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.
