99888, one single preceding nop isn't allowed on powerpc_elfv2,
> -   so overriding with two preceding nops to make it pass there.  */
> -/* { dg-additional-options "-fpatchable-function-entry=3,2" { target powerpc_elfv2 } } */
>  /* { dg-final { scan-assembler-times "nop|NOP|SWYM" 3 { target { ! { alpha*-*-* riscv*-*-* } } } } } */
>  /* { dg-final { scan-assembler-times "bis" 3 { target alpha*-*-* } } } */
>  /* { dg-final { scan-assembler-times "nop\n" 3 { target riscv*-*-* } } } */
> diff --git a/gcc/testsuite/gcc.target/powerpc/pr99888-4.c b/gcc/testsuite/gcc.target/powerpc/pr99888-4.c
> index 00a8d4d316e..6f23f2bb939 100644
> --- a/gcc/testsuite/gcc.target/powerpc/pr99888-4.c
> +++ b/gcc/testsuite/gcc.target/powerpc/pr99888-4.c
> @@ -2,12 +2,10 @@
>  /* There is no global entry point prologue with pcrel.  */
>  /* { dg-options "-mno-pcrel -fpatchable-function-entry=1,1" } */
>
> -/* Verify one error emitted for unexpected 1 nop before local
> -   entry.  */
> +/* Verify there is no error with 1 nop before local entry.  */
>
>  extern int a;
>
>  int test (int b) {
>    return a + b;
>  }
> -/* { dg-error "unsupported number of nops before function entry \\(1\\)" "" { target *-*-* } .-1 } */
> diff --git a/gcc/testsuite/gcc.target/powerpc/pr99888-5.c b/gcc/testsuite/gcc.target/powerpc/pr99888-5.c
> index 39d3b4465f1..13f192ebd20 100644
> --- a/gcc/testsuite/gcc.target/powerpc/pr99888-5.c
> +++ b/gcc/testsuite/gcc.target/powerpc/pr99888-5.c
> @@ -2,12 +2,10 @@
>  /* There is no global entry point prologue with pcrel.  */
>  /* { dg-options "-mno-pcrel -fpatchable-function-entry=7,3" } */
>
> -/* Verify one error emitted for unexpected 3 nops before local
> -   entry.  */
> +/* Verify no error emitted for 3 nops before local entry.  */
>
>  extern int a;
>
>  int test (int b) {
>    return a + b;
>  }
> -/* { dg-error "unsupported number of nops before function entry \\(3\\)" "" { target *-*-* } .-1 } */
> diff --git a/gcc/testsuite/gcc.target/powerpc/pr99888-6.c b/gcc/testsuite/gcc.target/powerpc/pr99888-6.c
> index c6c18dcc7ac..431c69cae9a 100644
> --- a/gcc/testsuite/gcc.target/powerpc/pr99888-6.c
> +++ b/gcc/testsuite/gcc.target/powerpc/pr99888-6.c
> @@ -2,8 +2,7 @@
>  /* There is no global entry point prologue with pcrel.  */
>  /* { dg-options "-mno-pcrel" } */
>
> -/* Verify one error emitted for unexpected 4 nops before local
> -   entry.  */
> +/* Verify no error emitted for 4 nops before local entry.  */
>
>  extern int a;
>
> @@ -11,4 +10,3 @@ __attribute__ ((patchable_function_entry (20, 4)))
>  int test (int b) {
>    return a + b;
>  }
> -/* { dg-error "unsupported number of nops before function entry \\(4\\)" "" { target *-*-* } .-1 } */
> --
> 2.39.1
  

Hi Richi,

>> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
>> index c584664e168..58e48f7dc55 100644
>> --- a/gcc/doc/invoke.texi
>> +++ b/gcc/doc/invoke.texi
>> @@ -18363,11 +18363,11 @@ If @code{N=0}, no pad location is recorded.
>>  The NOP instructions are inserted at---and maybe before, depending on
>>  @var{M}---the function entry address, even before the prologue.  On
>>  PowerPC with the ELFv2 ABI, for a function with dual entry points,
>> -the local entry point is this function entry address.
>> +@var{M} NOP instructions are inserted before the global entry point and
>> +@var{N} - @var{M} NOP instructions are inserted after the local entry
>> +point, which means the NOP instructions may not be consecutive.
> 
> Isn't it @var{M-1} NOP instructions before the global entry?  I suppose

No, the existing documentation is a bit confusing, sigh ...

> the existing
> 
> "... with the function entry point before the @var{M}th NOP.
> If @var{M} is omitted, it defaults to @code{0} so the
> function entry points to the address just at the first NOP."
> 
> wording is self-contradicting in a way since before the 0th NOP (default)
> to me is the same as before the 1st NOP (M == 1).  So maybe that should
> be _after_ the @var{M}th NOP instead which would be consistent with your
> ELFv2 docs?  Maybe the sentence should be re-worded similar to your
> ELVv2 one, specifying the number of NOPs before and after the entry point.
> 

... the current "with the function entry point before the Mth NOP."
has the 0th NOP assumption, so the default (0th) NOP and 1st NOP (M == 1)
are actually different, such as:

-fpatchable-function-entry=3,0

foo:
        nop
        nop
        nop

-fpatchable-function-entry=3,1

        nop
foo:
        nop
        nop

Alan also had the similar concern on this wording before:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99888#c8

" Alan Modra 2022-08-12 03:00:29 UTC
"
"(In reply to Segher Boessenkool from comment #7)
"> '-fpatchable-function-entry=N[,M]'
">      Generate N NOPs right at the beginning of each function, with the
">      function entry point before the Mth NOP.
"
" Bad doco.  Should be "after the Mth NOP" I think.
" Or better written to avoid the concept of a 0th nop.
" Default for M is zero, placing all nops after the function entry and
" before normal function prologue code.

BR,
Kewen

>> -The maximum value of @var{N} and @var{M} is 65535.  On PowerPC with the
>> -ELFv2 ABI, for a function with dual entry points, the supported values
>> -for @var{M} are 0, 2, 6 and 14.
>> +The maximum value of @var{N} and @var{M} is 65535.
>>  @end table
>>