Re: [PATCH] Add -fextra-libc-function=memcmpeq for __memcmpeq

[Fangrui Song <maskray@google.com>]

On Wed, Jun 15, 2022 at 2:44 PM H.J. Lu via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> On Mon, Jun 13, 2022 at 9:01 AM Richard Biener
> <richard.guenther@gmail.com> wrote:
> >
> >
> >
> > > Am 13.06.2022 um 16:36 schrieb H.J. Lu <hjl.tools@gmail.com>:
> > >
> > > On Mon, Jun 13, 2022 at 3:11 AM Richard Biener
> > > <richard.guenther@gmail.com> wrote:
> > >>
> > >>> On Tue, Jun 7, 2022 at 9:02 PM H.J. Lu via Gcc-patches
> > >>> <gcc-patches@gcc.gnu.org> wrote:
> > >>>
> > >>> Add -fextra-libc-function=memcmpeq to map
> > >>>
> > >>> extern int __memcmpeq (const void *, const void *, size_t);
> > >>>
> > >>> which was added to GLIBC 2.35, to __builtin_memcmp_eq.
> > >>
> > >> Humm.  Can't we instead use the presence of a declaration
> > >> of __memcmpeq with a GNU standard dialect as this instead of
> > >> adding a weird -fextra-libc-function= option?  Maybe that's even
> > >> reasonable with a non-GNU dialect standard in effect since
> > >> __ prefixed names are in the implementation namespace?
> > >
> > > But not all source codes include <string.h> and GCC may generate
> > > memcmp directly.  How should we handle these cases?
> >
> > Not.  Similar as to vectorized math functions.
> > I think it’s not worth optimizing for this case.
>
> Another question.  Should we consider any __memcmpeq prototype
> or just the one in the system header file?

An idea from https://reviews.llvm.org/D56593#3586673: -fbuiltin-__memcmpeq

This requires making -fbuiltin-function available, see
https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html
("There is no corresponding -fbuiltin-function option")

I prefer an option over a magic behavior about whether a declaration exists.

> > Richard.
> >
> > >
> > >> Richard.
> > >>
> > >>> gcc/
> > >>>
> > >>>        * builtins.cc: Include "opts.h".
> > >>>        (expand_builtin): Generate BUILT_IN_MEMCMP_EQ if __memcmpeq is
> > >>>        available.
> > >>>        * builtins.def (BUILT_IN___MEMCMPEQ): New.
> > >>>        * common.opt: Add -fextra-libc-function=.
> > >>>        * opts.cc (extra_libc_functions): New.
> > >>>        (parse_extra_libc_function): New function.
> > >>>        (common_handle_option): Handle -fextra-libc-function=.
> > >>>        * opts.h (extra_libc_function_list): New.
> > >>>        (extra_libc_functions): Likewise.
> > >>>        * doc/invoke.texi: Document -fextra-libc-function=memcmpeq.
> > >>>
> > >>> gcc/testsuite/
> > >>>
> > >>>        * c-c++-common/memcmpeq-1.c: New test.
> > >>>        * c-c++-common/memcmpeq-2.c: Likewise.
> > >>>        * c-c++-common/memcmpeq-3.c: Likewise.
> > >>>        * c-c++-common/memcmpeq-4.c: Likewise.
> > >>>        * c-c++-common/memcmpeq-5.c: Likewise.
> > >>>        * c-c++-common/memcmpeq-6.c: Likewise.
> > >>>        * c-c++-common/memcmpeq-7.c: Likewise.
> > >>> ---
> > >>> gcc/builtins.cc                         |  5 ++++-
> > >>> gcc/builtins.def                        |  4 ++++
> > >>> gcc/common.opt                          |  4 ++++
> > >>> gcc/doc/invoke.texi                     |  6 ++++++
> > >>> gcc/opts.cc                             | 23 +++++++++++++++++++++++
> > >>> gcc/opts.h                              |  7 +++++++
> > >>> gcc/testsuite/c-c++-common/memcmpeq-1.c | 11 +++++++++++
> > >>> gcc/testsuite/c-c++-common/memcmpeq-2.c | 11 +++++++++++
> > >>> gcc/testsuite/c-c++-common/memcmpeq-3.c | 11 +++++++++++
> > >>> gcc/testsuite/c-c++-common/memcmpeq-4.c | 11 +++++++++++
> > >>> gcc/testsuite/c-c++-common/memcmpeq-5.c | 11 +++++++++++
> > >>> gcc/testsuite/c-c++-common/memcmpeq-6.c | 11 +++++++++++
> > >>> gcc/testsuite/c-c++-common/memcmpeq-7.c | 11 +++++++++++
> > >>> 13 files changed, 125 insertions(+), 1 deletion(-)
> > >>> create mode 100644 gcc/testsuite/c-c++-common/memcmpeq-1.c
> > >>> create mode 100644 gcc/testsuite/c-c++-common/memcmpeq-2.c
> > >>> create mode 100644 gcc/testsuite/c-c++-common/memcmpeq-3.c
> > >>> create mode 100644 gcc/testsuite/c-c++-common/memcmpeq-4.c
> > >>> create mode 100644 gcc/testsuite/c-c++-common/memcmpeq-5.c
> > >>> create mode 100644 gcc/testsuite/c-c++-common/memcmpeq-6.c
> > >>> create mode 100644 gcc/testsuite/c-c++-common/memcmpeq-7.c
> > >>>
> > >>> diff --git a/gcc/builtins.cc b/gcc/builtins.cc
> > >>> index b9d89b409b8..22269318e8c 100644
> > >>> --- a/gcc/builtins.cc
> > >>> +++ b/gcc/builtins.cc
> > >>> @@ -81,6 +81,7 @@ along with GCC; see the file COPYING3.  If not see
> > >>> #include "demangle.h"
> > >>> #include "gimple-range.h"
> > >>> #include "pointer-query.h"
> > >>> +#include "opts.h"
> > >>>
> > >>> struct target_builtins default_target_builtins;
> > >>> #if SWITCHABLE_TARGET
> > >>> @@ -7410,7 +7411,9 @@ expand_builtin (tree exp, rtx target, rtx subtarget, machine_mode mode,
> > >>>        return target;
> > >>>       if (fcode == BUILT_IN_MEMCMP_EQ)
> > >>>        {
> > >>> -         tree newdecl = builtin_decl_explicit (BUILT_IN_MEMCMP);
> > >>> +         tree newdecl = builtin_decl_explicit
> > >>> +           (extra_libc_functions.has_memcmpeq
> > >>> +            ? BUILT_IN___MEMCMPEQ : BUILT_IN_MEMCMP);
> > >>>          TREE_OPERAND (exp, 1) = build_fold_addr_expr (newdecl);
> > >>>        }
> > >>>       break;
> > >>> diff --git a/gcc/builtins.def b/gcc/builtins.def
> > >>> index 005976f34e9..eb8d33b16e9 100644
> > >>> --- a/gcc/builtins.def
> > >>> +++ b/gcc/builtins.def
> > >>> @@ -965,6 +965,10 @@ DEF_BUILTIN_STUB (BUILT_IN_ALLOCA_WITH_ALIGN_AND_MAX, "__builtin_alloca_with_ali
> > >>>    equality with zero.  */
> > >>> DEF_BUILTIN_STUB (BUILT_IN_MEMCMP_EQ, "__builtin_memcmp_eq")
> > >>>
> > >>> +/* Similar to BUILT_IN_MEMCMP_EQ, but is mapped to __memcmpeq only with
> > >>> +   -fextra-libc-function=memcmpeq.  */
> > >>> +DEF_EXT_LIB_BUILTIN (BUILT_IN___MEMCMPEQ, "__memcmpeq", BT_FN_INT_CONST_PTR_CONST_PTR_SIZE, ATTR_PURE_NOTHROW_NONNULL_LEAF)
> > >>> +
> > >>> /* An internal version of strcmp/strncmp, used when the result is only
> > >>>    tested for equality with zero.  */
> > >>> DEF_BUILTIN_STUB (BUILT_IN_STRCMP_EQ, "__builtin_strcmp_eq")
> > >>> diff --git a/gcc/common.opt b/gcc/common.opt
> > >>> index 7ca0cceed82..7a7631682b0 100644
> > >>> --- a/gcc/common.opt
> > >>> +++ b/gcc/common.opt
> > >>> @@ -1587,6 +1587,10 @@ Enum(excess_precision) String(standard) Value(EXCESS_PRECISION_STANDARD)
> > >>> EnumValue
> > >>> Enum(excess_precision) String(16) Value(EXCESS_PRECISION_FLOAT16)
> > >>>
> > >>> +fextra-libc-function=
> > >>> +Common Driver Joined
> > >>> +Specify the extra function in the C library.
> > >>> +
> > >>> ; Whether we permit the extended set of values for FLT_EVAL_METHOD
> > >>> ; introduced in ISO/IEC TS 18661-3, or limit ourselves to those in C99/C11.
> > >>> fpermitted-flt-eval-methods=
> > >>> diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
> > >>> index 8cd5bdddc5d..fe1e3709953 100644
> > >>> --- a/gcc/doc/invoke.texi
> > >>> +++ b/gcc/doc/invoke.texi
> > >>> @@ -676,6 +676,7 @@ Objective-C and Objective-C++ Dialects}.
> > >>> -ffixed-@var{reg}  -fexceptions @gol
> > >>> -fnon-call-exceptions  -fdelete-dead-exceptions  -funwind-tables @gol
> > >>> -fasynchronous-unwind-tables @gol
> > >>> +-fextra-libc-function=memcmpeq @gol
> > >>> -fno-gnu-unique @gol
> > >>> -finhibit-size-directive  -fcommon  -fno-ident @gol
> > >>> -fpcc-struct-return  -fpic  -fPIC  -fpie  -fPIE  -fno-plt @gol
> > >>> @@ -17250,6 +17251,11 @@ Generate unwind table in DWARF format, if supported by target machine.  The
> > >>> table is exact at each instruction boundary, so it can be used for stack
> > >>> unwinding from asynchronous events (such as debugger or garbage collector).
> > >>>
> > >>> +@item -fextra-libc-function=memcmpeq
> > >>> +@opindex fextra-libc-function
> > >>> +Generate @code{__memcmpeq}, which was added to GLIBC 2.35, for
> > >>> +@code{__builtin_memcmp_eq}.
> > >>> +
> > >>> @item -fno-gnu-unique
> > >>> @opindex fno-gnu-unique
> > >>> @opindex fgnu-unique
> > >>> diff --git a/gcc/opts.cc b/gcc/opts.cc
> > >>> index bf06a55456a..e77f0922acc 100644
> > >>> --- a/gcc/opts.cc
> > >>> +++ b/gcc/opts.cc
> > >>> @@ -38,6 +38,9 @@ along with GCC; see the file COPYING3.  If not see
> > >>> /* In this file all option sets are explicit.  */
> > >>> #undef OPTION_SET_P
> > >>>
> > >>> +/* The list of extra functions in the C library.  */
> > >>> +extra_libc_function_list extra_libc_functions;
> > >>> +
> > >>> static void set_Wstrict_aliasing (struct gcc_options *opts, int onoff);
> > >>>
> > >>> /* Names of fundamental debug info formats indexed by enum
> > >>> @@ -2269,6 +2272,22 @@ parse_no_sanitize_attribute (char *value)
> > >>>   return flags;
> > >>> }
> > >>>
> > >>> +/* Parse -fextra-libc-function= suboptions from ARG.  */
> > >>> +
> > >>> +static void
> > >>> +parse_extra_libc_function (const char *arg)
> > >>> +{
> > >>> +  /* Check to see if the string matches a sub-option name.  */
> > >>> +  if (strcmp (arg, "memcmpeq") == 0)
> > >>> +    {
> > >>> +      extra_libc_functions.has_memcmpeq = 1;
> > >>> +      return;
> > >>> +    }
> > >>> +
> > >>> +  error ("unrecognized argument to %<-fextra-libc-function=%>: %qs",
> > >>> +        arg);
> > >>> +}
> > >>> +
> > >>> /* Parse -fzero-call-used-regs suboptions from ARG, return the FLAGS.  */
> > >>>
> > >>> unsigned int
> > >>> @@ -2940,6 +2959,10 @@ common_handle_option (struct gcc_options *opts,
> > >>>       SET_OPTION_IF_UNSET (opts, opts_set, flag_profile_correction, value);
> > >>>       break;
> > >>>
> > >>> +    case OPT_fextra_libc_function_:
> > >>> +      parse_extra_libc_function (arg);
> > >>> +      break;
> > >>> +
> > >>>     case OPT_fprofile_generate_:
> > >>>       opts->x_profile_data_prefix = xstrdup (arg);
> > >>>       value = true;
> > >>> diff --git a/gcc/opts.h b/gcc/opts.h
> > >>> index a43ce66cffe..e8d8835ba23 100644
> > >>> --- a/gcc/opts.h
> > >>> +++ b/gcc/opts.h
> > >>> @@ -345,6 +345,13 @@ struct cl_option_handlers
> > >>> extern const char *opt_fstack_limit_symbol_arg;
> > >>> extern int opt_fstack_limit_register_no;
> > >>>
> > >>> +/* The list of extra functions in the C library.  */
> > >>> +struct extra_libc_function_list
> > >>> +{
> > >>> +  unsigned int has_memcmpeq : 1;
> > >>> +};
> > >>> +extern extra_libc_function_list extra_libc_functions;
> > >>> +
> > >>> /* Input file names.  */
> > >>>
> > >>> extern const char **in_fnames;
> > >>> diff --git a/gcc/testsuite/c-c++-common/memcmpeq-1.c b/gcc/testsuite/c-c++-common/memcmpeq-1.c
> > >>> new file mode 100644
> > >>> index 00000000000..487f6cd98db
> > >>> --- /dev/null
> > >>> +++ b/gcc/testsuite/c-c++-common/memcmpeq-1.c
> > >>> @@ -0,0 +1,11 @@
> > >>> +/* { dg-do compile } */
> > >>> +/* { dg-options "-O2 -fextra-libc-function=memcmpeq" } */
> > >>> +/* { dg-final { scan-assembler "__memcmpeq" } } */
> > >>> +
> > >>> +#include <stddef.h>
> > >>> +
> > >>> +int
> > >>> +foo (const char *s1, const char *s2, size_t len)
> > >>> +{
> > >>> +  return __builtin_memcmp (s1, s2, len) != 0;
> > >>> +}
> > >>> diff --git a/gcc/testsuite/c-c++-common/memcmpeq-2.c b/gcc/testsuite/c-c++-common/memcmpeq-2.c
> > >>> new file mode 100644
> > >>> index 00000000000..1773dc2bee1
> > >>> --- /dev/null
> > >>> +++ b/gcc/testsuite/c-c++-common/memcmpeq-2.c
> > >>> @@ -0,0 +1,11 @@
> > >>> +/* { dg-do compile } */
> > >>> +/* { dg-options "-O2 -fextra-libc-function=memcmpeq" } */
> > >>> +/* { dg-final { scan-assembler "__memcmpeq" } } */
> > >>> +
> > >>> +#include <string.h>
> > >>> +
> > >>> +int
> > >>> +foo (const char *s1, const char *s2, size_t len)
> > >>> +{
> > >>> +  return memcmp (s1, s2, len) == 0;
> > >>> +}
> > >>> diff --git a/gcc/testsuite/c-c++-common/memcmpeq-3.c b/gcc/testsuite/c-c++-common/memcmpeq-3.c
> > >>> new file mode 100644
> > >>> index 00000000000..69c9537d572
> > >>> --- /dev/null
> > >>> +++ b/gcc/testsuite/c-c++-common/memcmpeq-3.c
> > >>> @@ -0,0 +1,11 @@
> > >>> +/* { dg-do compile } */
> > >>> +/* { dg-options "-O2 -fextra-libc-function=memcmpeq" } */
> > >>> +/* { dg-final { scan-assembler-not "__memcmpeq" } } */
> > >>> +
> > >>> +#include <string.h>
> > >>> +
> > >>> +int
> > >>> +foo (const char *s1, const char *s2, size_t len)
> > >>> +{
> > >>> +  return memcmp (s1, s2, len);
> > >>> +}
> > >>> diff --git a/gcc/testsuite/c-c++-common/memcmpeq-4.c b/gcc/testsuite/c-c++-common/memcmpeq-4.c
> > >>> new file mode 100644
> > >>> index 00000000000..a448312ea96
> > >>> --- /dev/null
> > >>> +++ b/gcc/testsuite/c-c++-common/memcmpeq-4.c
> > >>> @@ -0,0 +1,11 @@
> > >>> +/* { dg-do compile } */
> > >>> +/* { dg-options "-O2" } */
> > >>> +/* { dg-final { scan-assembler-not "__memcmpeq" } } */
> > >>> +
> > >>> +#include <stddef.h>
> > >>> +
> > >>> +int
> > >>> +foo (const char *s1, const char *s2, size_t len)
> > >>> +{
> > >>> +  return __builtin_memcmp (s1, s2, len) != 0;
> > >>> +}
> > >>> diff --git a/gcc/testsuite/c-c++-common/memcmpeq-5.c b/gcc/testsuite/c-c++-common/memcmpeq-5.c
> > >>> new file mode 100644
> > >>> index 00000000000..4ef33a1c238
> > >>> --- /dev/null
> > >>> +++ b/gcc/testsuite/c-c++-common/memcmpeq-5.c
> > >>> @@ -0,0 +1,11 @@
> > >>> +/* { dg-do compile } */
> > >>> +/* { dg-options "-O2" } */
> > >>> +/* { dg-final { scan-assembler-not "__memcmpeq" } } */
> > >>> +
> > >>> +#include <string.h>
> > >>> +
> > >>> +int
> > >>> +foo (const char *s1, const char *s2, size_t len)
> > >>> +{
> > >>> +  return memcmp (s1, s2, len) == 0;
> > >>> +}
> > >>> diff --git a/gcc/testsuite/c-c++-common/memcmpeq-6.c b/gcc/testsuite/c-c++-common/memcmpeq-6.c
> > >>> new file mode 100644
> > >>> index 00000000000..52304df7079
> > >>> --- /dev/null
> > >>> +++ b/gcc/testsuite/c-c++-common/memcmpeq-6.c
> > >>> @@ -0,0 +1,11 @@
> > >>> +/* { dg-do compile } */
> > >>> +/* { dg-options "-O2 -fextra-libc-function=memcmpeq" } */
> > >>> +/* { dg-final { scan-assembler "__memcmpeq" } } */
> > >>> +
> > >>> +#include <stddef.h>
> > >>> +
> > >>> +int
> > >>> +foo (const char *s1, const char *s2, size_t len)
> > >>> +{
> > >>> +  return __builtin_memcmp_eq (s1, s2, len) != 0;
> > >>> +}
> > >>> diff --git a/gcc/testsuite/c-c++-common/memcmpeq-7.c b/gcc/testsuite/c-c++-common/memcmpeq-7.c
> > >>> new file mode 100644
> > >>> index 00000000000..d59765894e7
> > >>> --- /dev/null
> > >>> +++ b/gcc/testsuite/c-c++-common/memcmpeq-7.c
> > >>> @@ -0,0 +1,11 @@
> > >>> +/* { dg-do compile } */
> > >>> +/* { dg-options "-O2" } */
> > >>> +/* { dg-final { scan-assembler-not "__memcmpeq" } } */
> > >>> +
> > >>> +#include <stddef.h>
> > >>> +
> > >>> +int
> > >>> +foo (const char *s1, const char *s2, size_t len)
> > >>> +{
> > >>> +  return __builtin_memcmp_eq (s1, s2, len) != 0;
> > >>> +}
> > >>> --
> > >>> 2.36.1
> > >>>
> > >
> > >
> > >
> > > --
> > > H.J.
>
>
>
> --
> H.J.



-- 
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