On Tue, 6 Oct 2020, Jakub Jelinek wrote:
> Hi!
>
> As written in the comment, tree-ssa-math-opts.c wouldn't create a DIVMOD
> ifn call for division + modulo by constant for the fear that during
> expansion we could generate better code for those cases.
> If the divisoris a power of two, that is certainly the case always,
> but otherwise expand_divmod can punt in many cases, e.g. if the division
> type's precision is above HOST_BITS_PER_WIDE_INT, we don't even call
> choose_multiplier, because it works on HOST_WIDE_INTs (true, something
> we should fix eventually now that we have wide_ints), or if pre/post shift
> is larger than BITS_PER_WORD.
>
> So, the following patch recognizes DIVMOD with constant last argument even
> when it is unclear if expand_divmod will be able to optimize it, and then
> during DIVMOD expansion if the divisor is constant attempts to expand it as
> division + modulo and if they actually don't contain any libcalls or
> division/modulo, they are kept as is, otherwise that sequence is thrown away
> and divmod optab or libcall is used.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
OK.
Richard.
> 2020-10-06 Jakub Jelinek <ja...@redhat.com>
>
> PR rtl-optimization/97282
> * tree-ssa-math-opts.c (divmod_candidate_p): Don't return false for
> constant op2 if it is not a power of two and the type has precision
> larger than HOST_BITS_PER_WIDE_INT or BITS_PER_WORD.
> * internal-fn.c (contains_call_div_mod): New function.
> (expand_DIVMOD): If last argument is a constant, try to expand it as
> TRUNC_DIV_EXPR followed by TRUNC_MOD_EXPR, but if the sequence
> contains any calls or {,U}{DIV,MOD} rtxes, throw it away and use
> divmod optab or divmod libfunc.
>
> * gcc.target/i386/pr97282.c: New test.
>
> --- gcc/tree-ssa-math-opts.c.jj 2020-10-01 10:40:10.104755999 +0200
> +++ gcc/tree-ssa-math-opts.c 2020-10-05 13:51:54.476628287 +0200
> @@ -3567,9 +3567,24 @@ divmod_candidate_p (gassign *stmt)
>
> /* Disable the transform if either is a constant, since
> division-by-constant
> may have specialized expansion. */
> - if (CONSTANT_CLASS_P (op1) || CONSTANT_CLASS_P (op2))
> + if (CONSTANT_CLASS_P (op1))
> return false;
>
> + if (CONSTANT_CLASS_P (op2))
> + {
> + if (integer_pow2p (op2))
> + return false;
> +
> + if (TYPE_PRECISION (type) <= HOST_BITS_PER_WIDE_INT
> + && TYPE_PRECISION (type) <= BITS_PER_WORD)
> + return false;
> +
> + /* If the divisor is not power of 2 and the precision wider than
> + HWI, expand_divmod punts on that, so in that case it is better
> + to use divmod optab or libfunc. Similarly if choose_multiplier
> + might need pre/post shifts of BITS_PER_WORD or more. */
> + }
> +
> /* Exclude the case where TYPE_OVERFLOW_TRAPS (type) as that should
> expand using the [su]divv optabs. */
> if (TYPE_OVERFLOW_TRAPS (type))
> --- gcc/internal-fn.c.jj 2020-10-02 10:36:43.272290992 +0200
> +++ gcc/internal-fn.c 2020-10-05 15:15:12.498349327 +0200
> @@ -50,6 +50,7 @@ along with GCC; see the file COPYING3.
> #include "tree-phinodes.h"
> #include "ssa-iterators.h"
> #include "explow.h"
> +#include "rtl-iter.h"
>
> /* The names of each internal function, indexed by function number. */
> const char *const internal_fn_name_array[] = {
> @@ -2985,6 +2986,32 @@ expand_gather_load_optab_fn (internal_fn
> emit_move_insn (lhs_rtx, ops[0].value);
> }
>
> +/* Helper for expand_DIVMOD. Return true if the sequence starting with
> + INSN contains any call insns or insns with {,U}{DIV,MOD} rtxes. */
> +
> +static bool
> +contains_call_div_mod (rtx_insn *insn)
> +{
> + subrtx_iterator::array_type array;
> + for (; insn; insn = NEXT_INSN (insn))
> + if (CALL_P (insn))
> + return true;
> + else if (INSN_P (insn))
> + FOR_EACH_SUBRTX (iter, array, PATTERN (insn), NONCONST)
> + switch (GET_CODE (*iter))
> + {
> + case CALL:
> + case DIV:
> + case UDIV:
> + case MOD:
> + case UMOD:
> + return true;
> + default:
> + break;
> + }
> + return false;
> + }
> +
> /* Expand DIVMOD() using:
> a) optab handler for udivmod/sdivmod if it is available.
> b) If optab_handler doesn't exist, generate call to
> @@ -3007,10 +3034,44 @@ expand_DIVMOD (internal_fn, gcall *call_
> rtx op1 = expand_normal (arg1);
> rtx target = expand_expr (lhs, NULL_RTX, VOIDmode, EXPAND_WRITE);
>
> - rtx quotient, remainder, libfunc;
> + rtx quotient = NULL_RTX, remainder = NULL_RTX;
> + rtx_insn *insns = NULL;
> +
> + if (TREE_CODE (arg1) == INTEGER_CST)
> + {
> + /* For DIVMOD by integral constants, there could be efficient code
> + expanded inline e.g. using shifts and plus/minus. Try to expand
> + the division and modulo and if it emits any library calls or any
> + {,U}{DIV,MOD} rtxes throw it away and use a divmod optab or
> + divmod libcall. */
> + struct separate_ops ops;
> + ops.code = TRUNC_DIV_EXPR;
> + ops.type = type;
> + ops.op0 = make_tree (ops.type, op0);
> + ops.op1 = arg1;
> + ops.op2 = NULL_TREE;
> + ops.location = gimple_location (call_stmt);
> + start_sequence ();
> + quotient = expand_expr_real_2 (&ops, NULL_RTX, mode, EXPAND_NORMAL);
> + if (contains_call_div_mod (get_insns ()))
> + quotient = NULL_RTX;
> + else
> + {
> + ops.code = TRUNC_MOD_EXPR;
> + remainder = expand_expr_real_2 (&ops, NULL_RTX, mode, EXPAND_NORMAL);
> + if (contains_call_div_mod (get_insns ()))
> + remainder = NULL_RTX;
> + }
> + if (remainder)
> + insns = get_insns ();
> + end_sequence ();
> + }
> +
> + if (remainder)
> + emit_insn (insns);
>
> /* Check if optab_handler exists for divmod_optab for given mode. */
> - if (optab_handler (tab, mode) != CODE_FOR_nothing)
> + else if (optab_handler (tab, mode) != CODE_FOR_nothing)
> {
> quotient = gen_reg_rtx (mode);
> remainder = gen_reg_rtx (mode);
> @@ -3018,7 +3079,7 @@ expand_DIVMOD (internal_fn, gcall *call_
> }
>
> /* Generate call to divmod libfunc if it exists. */
> - else if ((libfunc = optab_libfunc (tab, mode)) != NULL_RTX)
> + else if (rtx libfunc = optab_libfunc (tab, mode))
> targetm.expand_divmod_libfunc (libfunc, mode, op0, op1,
> "ient, &remainder);
>
> --- gcc/testsuite/gcc.target/i386/pr97282.c.jj 2020-10-05
> 15:31:19.500363710 +0200
> +++ gcc/testsuite/gcc.target/i386/pr97282.c 2020-10-05 15:28:51.864499619
> +0200
> @@ -0,0 +1,25 @@
> +/* PR rtl-optimization/97282 */
> +/* { dg-do compile } */
> +/* { dg-options "-O2" } */
> +/* { dg-final { scan-assembler "call\[^\n\r]*__udivmod\[dt]i4" } } */
> +
> +#ifdef __SIZEOF_INT128__
> +typedef __uint128_t T;
> +#else
> +typedef unsigned long long T;
> +#endif
> +
> +unsigned long
> +foo (T x)
> +{
> + if (x == 0)
> + return 0;
> +
> + unsigned long ret = 0;
> + while (x > 0)
> + {
> + ret = ret + x % 10;
> + x = x / 10;
> + }
> + return ret;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE Software Solutions Germany GmbH, Maxfeldstrasse 5, 90409 Nuernberg,
Germany; GF: Felix Imend
