On Sun, Sep 11, 2022 at 10:51 PM Takayuki 'January June' Suwa via
Gcc-patches <gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
>
> Many RISC machines, as we know, have some restrictions on placing
> register-width constants in the source of load-immediate machine
> instructions, so the target must provide a solution for that in the machine
> description.
>
> A naive way would be to solve it early, ie. to replace with read constants
> pooled in memory when expanding to RTL.
>
> Alternatively, a more fancy approach would be to forgo placement in the
> constant pool until somewhere before the reload/LRA eg. the "split1" pass to
> give the optimization passes that involve immediates a chance to work.
>
> If we choose the latter, we can expect better results with RTL if-conversion,
> constant folding, etc., but it often propagates constants that are too large
> in size to resolve to a simple load-immediate instruction.
>
> This is because constant propagation has no way of telling about it, so this
> patch provides it.
What does prevent other passes like fwprop, CSE and PRE from doing the
same propagation? Can that be the solution for
constant propagation as well?
Richard.
> ===
>
> This new target hook can be used to tell cprop whether or not to propagate
> a constant depending on its contents.
>
> For backwards compatibility, the default setting for this hook retains the
> old behavior.
>
> gcc/ChangeLog:
>
> * hooks.h (hook_bool_const_rtx_true): New prototype.
> * hooks.cc (hook_bool_const_rtx_true): New default hook.
> * target.def (constant_ok_for_cprop_p): New target hook.
> * cprop.cc (cprop_constant_p): Change to use the hook.
> * doc/tm.texi.in, (TARGET_CONSTANT_OK_FOR_CPROP_P): New @hook.
> * doc/tm.texi (TARGET_CONSTANT_OK_FOR_CPROP_P): New document.
> ---
> gcc/cprop.cc | 4 +++-
> gcc/doc/tm.texi | 12 ++++++++++++
> gcc/doc/tm.texi.in | 2 ++
> gcc/hooks.cc | 7 +++++++
> gcc/hooks.h | 1 +
> gcc/target.def | 14 ++++++++++++++
> 6 files changed, 39 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/cprop.cc b/gcc/cprop.cc
> index 580f811545d..dfb1e88e9b4 100644
> --- a/gcc/cprop.cc
> +++ b/gcc/cprop.cc
> @@ -40,6 +40,7 @@ along with GCC; see the file COPYING3. If not see
> #include "dbgcnt.h"
> #include "cfgloop.h"
> #include "gcse.h"
> +#include "target.h"
>
>
> /* An obstack for our working variables. */
> @@ -249,7 +250,8 @@ insert_set_in_table (rtx dest, rtx src, rtx_insn *insn,
> static bool
> cprop_constant_p (const_rtx x)
> {
> - return CONSTANT_P (x) && (GET_CODE (x) != CONST || shared_const_p (x));
> + return CONSTANT_P (x) && targetm.constant_ok_for_cprop_p (x)
> + && (GET_CODE (x) != CONST || shared_const_p (x));
> }
>
> /* Determine whether the rtx X should be treated as a register that can
> diff --git a/gcc/doc/tm.texi b/gcc/doc/tm.texi
> index 858bfb80cec..83151626a71 100644
> --- a/gcc/doc/tm.texi
> +++ b/gcc/doc/tm.texi
> @@ -12187,6 +12187,18 @@ MIPS, where add-immediate takes a 16-bit signed
> value,
> is zero, which disables this optimization.
> @end deftypevr
>
> +@deftypefn {Target Hook} bool TARGET_CONSTANT_OK_FOR_CPROP_P (const_rtx
> @var{cst})
> +On some target machines, such as RISC ones, load-immediate instructions
> +often have a limited range (for example, within signed 12 bits or less).
> +Because they will be typically placed into the constant pool,
> +unconditionally propagating constants that exceed such limit can lead to
> +increased number of instruction and/or memory read access.
> +This target hook should return @code{false} if @var{cst}, a candidate for
> +constant propagation, is undesirable as a source for load-immediate
> +instructions.
> +The default version of this hook always returns @code{true}.
> +@end deftypefn
> +
> @deftypefn {Target Hook} {unsigned HOST_WIDE_INT} TARGET_ASAN_SHADOW_OFFSET
> (void)
> Return the offset bitwise ored into shifted address to get corresponding
> Address Sanitizer shadow memory address. NULL if Address Sanitizer is not
> diff --git a/gcc/doc/tm.texi.in b/gcc/doc/tm.texi.in
> index 21b849ea32a..147331b0f53 100644
> --- a/gcc/doc/tm.texi.in
> +++ b/gcc/doc/tm.texi.in
> @@ -7887,6 +7887,8 @@ and the associated definitions of those functions.
>
> @hook TARGET_CONST_ANCHOR
>
> +@hook TARGET_CONSTANT_OK_FOR_CPROP_P
> +
> @hook TARGET_ASAN_SHADOW_OFFSET
>
> @hook TARGET_MEMMODEL_CHECK
> diff --git a/gcc/hooks.cc b/gcc/hooks.cc
> index b29233f4f85..67bf3553d26 100644
> --- a/gcc/hooks.cc
> +++ b/gcc/hooks.cc
> @@ -82,6 +82,13 @@ hook_bool_mode_true (machine_mode)
> return true;
> }
>
> +/* Generic hook that takes (const_rtx) and returns true. */
> +bool
> +hook_bool_const_rtx_true (const_rtx)
> +{
> + return true;
> +}
> +
> /* Generic hook that takes (machine_mode, machine_mode) and returns true. */
> bool
> hook_bool_mode_mode_true (machine_mode, machine_mode)
> diff --git a/gcc/hooks.h b/gcc/hooks.h
> index 1056e1e9e4d..d001f8fb9dc 100644
> --- a/gcc/hooks.h
> +++ b/gcc/hooks.h
> @@ -30,6 +30,7 @@ extern bool hook_bool_bool_gcc_optionsp_false (bool, struct
> gcc_options *);
> extern bool hook_bool_const_int_const_int_true (const int, const int);
> extern bool hook_bool_mode_false (machine_mode);
> extern bool hook_bool_mode_true (machine_mode);
> +extern bool hook_bool_const_rtx_true (const_rtx);
> extern bool hook_bool_mode_mode_true (machine_mode, machine_mode);
> extern bool hook_bool_mode_const_rtx_false (machine_mode, const_rtx);
> extern bool hook_bool_mode_const_rtx_true (machine_mode, const_rtx);
> diff --git a/gcc/target.def b/gcc/target.def
> index 4d49ffc2c88..8bf093ab0dc 100644
> --- a/gcc/target.def
> +++ b/gcc/target.def
> @@ -4510,6 +4510,20 @@ MIPS, where add-immediate takes a 16-bit signed
> value,\n\
> is zero, which disables this optimization.",
> unsigned HOST_WIDE_INT, 0)
>
> +DEFHOOK
> +(constant_ok_for_cprop_p,
> + "On some target machines, such as RISC ones, load-immediate instructions\n\
> +often have a limited range (for example, within signed 12 bits or less).\n\
> +Because they will be typically placed into the constant pool,\n\
> +unconditionally propagating constants that exceed such limit can lead to\n\
> +increased number of instruction and/or memory read access.\n\
> +This target hook should return @code{false} if @var{cst}, a candidate for\n\
> +constant propagation, is undesirable as a source for load-immediate\n\
> +instructions.\n\
> +The default version of this hook always returns @code{true}.",
> + bool, (const_rtx cst),
> + hook_bool_const_rtx_true)
> +
> /* Defines, which target-dependent bits (upper 16) are used by port */
> DEFHOOK
> (memmodel_check,
> --
> 2.20.1
