Re: [PATCH 5/8] middle-end: Add cltz_complement idiom recognition

[Andrew Carlotti <andrew.carlotti@arm.com>]

On Mon, Nov 14, 2022 at 04:10:22PM +0100, Richard Biener wrote:
> On Fri, Nov 11, 2022 at 7:53 PM Andrew Carlotti via Gcc-patches
> <gcc-patches@gcc.gnu.org> wrote:
> >
> > This recognises patterns of the form:
> > while (n) { n >>= 1 }
> >
> > This patch results in improved (but still suboptimal) codegen:
> >
> > foo (unsigned int b) {
> >     int c = 0;
> >
> >     while (b) {
> >         b >>= 1;
> >         c++;
> >     }
> >
> >     return c;
> > }
> >
> > foo:
> > .LFB11:
> >         .cfi_startproc
> >         cbz     w0, .L3
> >         clz     w1, w0
> >         tst     x0, 1
> >         mov     w0, 32
> >         sub     w0, w0, w1
> >         csel    w0, w0, wzr, ne
> >         ret
> >
> > The conditional is unnecessary. phiopt could recognise a redundant csel
> > (using cond_removal_in_builtin_zero_pattern) when one of the inputs is a
> > clz call, but it cannot recognise the redunancy when the input is (e.g.)
> > (32 - clz).
> >
> > I could perhaps extend this function to recognise this pattern in a later
> > patch, if this is a good place to recognise more patterns.
> >
> > gcc/ChangeLog:
> >
+           PR tree-optimization/94793
> >         * tree-scalar-evolution.cc (expression_expensive_p): Add checks
> >         for c[lt]z optabs.
> >         * tree-ssa-loop-niter.cc (build_cltz_expr): New.
> >         (number_of_iterations_cltz_complement): New.
> >         (number_of_iterations_bitcount): Add call to the above.
> >
> > gcc/testsuite/ChangeLog:
> >
> >         * lib/target-supports.exp (check_effective_target_clz)
> >         (check_effective_target_clzl, check_effective_target_clzll)
> >         (check_effective_target_ctz, check_effective_target_clzl)
> >         (check_effective_target_ctzll): New.
> >         * gcc.dg/tree-ssa/cltz-complement-max.c: New test.
> >         * gcc.dg/tree-ssa/clz-complement-char.c: New test.
> >         * gcc.dg/tree-ssa/clz-complement-int.c: New test.
> >         * gcc.dg/tree-ssa/clz-complement-long-long.c: New test.
> >         * gcc.dg/tree-ssa/clz-complement-long.c: New test.
> >         * gcc.dg/tree-ssa/ctz-complement-char.c: New test.
> >         * gcc.dg/tree-ssa/ctz-complement-int.c: New test.
> >         * gcc.dg/tree-ssa/ctz-complement-long-long.c: New test.
> >         * gcc.dg/tree-ssa/ctz-complement-long.c: New test.
> >
> >
> > --
> >
> >

[snip test diffs]

> > diff --git a/gcc/tree-scalar-evolution.cc b/gcc/tree-scalar-evolution.cc
> > index 7e2a3e986619de87e4ae9daf16198be1f13b917c..1ac9526c69b5fe80c26022f2fa1176d222e2dfb9 100644
> > --- a/gcc/tree-scalar-evolution.cc
> > +++ b/gcc/tree-scalar-evolution.cc
> > @@ -3406,12 +3406,21 @@ expression_expensive_p (tree expr, hash_map<tree, uint64_t> &cache,
> >          library call for popcount when backend does not have an instruction
> >          to do so.  We consider this to be expensive and generate
> >          __builtin_popcount only when backend defines it.  */
> > +      optab optab;
> >        combined_fn cfn = get_call_combined_fn (expr);
> >        switch (cfn)
> >         {
> >         CASE_CFN_POPCOUNT:
> > +         optab = popcount_optab;
> > +         goto bitcount_call;
> > +       CASE_CFN_CLZ:
> > +         optab = clz_optab;
> > +         goto bitcount_call;
> > +       CASE_CFN_CTZ:
> > +         optab = ctz_optab;
> > +bitcount_call:
> >           /* Check if opcode for popcount is available in the mode required.  */
> > -         if (optab_handler (popcount_optab,
> > +         if (optab_handler (optab,
> >                              TYPE_MODE (TREE_TYPE (CALL_EXPR_ARG (expr, 0))))
> >               == CODE_FOR_nothing)
> >             {
> > @@ -3424,7 +3433,7 @@ expression_expensive_p (tree expr, hash_map<tree, uint64_t> &cache,
> >                  instructions.  */
> >               if (is_a <scalar_int_mode> (mode, &int_mode)
> >                   && GET_MODE_SIZE (int_mode) == 2 * UNITS_PER_WORD
> > -                 && (optab_handler (popcount_optab, word_mode)
> > +                 && (optab_handler (optab, word_mode)
> >                       != CODE_FOR_nothing))
> >                   break;
> >               return true;
> > diff --git a/gcc/tree-ssa-loop-niter.cc b/gcc/tree-ssa-loop-niter.cc
> > index fece876099c1687569d6351e7d2416ea6acae5b5..16e8e25919d808cea27adbd72f0be01ae2f0e547 100644
> > --- a/gcc/tree-ssa-loop-niter.cc
> > +++ b/gcc/tree-ssa-loop-niter.cc
> > @@ -2198,6 +2198,195 @@ number_of_iterations_popcount (loop_p loop, edge exit,
> >    return true;
> >  }
> >
> > +/* Return an expression that counts the leading/trailing zeroes of src.  */
> 
> Can you expand the comment on how you handle defined_at_zero and how
> that relates to the C[LT]Z_DEFINED_VALUE_AT_ZERO target macros?
> The loop examples you gave above all have a defined value for zero, I'm
> not sure how you'd write a C loop which has that undefined.
> 

How about:

/* Return an expression that counts the leading/trailing zeroes of src.

   If defined_at_zero is true, then the built expression uses a conditonal
   expression to return the precision of src when src == 0.
   Otherwise, we can elide the conditional expression and let src = 0 invoke
   undefined behaviour.  */


> > +static tree
> > +build_cltz_expr (tree src, bool leading, bool defined_at_zero)
> > +{
> > +  tree fn;
> > +  int prec = TYPE_PRECISION (TREE_TYPE (src));
> > +  int i_prec = TYPE_PRECISION (integer_type_node);
> > +  int li_prec = TYPE_PRECISION (long_integer_type_node);
> > +  int lli_prec = TYPE_PRECISION (long_long_integer_type_node);
> > +  if (prec <= i_prec)
> 
> I don't think we can use <= for both CLZ and CTZ, no?  You probably
> need a GIMPLE testcase or a language that doesn't promote char/short
> to int for a testcase that fails though.

I don't see an issue here.  I've ensured that src is the mode used in
the iterator, and if a longer mode is used for the __builtin_clz
call then I account for the difference below...

> > +    fn = leading ? builtin_decl_implicit (BUILT_IN_CLZ)
> > +                : builtin_decl_implicit (BUILT_IN_CTZ);
> > +  else if (prec == li_prec)
> > +    fn = leading ? builtin_decl_implicit (BUILT_IN_CLZL)
> > +                : builtin_decl_implicit (BUILT_IN_CTZL);
> > +  else if (prec == lli_prec || prec == 2 * lli_prec)
> > +    fn = leading ? builtin_decl_implicit (BUILT_IN_CLZLL)
> > +                : builtin_decl_implicit (BUILT_IN_CTZLL);
> > +  else
> > +    return NULL_TREE;
> > +
> > +  tree utype = unsigned_type_for (TREE_TYPE (src));
> > +  src = fold_convert (utype, src);
> > +  if (prec < i_prec)
> > +    src = fold_convert (unsigned_type_node, src);
> > +
> > +  tree call;
> > +  if (prec == 2 * lli_prec)
> > +    {
> > +      tree src1 = fold_convert (long_long_unsigned_type_node,
> > +                               fold_build2 (RSHIFT_EXPR, TREE_TYPE (src),
> > +                                            unshare_expr (src),
> > +                                            build_int_cst (integer_type_node,
> > +                                                           lli_prec)));
> > +      tree src2 = fold_convert (long_long_unsigned_type_node, src);
> > +      /* We count the zeroes in src1, and add the number in src2 when src1
> > +        is 0.  */
> > +      if (!leading)
> > +       std::swap(src1, src2);
> > +      tree call1 = build_call_expr (fn, 1, src1);
> > +      tree call2 = build_call_expr (fn, 1, src2);
> > +      if (defined_at_zero)
> > +       {
> > +         tree is_zero2 = fold_build2 (NE_EXPR, boolean_type_node, src2,
> > +                                      build_zero_cst (TREE_TYPE (src2)));
> > +         call2 = fold_build3(COND_EXPR, integer_type_node, is_zero2, call2,
> > +                             build_int_cst (integer_type_node, lli_prec));
> > +       }
> > +      tree is_zero1 = fold_build2 (NE_EXPR, boolean_type_node, src1,
> > +                                  build_zero_cst (TREE_TYPE (src1)));
> > +      call = fold_build3(COND_EXPR, integer_type_node, is_zero1, call1,
> > +                        fold_build2 (PLUS_EXPR, integer_type_node, call2,
> > +                                     build_int_cst (integer_type_node,
> > +                                                    lli_prec)));
> > +    }
> > +  else
> > +    {
> > +      call = build_call_expr (fn, 1, src);
> > +      if (defined_at_zero)
> > +       {
> > +         tree is_zero = fold_build2 (NE_EXPR, boolean_type_node, src,
> > +                                     build_zero_cst (TREE_TYPE (src)));
> > +         call = fold_build3(COND_EXPR, integer_type_node, is_zero, call,
> > +                            build_int_cst (integer_type_node, prec));
> > +       }
> > +    }
> > +

...with this code:

> > +  if (leading && prec < i_prec)
> > +    call = fold_build2(MINUS_EXPR, integer_type_node, call,
> > +                      build_int_cst (integer_type_node,
> > +                                     i_prec - prec));
> > +
> > +  return call;
> > +}
> > +
> > +/* See comment below for number_of_iterations_bitcount.
> > +   For c[lt]z complement, we have:
> > +
> > +   modify:
> > +   iv_2 = iv_1 >> 1 OR iv_1 << 1
> > +
> > +   test:
> > +   if (iv != 0)
> > +
> > +   modification count:
> > +   src precision - c[lt]z (src)
> > +
> > + */
> > +
> > +static bool
> > +number_of_iterations_cltz_complement (loop_p loop, edge exit,
> > +                              enum tree_code code,
> > +                              class tree_niter_desc *niter)
> > +{
> > +  bool modify_before_test = true;
> > +  HOST_WIDE_INT max;
> > +
> > +  /* Check that condition for staying inside the loop is like
> > +     if (iv != 0).  */
> > +  gimple *cond_stmt = last_stmt (exit->src);
> > +  if (!cond_stmt
> > +      || gimple_code (cond_stmt) != GIMPLE_COND
> > +      || code != NE_EXPR
> > +      || !integer_zerop (gimple_cond_rhs (cond_stmt))
> > +      || TREE_CODE (gimple_cond_lhs (cond_stmt)) != SSA_NAME)
> > +    return false;
> > +
> > +  tree iv_2 = gimple_cond_lhs (cond_stmt);
> > +  gimple *iv_2_stmt = SSA_NAME_DEF_STMT (iv_2);
> > +
> > +  /* If the test comes before the iv modification, then these will actually be
> > +     iv_1 and a phi node.  */
> > +  if (gimple_code (iv_2_stmt) == GIMPLE_PHI
> > +      && gimple_bb (iv_2_stmt) == loop->header
> > +      && gimple_phi_num_args (iv_2_stmt) == 2
> > +      && (TREE_CODE (gimple_phi_arg_def (iv_2_stmt,
> > +                                        loop_latch_edge (loop)->dest_idx))
> > +         == SSA_NAME))
> > +    {
> > +      /* iv_2 is actually one of the inputs to the phi.  */
> > +      iv_2 = gimple_phi_arg_def (iv_2_stmt, loop_latch_edge (loop)->dest_idx);
> > +      iv_2_stmt = SSA_NAME_DEF_STMT (iv_2);
> > +      modify_before_test = false;
> > +    }
> > +
> > +  /* Make sure iv_2_stmt is a logical shift by one stmt:
> > +     iv_2 = iv_1 {>>|<<} 1  */
> > +  if (!is_gimple_assign (iv_2_stmt)
> > +      || (gimple_assign_rhs_code (iv_2_stmt) != LSHIFT_EXPR
> > +         && (gimple_assign_rhs_code (iv_2_stmt) != RSHIFT_EXPR
> > +             || !TYPE_UNSIGNED (TREE_TYPE (gimple_assign_lhs (iv_2_stmt)))))
> > +      || !integer_onep (gimple_assign_rhs2 (iv_2_stmt)))
> > +    return false;
> > +
> > +  bool left_shift = (gimple_assign_rhs_code (iv_2_stmt) == LSHIFT_EXPR);
> > +
> > +  tree iv_1 = gimple_assign_rhs1 (iv_2_stmt);
> > +
> > +  /* Check the recurrence.  */
> > +  gimple *phi = SSA_NAME_DEF_STMT (iv_1);
> > +  if (gimple_code (phi) != GIMPLE_PHI
> > +      || (gimple_bb (phi) != loop_latch_edge (loop)->dest)
> > +      || (iv_2 != gimple_phi_arg_def (phi, loop_latch_edge (loop)->dest_idx)))
> > +    return false;
> > +
> > +  /* We found a match.  */
> > +  tree src = gimple_phi_arg_def (phi, loop_preheader_edge (loop)->dest_idx);
> > +  int src_precision = TYPE_PRECISION (TREE_TYPE (src));
> > +
> > +  /* Get the corresponding c[lt]z builtin.  */
> > +  tree expr = build_cltz_expr (src, !left_shift, true);
> 
> So we always have defined_at_zero == true?
 
In the patch: yes.  The next patch uses defined_at_zero == false, and I
don't think there's any point in submitting a simpler build_cltz_expr
for just this patch.

> > +
> > +  if (!expr)
> > +    return false;
> > +
> > +  expr = fold_build2 (MINUS_EXPR, integer_type_node,
> > +                     build_int_cst (integer_type_node, src_precision),
> > +                     expr);
> > +
> > +  max = src_precision;
> > +
> > +  tree may_be_zero = boolean_false_node;
> > +
> > +  if (modify_before_test)
> > +    {
> > +      expr = fold_build2 (MINUS_EXPR, integer_type_node, expr,
> > +                         integer_one_node);
> > +      max = max - 1;
> > +      may_be_zero = fold_build2 (EQ_EXPR, boolean_type_node, src,
> > +                                     build_zero_cst (TREE_TYPE (src)));
> > +    }
> > +
> > +  expr = fold_convert (unsigned_type_node, expr);
> > +
> > +  niter->assumptions = boolean_true_node;
> > +  niter->may_be_zero = simplify_using_initial_conditions (loop, may_be_zero);
> > +  niter->niter = simplify_using_initial_conditions (loop, expr);
> > +
> > +  if (TREE_CODE (niter->niter) == INTEGER_CST)
> > +    niter->max = tree_to_uhwi (niter->niter);
> > +  else
> > +    niter->max = max;
> > +
> > +  niter->bound = NULL_TREE;
> > +  niter->cmp = ERROR_MARK;
> > +  return true;
> > +}
> > +
> >  /* See if LOOP contains a bit counting idiom. The idiom consists of two parts:
> >     1. A modification to the induction variabler;.
> >     2. A test to determine whether or not to exit the loop.
> > @@ -2244,7 +2433,8 @@ number_of_iterations_bitcount (loop_p loop, edge exit,
> >                                enum tree_code code,
> >                                class tree_niter_desc *niter)
> >  {
> > -  return number_of_iterations_popcount (loop, exit, code, niter);
> > +  return (number_of_iterations_popcount (loop, exit, code, niter)
> > +         || number_of_iterations_cltz_complement (loop, exit, code, niter));
> 
> I'm kind-of missing the non-complement variant ;)

See next patch :)

> Otherwise looks OK to me.
> 
> Thanks,
> Richard.
> 
> >  }
> >
> >  /* Substitute NEW_TREE for OLD in EXPR and fold the result.