Re: [PATCH 7/8] middle-end: Add c[lt]z idiom recognition

[Richard Biener <richard.guenther@gmail.com>]

On Fri, Nov 11, 2022 at 8:06 PM Andrew Carlotti via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> This recognises the patterns of the form:
>   while (n & 1) { n >>= 1 }
>
> Unfortunately there are currently two issues relating to this patch.
>
> Firstly, simplify_using_initial_conditions does not recognise that
>         (n != 0) and ((n & 1) == 0) implies that ((n >> 1) != 0).
>
> This preconditions arise following the loop copy-header pass, and the
> assumptions returned by number_of_iterations_exit_assumptions then
> prevent final value replacement from using the niter result.
>
> I'm not sure what is the best way to fix this - one approach could be to
> modify simplify_using_initial_conditions to handle this sort of case,
> but it seems that it basically wants the information that ranger could
> give anway, so would something like that be a better option?

I've noted elsewhere that simplify_using_initial_conditions should be
rewritten to use (path) ranger somehow.  But I've also worked around
that for the case in PR100756, though not in this function.

> The second issue arises in the vectoriser, which is able to determine
> that the niter->assumptions are always true.
> When building with -march=armv8.4-a+sve -S -O3, we get this codegen:
>
> foo (unsigned int b) {
>     int c = 0;
>
>     if (b == 0)
>       return PREC;
>
>     while (!(b & (1 << (PREC - 1)))) {
>         b <<= 1;
>         c++;
>     }
>
>     return c;
> }
>
> foo:
> .LFB0:
>         .cfi_startproc
>         cmp     w0, 0
>         cbz     w0, .L6
>         blt     .L7
>         lsl     w1, w0, 1
>         clz     w2, w1
>         cmp     w2, 14
>         bls     .L8
>         mov     x0, 0
>         cntw    x3
>         add     w1, w2, 1
>         index   z1.s, #0, #1
>         whilelo p0.s, wzr, w1
> .L4:
>         add     x0, x0, x3
>         mov     p1.b, p0.b
>         mov     z0.d, z1.d
>         whilelo p0.s, w0, w1
>         incw    z1.s
>         b.any   .L4
>         add     z0.s, z0.s, #1
>         lastb   w0, p1, z0.s
>         ret
>         .p2align 2,,3
> .L8:
>         mov     w0, 0
>         b       .L3
>         .p2align 2,,3
> .L13:
>         lsl     w1, w1, 1
> .L3:
>         add     w0, w0, 1
>         tbz     w1, #31, .L13
>         ret
>         .p2align 2,,3
> .L6:
>         mov     w0, 32
>         ret
>         .p2align 2,,3
> .L7:
>         mov     w0, 0
>         ret
>         .cfi_endproc
>
> In essence, the vectoriser uses the niter information to determine
> exactly how many iterations of the loop it needs to run. It then uses
> SVE whilelo instructions to run this number of iterations. The original
> loop counter is also vectorised, despite only being used in the final
> iteration, and then the final value of this counter is used as the
> return value (which is the same as the number of iterations it computed
> in the first place).
>
> This vectorisation is obviously bad, and I think it exposes a latent
> bug in the vectoriser, rather than being an issue caused by this
> specific patch.

The main issue is that we use niter analysis to detect popcount and
friends but final value replacement doesn't always apply.  When other
optimizations pick up this niter result the final values are not
replaced as aggressively.  Ideally we'd replace the loops IV with
a counting one, but sometimes the intermediate values of the
popcounted variable are still used.

This patch looks OK.

Thanks,
Richard.

> gcc/ChangeLog:
>
>         * tree-ssa-loop-niter.cc (number_of_iterations_cltz): New.
>         (number_of_iterations_bitcount): Add call to the above.
>         (number_of_iterations_exit_assumptions): Add EQ_EXPR case for
>         c[lt]z idiom recognition.
>
> gcc/testsuite/ChangeLog:
>
>         * gcc.dg/tree-ssa/cltz-max.c: New test.
>         * gcc.dg/tree-ssa/clz-char.c: New test.
>         * gcc.dg/tree-ssa/clz-int.c: New test.
>         * gcc.dg/tree-ssa/clz-long-long.c: New test.
>         * gcc.dg/tree-ssa/clz-long.c: New test.
>         * gcc.dg/tree-ssa/ctz-char.c: New test.
>         * gcc.dg/tree-ssa/ctz-int.c: New test.
>         * gcc.dg/tree-ssa/ctz-long-long.c: New test.
>         * gcc.dg/tree-ssa/ctz-long.c: New test.
>
>
> --
>
>
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/cltz-max.c b/gcc/testsuite/gcc.dg/tree-ssa/cltz-max.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..a6bea3d338940efee2e7e1c95a5941525945af9e
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/cltz-max.c
> @@ -0,0 +1,72 @@
> +/* { dg-do compile } */
> +/* { dg-options "-O2 -fno-tree-loop-optimize -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__)
> +
> +int clz_count1 (unsigned char b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return 0;
> +
> +    while (!(b & (1 << (PREC - 1)))) {
> +       b <<= 1;
> +       c++;
> +    }
> +    if (c <= PREC - 1)
> +      return 0;
> +    else
> +      return 34567;
> +}
> +
> +int clz_count2 (unsigned char b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return 0;
> +
> +    while (!(b & (1 << PREC - 1))) {
> +       b <<= 1;
> +       c++;
> +    }
> +    if (c <= PREC - 2)
> +      return 0;
> +    else
> +      return 76543;
> +}
> +
> +int ctz_count1 (unsigned char b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return 0;
> +
> +    while (!(b & 1)) {
> +       b >>= 1;
> +       c++;
> +    }
> +    if (c <= PREC - 1)
> +      return 0;
> +    else
> +      return 23456;
> +}
> +
> +int ctz_count2 (unsigned char b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return 0;
> +
> +    while (!(b & 1)) {
> +       b >>= 1;
> +       c++;
> +    }
> +    if (c <= PREC - 2)
> +      return 0;
> +    else
> +      return 65432;
> +}
> +/* { dg-final { scan-tree-dump-times "34567" 0 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "76543" 1 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "23456" 0 "optimized" } } */
> +/* { dg-final { scan-tree-dump-times "65432" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/clz-char.c b/gcc/testsuite/gcc.dg/tree-ssa/clz-char.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..4a122db95bbb576b4ade706bd3b1ca809d2f1e3b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/clz-char.c
> @@ -0,0 +1,34 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target clzl } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned char b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & (1 << (PREC - 1)))) {
> +       b <<= 1;
> +       c++;
> +    }
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(1 << (PREC - 1)) != 0)
> +    __builtin_abort ();
> +  if (foo(35) != PREC - 6)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_clz|\\.CLZ" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/clz-int.c b/gcc/testsuite/gcc.dg/tree-ssa/clz-int.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..96646f8e19cd5b2342acb88949b3ef6e3e2abd5a
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/clz-int.c
> @@ -0,0 +1,34 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target clzl } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__ * __SIZEOF_INT__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned int b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & (1 << (PREC - 1)))) {
> +       b <<= 1;
> +       c++;
> +    }
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(1 << (PREC - 1)) != 0)
> +    __builtin_abort ();
> +  if (foo(35) != PREC - 6)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_clz|\\.CLZ" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/clz-long-long.c b/gcc/testsuite/gcc.dg/tree-ssa/clz-long-long.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..80d3edc1dab2e74fc3271ba9d97640839b3a3786
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/clz-long-long.c
> @@ -0,0 +1,34 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target clzll } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__ * __SIZEOF_LONG_LONG__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned long long b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & (1LL << (PREC - 1)))) {
> +       b <<= 1;
> +       c++;
> +    }
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(1LL << (PREC - 1)) != 0)
> +    __builtin_abort ();
> +  if (foo(35) != PREC - 6)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_clz|\\.CLZ" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/clz-long.c b/gcc/testsuite/gcc.dg/tree-ssa/clz-long.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..1c8037f93b9c9d42f580a172267c65723a46ef8b
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/clz-long.c
> @@ -0,0 +1,34 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target clzl } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__ * __SIZEOF_LONG__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned long b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & (1L << (PREC - 1)))) {
> +       b <<= 1;
> +       c++;
> +}
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(1L << (PREC - 1)) != 0)
> +    __builtin_abort ();
> +  if (foo(35) != PREC - 6)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_clz|\\.CLZ" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ctz-char.c b/gcc/testsuite/gcc.dg/tree-ssa/ctz-char.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..3cd166acbd4670e175d79a2403de2d5a4fd38665
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ctz-char.c
> @@ -0,0 +1,36 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target ctz } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned char b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & 1)) {
> +       b >>= 1;
> +       c++;
> +    }
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(128) != 7)
> +    __builtin_abort ();
> +  if (foo(96) != 5)
> +    __builtin_abort ();
> +  if (foo(35) != 0)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_ctz|\\.CTZ" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ctz-int.c b/gcc/testsuite/gcc.dg/tree-ssa/ctz-int.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..7f63493eb7389a18516f8f126c3c55dc80f0bde6
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ctz-int.c
> @@ -0,0 +1,36 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target ctz } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__ * __SIZEOF_INT__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned int b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & 1)) {
> +       b >>= 1;
> +       c++;
> +    }
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(1 << (PREC - 1)) != PREC - 1)
> +    __builtin_abort ();
> +  if (foo(96) != 5)
> +    __builtin_abort ();
> +  if (foo(35) != 0)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_ctz|\\.CTZ" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ctz-long-long.c b/gcc/testsuite/gcc.dg/tree-ssa/ctz-long-long.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..924f61b76f01c77a40b9fff64af3b629ab1418c0
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ctz-long-long.c
> @@ -0,0 +1,36 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target ctzll } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__ * __SIZEOF_LONG_LONG__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned long long b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & 1)) {
> +       b >>= 1;
> +       c++;
> +    }
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(1LL << (PREC - 1)) != PREC - 1)
> +    __builtin_abort ();
> +  if (foo(96) != 5)
> +    __builtin_abort ();
> +  if (foo(35) != 0)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_ctz|\\.CTZ" 1 "optimized" } } */
> diff --git a/gcc/testsuite/gcc.dg/tree-ssa/ctz-long.c b/gcc/testsuite/gcc.dg/tree-ssa/ctz-long.c
> new file mode 100644
> index 0000000000000000000000000000000000000000..178945daa8a2697989f1a1a0804ce33d768dcc55
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/tree-ssa/ctz-long.c
> @@ -0,0 +1,36 @@
> +/* { dg-do run } */
> +/* { dg-require-effective-target ctzl } */
> +/* { dg-options "-O2 -fno-tree-ch -fdump-tree-optimized" } */
> +
> +#define PREC (__CHAR_BIT__ * __SIZEOF_LONG__)
> +
> +int
> +__attribute__ ((noinline, noclone))
> +foo (unsigned long b) {
> +    int c = 0;
> +
> +    if (b == 0)
> +      return PREC;
> +
> +    while (!(b & 1)) {
> +       b >>= 1;
> +       c++;
> +    }
> +
> +    return c;
> +}
> +
> +int main()
> +{
> +  if (foo(0) != PREC)
> +    __builtin_abort ();
> +  if (foo(1L << (PREC - 1)) != PREC - 1)
> +    __builtin_abort ();
> +  if (foo(96) != 5)
> +    __builtin_abort ();
> +  if (foo(35) != 0)
> +    __builtin_abort ();
> +  return 0;
> +}
> +
> +/* { dg-final { scan-tree-dump-times "__builtin_ctz|\\.CTZ" 1 "optimized" } } */
> diff --git a/gcc/tree-ssa-loop-niter.cc b/gcc/tree-ssa-loop-niter.cc
> index 16e8e25919d808cea27adbd72f0be01ae2f0e547..87e6fe81d68fc3352e450688ef79e6fc68854d8a 100644
> --- a/gcc/tree-ssa-loop-niter.cc
> +++ b/gcc/tree-ssa-loop-niter.cc
> @@ -2274,6 +2274,167 @@ build_cltz_expr (tree src, bool leading, bool defined_at_zero)
>    return call;
>  }
>
> +/* See comment below for number_of_iterations_bitcount.
> +   For c[lt]z, we have:
> +
> +   modify:
> +   iv_2 = iv_1 << 1 OR iv_1 >> 1
> +
> +   test:
> +   if (iv & 1 << (prec-1)) OR (iv & 1)
> +
> +   modification count:
> +   src precision - c[lt]z (src)
> +
> + */
> +
> +static bool
> +number_of_iterations_cltz (loop_p loop, edge exit,
> +                              enum tree_code code,
> +                              class tree_niter_desc *niter)
> +{
> +  bool modify_before_test = true;
> +  HOST_WIDE_INT max;
> +  int checked_bit;
> +  tree iv_2;
> +
> +  /* 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 != EQ_EXPR && code != GE_EXPR)
> +      || !integer_zerop (gimple_cond_rhs (cond_stmt))
> +      || TREE_CODE (gimple_cond_lhs (cond_stmt)) != SSA_NAME)
> +    return false;
> +
> +  if (code == EQ_EXPR)
> +    {
> +      /* Make sure we check a bitwise and with a suitable constant */
> +      gimple *and_stmt = SSA_NAME_DEF_STMT (gimple_cond_lhs (cond_stmt));
> +      if (!is_gimple_assign (and_stmt)
> +         || gimple_assign_rhs_code (and_stmt) != BIT_AND_EXPR
> +         || !integer_pow2p (gimple_assign_rhs2 (and_stmt)))
> +       return false;
> +
> +      checked_bit = tree_log2 (gimple_assign_rhs2 (and_stmt));
> +
> +      iv_2 = gimple_assign_rhs1 (and_stmt);
> +    }
> +  else
> +    {
> +      /* We have a GE_EXPR - a signed comparison with zero is equivalent to
> +        testing the leading bit, so check for this pattern too.  */
> +
> +      iv_2 = gimple_cond_lhs (cond_stmt);
> +      tree test_value_type = TREE_TYPE (iv_2);
> +
> +      if (TYPE_UNSIGNED (test_value_type))
> +       return false;
> +
> +      gimple *test_value_stmt = SSA_NAME_DEF_STMT (iv_2);
> +
> +      if (is_gimple_assign (test_value_stmt)
> +         && gimple_assign_rhs_code (test_value_stmt) == NOP_EXPR)
> +       {
> +         /* If the test value comes from a NOP_EXPR, then we need to unwrap
> +            this.  We conservatively require that both types have the same
> +            precision.  */
> +         iv_2 = gimple_assign_rhs1 (test_value_stmt);
> +         tree rhs_type = TREE_TYPE (iv_2);
> +         if (TREE_CODE (rhs_type) != INTEGER_TYPE
> +             || (TYPE_PRECISION (rhs_type)
> +                 != TYPE_PRECISION (test_value_type)))
> +           return false;
> +       }
> +
> +      checked_bit = TYPE_PRECISION (test_value_type) - 1;
> +    }
> +
> +  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));
> +
> +  /* Apply any needed preprocessing to src.  */
> +  int num_ignored_bits;
> +  if (left_shift)
> +    num_ignored_bits = src_precision - checked_bit - 1;
> +  else
> +    num_ignored_bits = checked_bit;
> +
> +  if (modify_before_test)
> +    num_ignored_bits++;
> +
> +  if (num_ignored_bits != 0)
> +    src = fold_build2 (left_shift ? LSHIFT_EXPR : RSHIFT_EXPR,
> +                      TREE_TYPE (src), src,
> +                      build_int_cst (integer_type_node, num_ignored_bits));
> +
> +  /* Get the corresponding c[lt]z builtin.  */
> +  tree expr = build_cltz_expr (src, left_shift, false);
> +
> +  if (!expr)
> +    return false;
> +
> +  max = src_precision - num_ignored_bits - 1;
> +
> +  expr = fold_convert (unsigned_type_node, expr);
> +
> +  tree assumptions = fold_build2 (NE_EXPR, boolean_type_node, src,
> +                                 build_zero_cst (TREE_TYPE (src)));
> +
> +  niter->assumptions = simplify_using_initial_conditions (loop, assumptions);
> +  niter->may_be_zero = boolean_false_node;
> +  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 comment below for number_of_iterations_bitcount.
>     For c[lt]z complement, we have:
>
> @@ -2434,6 +2595,7 @@ number_of_iterations_bitcount (loop_p loop, edge exit,
>                                class tree_niter_desc *niter)
>  {
>    return (number_of_iterations_popcount (loop, exit, code, niter)
> +         || number_of_iterations_cltz (loop, exit, code, niter)
>           || number_of_iterations_cltz_complement (loop, exit, code, niter));
>  }
>
> @@ -2960,6 +3122,9 @@ number_of_iterations_exit_assumptions (class loop *loop, edge exit,
>      case NE_EXPR:
>        break;
>
> +    case EQ_EXPR:
> +      return number_of_iterations_cltz (loop, exit, code, niter);
> +
>      default:
>        return false;
>      }