[PATCH] Add TARGET_FOLD_MEMCPY_MAX

Richard Biener richard.guenther@gmail.com
Wed Mar 2 08:51:26 GMT 2022


On Tue, Mar 1, 2022 at 11:41 PM H.J. Lu via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Add TARGET_FOLD_MEMCPY_MAX for the maximum number of bytes to fold memcpy.
> The default is
>
> MOVE_MAX * MOVE_RATIO (optimize_function_for_size_p (cfun))
>
> For x86, it is MOVE_MAX to restore the old behavior before

I know we've discussed this to death in the PR, I just want to repeat here
that the GIMPLE folding expects to generate a single load and a single
store (that is what it does on the GIMPLE level) which is why MOVE_MAX
was chosen originally (it's documented to what a "single instruction" does).
In practice MOVE_MAX does not seem to cover vector register sizes
so Richard pulled MOVE_RATIO which is really intended to cover
the case of using multiple instructions for moving memory (but then I
don't remember whether for the ARM case the single load/store GIMPLE
will be expanded to multiple load/store instructions).

TARGET_FOLD_MEMCPY_MAX sounds like a stop-gap solution,
being very specific for memcpy folding (we also fold memmove btw).

There is also MOVE_MAX_PIECES which _might_ be more appropriate
than MOVE_MAX here and still honor the idea of single instructions.
Now neither arm nor aarch64 define this and it defaults to MOVE_MAX,
not MOVE_MAX * MOVE_RATIO.

So if we need a new hook then that hook should at least get the
'speed' argument of MOVE_RATIO and it should get a better name.

I still think that it should be possible to improve the insn check to
avoid use of "disabled" modes, maybe that's also a point to add
a new hook like .move_with_mode_p or so?  To quote, we do

              scalar_int_mode mode;
              if (int_mode_for_size (ilen * 8, 0).exists (&mode)
                  && GET_MODE_SIZE (mode) * BITS_PER_UNIT == ilen * 8
                  && have_insn_for (SET, mode)
                  /* If the destination pointer is not aligned we must be able
                     to emit an unaligned store.  */
                  && (dest_align >= GET_MODE_ALIGNMENT (mode)
                      || !targetm.slow_unaligned_access (mode, dest_align)
                      || (optab_handler (movmisalign_optab, mode)
                          != CODE_FOR_nothing)))

where I understand the ISA is enabled and if the user explicitely
uses it that's OK but -mprefer-avx128 should tell GCC to never
generate AVX256 code where the user was not explicitely using it
(still for example glibc might happily use AVX256 code to implement
the memcpy we are folding!)

Note the BB vectorizer also might end up with using AVX256 because
in places it also relies on optab queries and the vector_mode_supported_p
check (but the memcpy folding uses the fake integer modes).  So
x86 might need to implement the related_mode hook to avoid "auto"-using
a larger vector mode which the default implementation would happily do.

Richard.

> commit 5f6a6c91d7c592cb49f7c519f289777eac09bb74
> Author: Richard Earnshaw <rearnsha@arm.com>
> Date:   Fri Sep 3 17:06:15 2021 +0100
>
>     gimple: allow more folding of memcpy [PR102125]
>
> gcc/
>
>         PR target/103393
>         * gimple-fold.cc (gimple_fold_builtin_memory_op): Use
>         targetm.fold_memcpy_max instead of MOVE_MAX * MOVE_RATIO.
>         * target.def: Add fold_memcpy_max.
>         * varasm.cc (default_fold_memcpy_max): New.
>         * varasm.h (default_fold_memcpy_max): Likewise.
>         * config/i386/i386.cc (ix86_fold_memcpy_max): New.
>         (TARGET_FOLD_MEMCPY_MAX): Likewise.
>         * doc/tm.texi.in: Add TARGET_FOLD_MEMCPY_MAX.
>         * doc/tm.texi: Regenerate.
>
> gcc/testsuite/
>
>         PR target/103393
>         * gcc.target/i386/pr103393.c: New test.
> ---
>  gcc/config/i386/i386.cc                  | 12 ++++++++++++
>  gcc/doc/tm.texi                          |  4 ++++
>  gcc/doc/tm.texi.in                       |  2 ++
>  gcc/gimple-fold.cc                       |  7 ++-----
>  gcc/target.def                           |  6 ++++++
>  gcc/testsuite/gcc.target/i386/pr103393.c | 16 ++++++++++++++++
>  gcc/varasm.cc                            |  9 +++++++++
>  gcc/varasm.h                             |  2 ++
>  8 files changed, 53 insertions(+), 5 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.target/i386/pr103393.c
>
> diff --git a/gcc/config/i386/i386.cc b/gcc/config/i386/i386.cc
> index b2bf90576d5..2198db14dc6 100644
> --- a/gcc/config/i386/i386.cc
> +++ b/gcc/config/i386/i386.cc
> @@ -23918,6 +23918,15 @@ ix86_push_rounding (poly_int64 bytes)
>    return ROUND_UP (bytes, UNITS_PER_WORD);
>  }
>
> +/* Implement the TARGET_FOLD_MEMCPY_MAX hook.  Return the maximum number
> +   of bytes to fold memcpy.  */
> +
> +static unsigned int
> +ix86_fold_memcpy_max (void)
> +{
> +  return MOVE_MAX;
> +}
> +
>  /* Target-specific selftests.  */
>
>  #if CHECKING_P
> @@ -24735,6 +24744,9 @@ static bool ix86_libc_has_fast_function (int fcode ATTRIBUTE_UNUSED)
>  #define TARGET_RUN_TARGET_SELFTESTS selftest::ix86_run_selftests
>  #endif /* #if CHECKING_P */
>
> +#undef TARGET_FOLD_MEMCPY_MAX
> +#define TARGET_FOLD_MEMCPY_MAX ix86_fold_memcpy_max
> +
>  struct gcc_target targetm = TARGET_INITIALIZER;
>
>  #include "gt-i386.h"
> diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
> index 49864dd79f8..5c3b65b176e 100644
> --- a/gcc/doc/tm.texi
> +++ b/gcc/doc/tm.texi
> @@ -11924,6 +11924,10 @@ statement holding the function call.  Returns true if any change
>  was made to the GIMPLE stream.
>  @end deftypefn
>
> +@deftypefn {Target Hook} {unsigned int} TARGET_FOLD_MEMCPY_MAX (void)
> +This target hook returns the maximum number of bytes to fold memcpy.
> +@end deftypefn
> +
>  @deftypefn {Target Hook} int TARGET_COMPARE_VERSION_PRIORITY (tree @var{decl1}, tree @var{decl2})
>  This hook is used to compare the target attributes in two functions to
>  determine which function's features get higher priority.  This is used
> diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
> index 95e5e341f07..169145d0d45 100644
> --- a/gcc/doc/tm.texi.in
> +++ b/gcc/doc/tm.texi.in
> @@ -7924,6 +7924,8 @@ to by @var{ce_info}.
>
>  @hook TARGET_GIMPLE_FOLD_BUILTIN
>
> +@hook TARGET_FOLD_MEMCPY_MAX
> +
>  @hook TARGET_COMPARE_VERSION_PRIORITY
>
>  @hook TARGET_GET_FUNCTION_VERSIONS_DISPATCHER
> diff --git a/gcc/gimple-fold.cc b/gcc/gimple-fold.cc
> index c9179abb27e..31a9684fea4 100644
> --- a/gcc/gimple-fold.cc
> +++ b/gcc/gimple-fold.cc
> @@ -965,14 +965,11 @@ gimple_fold_builtin_memory_op (gimple_stmt_iterator *gsi,
>        /* If we can perform the copy efficiently with first doing all loads and
>          then all stores inline it that way.  Currently efficiently means that
>          we can load all the memory with a single set operation and that the
> -        total size is less than MOVE_MAX * MOVE_RATIO.  */
> +        total size is less than TARGET_FOLD_MEMCPY_MAX.  */
>        src_align = get_pointer_alignment (src);
>        dest_align = get_pointer_alignment (dest);
>        if (tree_fits_uhwi_p (len)
> -         && (compare_tree_int
> -             (len, (MOVE_MAX
> -                    * MOVE_RATIO (optimize_function_for_size_p (cfun))))
> -             <= 0)
> +         && compare_tree_int (len, targetm.fold_memcpy_max ()) <= 0
>           /* FIXME: Don't transform copies from strings with known length.
>              Until GCC 9 this prevented a case in gcc.dg/strlenopt-8.c
>              from being handled, and the case was XFAILed for that reason.
> diff --git a/gcc/target.def b/gcc/target.def
> index 72c2e1ef756..b88338f5003 100644
> --- a/gcc/target.def
> +++ b/gcc/target.def
> @@ -2489,6 +2489,12 @@ was made to the GIMPLE stream.",
>   bool, (gimple_stmt_iterator *gsi),
>   hook_bool_gsiptr_false)
>
> +DEFHOOK
> +(fold_memcpy_max,
> + "This target hook returns the maximum number of bytes to fold memcpy.",
> + unsigned int, (void),
> + default_fold_memcpy_max)
> +
>  /* Target hook is used to compare the target attributes in two functions to
>     determine which function's features get higher priority.  This is used
>     during function multi-versioning to figure out the order in which two
> diff --git a/gcc/testsuite/gcc.target/i386/pr103393.c b/gcc/testsuite/gcc.target/i386/pr103393.c
> new file mode 100644
> index 00000000000..7d54ae76561
> --- /dev/null
> +++ b/gcc/testsuite/gcc.target/i386/pr103393.c
> @@ -0,0 +1,16 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -march=skylake-avx512" } */
> +
> +struct TestData {
> +  float arr[8];
> +};
> +
> +void
> +cpy (struct TestData *s1, struct TestData *s2 )
> +{
> +  for(int i=0; i<16; ++i)
> +    s1->arr[i] = s2->arr[i];
> +}
> +
> +/* { dg-final { scan-assembler "jmp\[\\t \]*_?memmove" { target { ! ia32 } } } } */
> +/* { dg-final { scan-assembler "call\[\\t \]*_?memmove" { target ia32 } } } */
> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
> index d3d9daffb5d..49cd05146c0 100644
> --- a/gcc/varasm.cc
> +++ b/gcc/varasm.cc
> @@ -8509,4 +8509,13 @@ handle_vtv_comdat_section (section *sect, const_tree decl ATTRIBUTE_UNUSED)
>  #endif
>  }
>
> +
> +/* The default implementation of TARGET_FOLD_MEMCPY_MAX.  */
> +
> +unsigned int
> +default_fold_memcpy_max (void)
> +{
> +  return MOVE_MAX * MOVE_RATIO (optimize_function_for_size_p (cfun));
> +}
> +
>  #include "gt-varasm.h"
> diff --git a/gcc/varasm.h b/gcc/varasm.h
> index d5d8c4e5578..1fcb37e1f66 100644
> --- a/gcc/varasm.h
> +++ b/gcc/varasm.h
> @@ -79,4 +79,6 @@ extern rtx assemble_static_space (unsigned HOST_WIDE_INT);
>
>  extern rtx assemble_trampoline_template (void);
>
> +extern unsigned int default_fold_memcpy_max (void);
> +
>  #endif  // GCC_VARASM_H
> --
> 2.35.1
>


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