Patchwork Add powi-to-multiply expansion to cse_sincos pass

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Submitter William J. Schmidt
Date May 19, 2011, 2:04 p.m.
Message ID <1305813841.29239.3.camel@gnopaine>
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Permalink /patch/96383/
State New
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Comments

William J. Schmidt - May 19, 2011, 2:04 p.m.
As Richard Guenther suggested, I'm submitting a separate patch to add
powi expansion to multiplies during the cse_sincos pass.  This was
originally submitted as part of a larger patch to fix PR46728.

Richard noted that tree_expand_builtin_powi, powi_as_mults, and
powi_as_mults_1 more properly belong in tree-ssa-math-opts.c.  I've
moved those here.  These were originally in builtins.c because they
require some #defines, a static table of values, and a couple of static
functions in builtins.c.  Those items will also belong in
tree-ssa-math-opts.c once the similar RTL logic is removed.

I chose to duplicate the required items into tree-ssa-math-opts.c for
now.  They will be removed from builtins.c in a future patch.  This
seemed more reasonable than exposing everything in a .h temporarily, and
then removing the .h code later.  But I can change to go that route if
you prefer. 

Bootstrap/regtest complete on powerpc target.

Seeking approval for mainline -- thanks!

Bill


2011-05-19  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>

	* tree-ssa-math-opts.c (POWI_MAX_MULTS): Copied from builtins.c;
	will be deleted from builtins.c in a later patch.
	(POWI_TABLE_SIZE): Likewise.
	(POWI_WINDOW_SIZE): Likewise.
	(powi_table): Likewise.
	(powi_lookup_cost): Likewise.
	(powi_cost): Likewise.
	(powi_as_mults_1): New.
	(powi_as_mults): New.
	(tree_expand_builtin_powi): New.
	(execute_cse_sincos): Add switch case for BUILT_IN_POWI.
	(gate_cse_sincos): Remove sincos/cexp restriction.
Richard Guenther - May 23, 2011, 11:26 a.m.
On Thu, May 19, 2011 at 4:04 PM, William J. Schmidt
<wschmidt@linux.vnet.ibm.com> wrote:
> As Richard Guenther suggested, I'm submitting a separate patch to add
> powi expansion to multiplies during the cse_sincos pass.  This was
> originally submitted as part of a larger patch to fix PR46728.
>
> Richard noted that tree_expand_builtin_powi, powi_as_mults, and
> powi_as_mults_1 more properly belong in tree-ssa-math-opts.c.  I've
> moved those here.  These were originally in builtins.c because they
> require some #defines, a static table of values, and a couple of static
> functions in builtins.c.  Those items will also belong in
> tree-ssa-math-opts.c once the similar RTL logic is removed.
>
> I chose to duplicate the required items into tree-ssa-math-opts.c for
> now.  They will be removed from builtins.c in a future patch.  This
> seemed more reasonable than exposing everything in a .h temporarily, and
> then removing the .h code later.  But I can change to go that route if
> you prefer.
>
> Bootstrap/regtest complete on powerpc target.
>
> Seeking approval for mainline -- thanks!
>
> Bill
>
>
> 2011-05-19  Bill Schmidt  <wschmidt@linux.vnet.ibm.com>
>
>        * tree-ssa-math-opts.c (POWI_MAX_MULTS): Copied from builtins.c;
>        will be deleted from builtins.c in a later patch.

No need to mention this.

>        (POWI_TABLE_SIZE): Likewise.
>        (POWI_WINDOW_SIZE): Likewise.
>        (powi_table): Likewise.
>        (powi_lookup_cost): Likewise.
>        (powi_cost): Likewise.
>        (powi_as_mults_1): New.
>        (powi_as_mults): New.
>        (tree_expand_builtin_powi): New.
>        (execute_cse_sincos): Add switch case for BUILT_IN_POWI.
>        (gate_cse_sincos): Remove sincos/cexp restriction.
>
>
> Index: gcc/tree-ssa-math-opts.c
> ===================================================================
> --- gcc/tree-ssa-math-opts.c    (revision 173908)
> +++ gcc/tree-ssa-math-opts.c    (working copy)
> @@ -795,8 +795,239 @@ execute_cse_sincos_1 (tree name)
>   return cfg_changed;
>  }
>
> +/* To evaluate powi(x,n), the floating point value x raised to the
> +   constant integer exponent n, we use a hybrid algorithm that
> +   combines the "window method" with look-up tables.  For an
> +   introduction to exponentiation algorithms and "addition chains",
> +   see section 4.6.3, "Evaluation of Powers" of Donald E. Knuth,
> +   "Seminumerical Algorithms", Vol. 2, "The Art of Computer Programming",
> +   3rd Edition, 1998, and Daniel M. Gordon, "A Survey of Fast Exponentiation
> +   Methods", Journal of Algorithms, Vol. 27, pp. 129-146, 1998.  */
> +
> +/* Provide a default value for POWI_MAX_MULTS, the maximum number of
> +   multiplications to inline before calling the system library's pow
> +   function.  powi(x,n) requires at worst 2*bits(n)-2 multiplications,
> +   so this default never requires calling pow, powf or powl.  */
> +
> +#ifndef POWI_MAX_MULTS
> +#define POWI_MAX_MULTS  (2*HOST_BITS_PER_WIDE_INT-2)
> +#endif
> +
> +/* The size of the "optimal power tree" lookup table.  All
> +   exponents less than this value are simply looked up in the
> +   powi_table below.  This threshold is also used to size the
> +   cache of pseudo registers that hold intermediate results.  */
> +#define POWI_TABLE_SIZE 256
> +
> +/* The size, in bits of the window, used in the "window method"
> +   exponentiation algorithm.  This is equivalent to a radix of
> +   (1<<POWI_WINDOW_SIZE) in the corresponding "m-ary method".  */
> +#define POWI_WINDOW_SIZE 3
> +
> +/* The following table is an efficient representation of an
> +   "optimal power tree".  For each value, i, the corresponding
> +   value, j, in the table states than an optimal evaluation
> +   sequence for calculating pow(x,i) can be found by evaluating
> +   pow(x,j)*pow(x,i-j).  An optimal power tree for the first
> +   100 integers is given in Knuth's "Seminumerical algorithms".  */
> +
> +static const unsigned char powi_table[POWI_TABLE_SIZE] =
> +  {
> +      0,   1,   1,   2,   2,   3,   3,   4,  /*   0 -   7 */
> +      4,   6,   5,   6,   6,  10,   7,   9,  /*   8 -  15 */
> +      8,  16,   9,  16,  10,  12,  11,  13,  /*  16 -  23 */
> +     12,  17,  13,  18,  14,  24,  15,  26,  /*  24 -  31 */
> +     16,  17,  17,  19,  18,  33,  19,  26,  /*  32 -  39 */
> +     20,  25,  21,  40,  22,  27,  23,  44,  /*  40 -  47 */
> +     24,  32,  25,  34,  26,  29,  27,  44,  /*  48 -  55 */
> +     28,  31,  29,  34,  30,  60,  31,  36,  /*  56 -  63 */
> +     32,  64,  33,  34,  34,  46,  35,  37,  /*  64 -  71 */
> +     36,  65,  37,  50,  38,  48,  39,  69,  /*  72 -  79 */
> +     40,  49,  41,  43,  42,  51,  43,  58,  /*  80 -  87 */
> +     44,  64,  45,  47,  46,  59,  47,  76,  /*  88 -  95 */
> +     48,  65,  49,  66,  50,  67,  51,  66,  /*  96 - 103 */
> +     52,  70,  53,  74,  54, 104,  55,  74,  /* 104 - 111 */
> +     56,  64,  57,  69,  58,  78,  59,  68,  /* 112 - 119 */
> +     60,  61,  61,  80,  62,  75,  63,  68,  /* 120 - 127 */
> +     64,  65,  65, 128,  66, 129,  67,  90,  /* 128 - 135 */
> +     68,  73,  69, 131,  70,  94,  71,  88,  /* 136 - 143 */
> +     72, 128,  73,  98,  74, 132,  75, 121,  /* 144 - 151 */
> +     76, 102,  77, 124,  78, 132,  79, 106,  /* 152 - 159 */
> +     80,  97,  81, 160,  82,  99,  83, 134,  /* 160 - 167 */
> +     84,  86,  85,  95,  86, 160,  87, 100,  /* 168 - 175 */
> +     88, 113,  89,  98,  90, 107,  91, 122,  /* 176 - 183 */
> +     92, 111,  93, 102,  94, 126,  95, 150,  /* 184 - 191 */
> +     96, 128,  97, 130,  98, 133,  99, 195,  /* 192 - 199 */
> +    100, 128, 101, 123, 102, 164, 103, 138,  /* 200 - 207 */
> +    104, 145, 105, 146, 106, 109, 107, 149,  /* 208 - 215 */
> +    108, 200, 109, 146, 110, 170, 111, 157,  /* 216 - 223 */
> +    112, 128, 113, 130, 114, 182, 115, 132,  /* 224 - 231 */
> +    116, 200, 117, 132, 118, 158, 119, 206,  /* 232 - 239 */
> +    120, 240, 121, 162, 122, 147, 123, 152,  /* 240 - 247 */
> +    124, 166, 125, 214, 126, 138, 127, 153,  /* 248 - 255 */
> +  };
> +
> +
> +/* Return the number of multiplications required to calculate
> +   powi(x,n) where n is less than POWI_TABLE_SIZE.  This is a
> +   subroutine of powi_cost.  CACHE is an array indicating
> +   which exponents have already been calculated.  */
> +
> +static int
> +powi_lookup_cost (unsigned HOST_WIDE_INT n, bool *cache)
> +{
> +  /* If we've already calculated this exponent, then this evaluation
> +     doesn't require any additional multiplications.  */
> +  if (cache[n])
> +    return 0;
> +
> +  cache[n] = true;
> +  return powi_lookup_cost (n - powi_table[n], cache)
> +        + powi_lookup_cost (powi_table[n], cache) + 1;
> +}
> +
> +/* Return the number of multiplications required to calculate
> +   powi(x,n) for an arbitrary x, given the exponent N.  This
> +   function needs to be kept in sync with expand_powi below.  */

Please double-check the function comments if they still refer
to the correct functions.

> +static int
> +powi_cost (HOST_WIDE_INT n)
> +{
> +  bool cache[POWI_TABLE_SIZE];
> +  unsigned HOST_WIDE_INT digit;
> +  unsigned HOST_WIDE_INT val;
> +  int result;
> +
> +  if (n == 0)
> +    return 0;
> +
> +  /* Ignore the reciprocal when calculating the cost.  */
> +  val = (n < 0) ? -n : n;
> +
> +  /* Initialize the exponent cache.  */
> +  memset (cache, 0, POWI_TABLE_SIZE * sizeof (bool));
> +  cache[1] = true;
> +
> +  result = 0;
> +
> +  while (val >= POWI_TABLE_SIZE)
> +    {
> +      if (val & 1)
> +       {
> +         digit = val & ((1 << POWI_WINDOW_SIZE) - 1);
> +         result += powi_lookup_cost (digit, cache)
> +                   + POWI_WINDOW_SIZE + 1;
> +         val >>= POWI_WINDOW_SIZE;
> +       }
> +      else
> +       {
> +         val >>= 1;
> +         result++;
> +       }
> +    }
> +
> +  return result + powi_lookup_cost (val, cache);
> +}
> +
> +/* Recursive subroutine of powi_as_mults.  This function takes the
> +   array, CACHE, of already calculated exponents and an exponent N and
> +   returns a tree that corresponds to CACHE[1]**N, with type TYPE.  */
> +
> +static tree
> +powi_as_mults_1 (gimple_stmt_iterator *gsi, location_t loc, tree type,
> +                HOST_WIDE_INT n, tree *cache)
> +{
> +  tree op0, op1, target;
> +  unsigned HOST_WIDE_INT digit;
> +  gimple mult_stmt;
> +
> +  if (n < POWI_TABLE_SIZE)
> +    {
> +      if (cache[n])
> +       return cache[n];
> +
> +      target = create_tmp_var (type, "powmult");
> +      add_referenced_var (target);

Avoid creating multiple variables, just re-use a single temporary
(just pass it along here as an additional argument).

> +      target = make_ssa_name (target, NULL);
> +      cache[n] = target;
> +
> +      op0 = powi_as_mults_1 (gsi, loc, type, n - powi_table[n], cache);
> +      op1 = powi_as_mults_1 (gsi, loc, type, powi_table[n], cache);
> +    }
> +  else if (n & 1)
> +    {
> +      target = create_tmp_var (type, "powmult");
> +      add_referenced_var (target);

Likewise.

> +      target = make_ssa_name (target, NULL);
> +      digit = n & ((1 << POWI_WINDOW_SIZE) - 1);
> +      op0 = powi_as_mults_1 (gsi, loc, type, n - digit, cache);
> +      op1 = powi_as_mults_1 (gsi, loc, type, digit, cache);
> +    }
> +  else
> +    {
> +      target = create_tmp_var (type, "powmult");
> +      add_referenced_var (target);

Likewise.

> +      target = make_ssa_name (target, NULL);
> +      op0 = powi_as_mults_1 (gsi, loc, type, n >> 1, cache);
> +      op1 = op0;
> +    }
> +
> +  mult_stmt = gimple_build_assign_with_ops (MULT_EXPR, target, op0, op1);
> +  SSA_NAME_DEF_STMT (target) = mult_stmt;

I think setting SSA_NAME_DEF_STMT is already done by
gimple_build_assign_with_ops.

> +  gsi_insert_before (gsi, mult_stmt, GSI_SAME_STMT);
> +
> +  return target;
> +}
> +
> +/* Convert ARG0**N to a tree of multiplications of ARG0 with itself.
> +   This function needs to be kept in sync with powi_cost in builtins.c.  */
> +
> +static tree
> +powi_as_mults (gimple_stmt_iterator *gsi, location_t loc,
> +              tree arg0, HOST_WIDE_INT n)
> +{
> +  tree cache[POWI_TABLE_SIZE], result, type = TREE_TYPE (arg0);
> +
> +  if (n == 0)
> +    return omit_one_operand_loc (loc, type, build_real (type, dconst1), arg0);

No need for omit_one_operand - simply return build_real (type, dconst1).

> +  memset (cache, 0,  sizeof (cache));
> +  cache[1] = arg0;

Allocate the temporary var here

> +  result = powi_as_mults_1 (gsi, loc, type, (n < 0) ? -n : n, cache);
> +
> +  /* If the original exponent was negative, reciprocate the result.  */
> +  if (n < 0)
> +    result = build2_loc (loc, RDIV_EXPR, type,
> +                        build_real (type, dconst1), result);

Please build a final division statement and assign it to a new SSA name
which you return.

> +  return result;
> +}
> +
> +/* ARGS are the two arguments to a powi builtin in GSI with location info
> +   LOC.  If the arguments are appropriate, create an equivalent set of
> +   statements prior to GSI using an optimal number of multiplications,
> +   and return an expession holding the result.  */
> +
> +static tree
> +tree_expand_builtin_powi (gimple_stmt_iterator *gsi, location_t loc, tree *args)

A better name would be gimple_expand_builtin_powi now.

> +{
> +  HOST_WIDE_INT n = TREE_INT_CST_LOW (args[1]);
> +  HOST_WIDE_INT n_hi = TREE_INT_CST_HIGH (args[1]);
> +
> +  if ((n_hi == 0 || n_hi == -1)
> +      /* Avoid largest negative number.  */
> +      && (n != -n)
> +      && ((n >= -1 && n <= 2)
> +         || (optimize_function_for_speed_p (cfun)
> +             && powi_cost (n) <= POWI_MAX_MULTS)))
> +    return powi_as_mults (gsi, loc, args[0], n);
> +
> +  return NULL_TREE;
> +}
> +
>  /* Go through all calls to sin, cos and cexpi and call execute_cse_sincos_1
> -   on the SSA_NAME argument of each of them.  */
> +   on the SSA_NAME argument of each of them.  Also expand powi(x,n) into
> +   an optimal number of multiplies, when n is a constant.  */
>
>  static unsigned int
>  execute_cse_sincos (void)
> @@ -821,7 +1052,7 @@ execute_cse_sincos (void)
>              && (fndecl = gimple_call_fndecl (stmt))
>              && DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL)
>            {
> -             tree arg;
> +             tree arg, *args;
>
>              switch (DECL_FUNCTION_CODE (fndecl))
>                {
> @@ -833,6 +1064,23 @@ execute_cse_sincos (void)
>                    cfg_changed |= execute_cse_sincos_1 (arg);
>                  break;
>
> +               CASE_FLT_FN (BUILT_IN_POWI):
> +                 args = gimple_call_arg_ptr (stmt, 0);
> +                 if (host_integerp (args[1], 0) && !TREE_OVERFLOW (args[1]))

Please access arguments via gimple_call_arg (stmt, N), simply assign
both to tempoaries and pass them down separately to
tree_expand_builtin_pow, the exponent as HOST_WIDE_INT instead
of as tree. We also do not care for TREE_OVERFLOW
(I realize this is pre-existing code).

> +                   {
> +                     tree result = NULL_TREE;
> +                     location_t loc = gimple_location (stmt);
> +                     result = tree_expand_builtin_powi (&gsi, loc, args);
> +                     if (result)
> +                       {
> +                         tree lhs = gimple_get_lhs (stmt);
> +                         gimple new_stmt = gimple_build_assign (lhs, result);
> +                         gimple_set_location (new_stmt, loc);
> +                         gsi_replace (&gsi, new_stmt, true);
> +                       }
> +                   }
> +                 break;
> +
>                default:;
>                }
>            }
> @@ -849,10 +1097,9 @@ execute_cse_sincos (void)
>  static bool
>  gate_cse_sincos (void)
>  {
> -  /* Make sure we have either sincos or cexp.  */
> -  return (TARGET_HAS_SINCOS
> -         || TARGET_C99_FUNCTIONS)
> -        && optimize;
> +  /* We no longer require either sincos or cexp, since powi expansion
> +     piggybacks on this pass.  */
> +  return optimize;
>  }

Otherwise this patch looks good to me.  Please re-post it with the
suggested adjustments.

The next patch to attack would be to make sure integer valued exponent
pows are folded to powi for -funsafe-math-optimizations.

Thanks,
Richard.

>  struct gimple_opt_pass pass_cse_sincos =
>
>
>

Patch

Index: gcc/tree-ssa-math-opts.c
===================================================================
--- gcc/tree-ssa-math-opts.c	(revision 173908)
+++ gcc/tree-ssa-math-opts.c	(working copy)
@@ -795,8 +795,239 @@  execute_cse_sincos_1 (tree name)
   return cfg_changed;
 }
 
+/* To evaluate powi(x,n), the floating point value x raised to the
+   constant integer exponent n, we use a hybrid algorithm that
+   combines the "window method" with look-up tables.  For an
+   introduction to exponentiation algorithms and "addition chains",
+   see section 4.6.3, "Evaluation of Powers" of Donald E. Knuth,
+   "Seminumerical Algorithms", Vol. 2, "The Art of Computer Programming",
+   3rd Edition, 1998, and Daniel M. Gordon, "A Survey of Fast Exponentiation
+   Methods", Journal of Algorithms, Vol. 27, pp. 129-146, 1998.  */
+
+/* Provide a default value for POWI_MAX_MULTS, the maximum number of
+   multiplications to inline before calling the system library's pow
+   function.  powi(x,n) requires at worst 2*bits(n)-2 multiplications,
+   so this default never requires calling pow, powf or powl.  */
+
+#ifndef POWI_MAX_MULTS
+#define POWI_MAX_MULTS  (2*HOST_BITS_PER_WIDE_INT-2)
+#endif
+
+/* The size of the "optimal power tree" lookup table.  All
+   exponents less than this value are simply looked up in the
+   powi_table below.  This threshold is also used to size the
+   cache of pseudo registers that hold intermediate results.  */
+#define POWI_TABLE_SIZE 256
+
+/* The size, in bits of the window, used in the "window method"
+   exponentiation algorithm.  This is equivalent to a radix of
+   (1<<POWI_WINDOW_SIZE) in the corresponding "m-ary method".  */
+#define POWI_WINDOW_SIZE 3
+
+/* The following table is an efficient representation of an
+   "optimal power tree".  For each value, i, the corresponding
+   value, j, in the table states than an optimal evaluation
+   sequence for calculating pow(x,i) can be found by evaluating
+   pow(x,j)*pow(x,i-j).  An optimal power tree for the first
+   100 integers is given in Knuth's "Seminumerical algorithms".  */
+
+static const unsigned char powi_table[POWI_TABLE_SIZE] =
+  {
+      0,   1,   1,   2,   2,   3,   3,   4,  /*   0 -   7 */
+      4,   6,   5,   6,   6,  10,   7,   9,  /*   8 -  15 */
+      8,  16,   9,  16,  10,  12,  11,  13,  /*  16 -  23 */
+     12,  17,  13,  18,  14,  24,  15,  26,  /*  24 -  31 */
+     16,  17,  17,  19,  18,  33,  19,  26,  /*  32 -  39 */
+     20,  25,  21,  40,  22,  27,  23,  44,  /*  40 -  47 */
+     24,  32,  25,  34,  26,  29,  27,  44,  /*  48 -  55 */
+     28,  31,  29,  34,  30,  60,  31,  36,  /*  56 -  63 */
+     32,  64,  33,  34,  34,  46,  35,  37,  /*  64 -  71 */
+     36,  65,  37,  50,  38,  48,  39,  69,  /*  72 -  79 */
+     40,  49,  41,  43,  42,  51,  43,  58,  /*  80 -  87 */
+     44,  64,  45,  47,  46,  59,  47,  76,  /*  88 -  95 */
+     48,  65,  49,  66,  50,  67,  51,  66,  /*  96 - 103 */
+     52,  70,  53,  74,  54, 104,  55,  74,  /* 104 - 111 */
+     56,  64,  57,  69,  58,  78,  59,  68,  /* 112 - 119 */
+     60,  61,  61,  80,  62,  75,  63,  68,  /* 120 - 127 */
+     64,  65,  65, 128,  66, 129,  67,  90,  /* 128 - 135 */
+     68,  73,  69, 131,  70,  94,  71,  88,  /* 136 - 143 */
+     72, 128,  73,  98,  74, 132,  75, 121,  /* 144 - 151 */
+     76, 102,  77, 124,  78, 132,  79, 106,  /* 152 - 159 */
+     80,  97,  81, 160,  82,  99,  83, 134,  /* 160 - 167 */
+     84,  86,  85,  95,  86, 160,  87, 100,  /* 168 - 175 */
+     88, 113,  89,  98,  90, 107,  91, 122,  /* 176 - 183 */
+     92, 111,  93, 102,  94, 126,  95, 150,  /* 184 - 191 */
+     96, 128,  97, 130,  98, 133,  99, 195,  /* 192 - 199 */
+    100, 128, 101, 123, 102, 164, 103, 138,  /* 200 - 207 */
+    104, 145, 105, 146, 106, 109, 107, 149,  /* 208 - 215 */
+    108, 200, 109, 146, 110, 170, 111, 157,  /* 216 - 223 */
+    112, 128, 113, 130, 114, 182, 115, 132,  /* 224 - 231 */
+    116, 200, 117, 132, 118, 158, 119, 206,  /* 232 - 239 */
+    120, 240, 121, 162, 122, 147, 123, 152,  /* 240 - 247 */
+    124, 166, 125, 214, 126, 138, 127, 153,  /* 248 - 255 */
+  };
+
+
+/* Return the number of multiplications required to calculate
+   powi(x,n) where n is less than POWI_TABLE_SIZE.  This is a
+   subroutine of powi_cost.  CACHE is an array indicating
+   which exponents have already been calculated.  */
+
+static int
+powi_lookup_cost (unsigned HOST_WIDE_INT n, bool *cache)
+{
+  /* If we've already calculated this exponent, then this evaluation
+     doesn't require any additional multiplications.  */
+  if (cache[n])
+    return 0;
+
+  cache[n] = true;
+  return powi_lookup_cost (n - powi_table[n], cache)
+	 + powi_lookup_cost (powi_table[n], cache) + 1;
+}
+
+/* Return the number of multiplications required to calculate
+   powi(x,n) for an arbitrary x, given the exponent N.  This
+   function needs to be kept in sync with expand_powi below.  */
+
+static int
+powi_cost (HOST_WIDE_INT n)
+{
+  bool cache[POWI_TABLE_SIZE];
+  unsigned HOST_WIDE_INT digit;
+  unsigned HOST_WIDE_INT val;
+  int result;
+
+  if (n == 0)
+    return 0;
+
+  /* Ignore the reciprocal when calculating the cost.  */
+  val = (n < 0) ? -n : n;
+
+  /* Initialize the exponent cache.  */
+  memset (cache, 0, POWI_TABLE_SIZE * sizeof (bool));
+  cache[1] = true;
+
+  result = 0;
+
+  while (val >= POWI_TABLE_SIZE)
+    {
+      if (val & 1)
+	{
+	  digit = val & ((1 << POWI_WINDOW_SIZE) - 1);
+	  result += powi_lookup_cost (digit, cache)
+		    + POWI_WINDOW_SIZE + 1;
+	  val >>= POWI_WINDOW_SIZE;
+	}
+      else
+	{
+	  val >>= 1;
+	  result++;
+	}
+    }
+
+  return result + powi_lookup_cost (val, cache);
+}
+
+/* Recursive subroutine of powi_as_mults.  This function takes the
+   array, CACHE, of already calculated exponents and an exponent N and
+   returns a tree that corresponds to CACHE[1]**N, with type TYPE.  */
+
+static tree
+powi_as_mults_1 (gimple_stmt_iterator *gsi, location_t loc, tree type,
+		 HOST_WIDE_INT n, tree *cache)
+{
+  tree op0, op1, target;
+  unsigned HOST_WIDE_INT digit;
+  gimple mult_stmt;
+
+  if (n < POWI_TABLE_SIZE)
+    {
+      if (cache[n])
+	return cache[n];
+
+      target = create_tmp_var (type, "powmult");
+      add_referenced_var (target);
+      target = make_ssa_name (target, NULL);
+      cache[n] = target;
+
+      op0 = powi_as_mults_1 (gsi, loc, type, n - powi_table[n], cache);
+      op1 = powi_as_mults_1 (gsi, loc, type, powi_table[n], cache);
+    }
+  else if (n & 1)
+    {
+      target = create_tmp_var (type, "powmult");
+      add_referenced_var (target);
+      target = make_ssa_name (target, NULL);
+      digit = n & ((1 << POWI_WINDOW_SIZE) - 1);
+      op0 = powi_as_mults_1 (gsi, loc, type, n - digit, cache);
+      op1 = powi_as_mults_1 (gsi, loc, type, digit, cache);
+    }
+  else
+    {
+      target = create_tmp_var (type, "powmult");
+      add_referenced_var (target);
+      target = make_ssa_name (target, NULL);
+      op0 = powi_as_mults_1 (gsi, loc, type, n >> 1, cache);
+      op1 = op0;
+    }
+
+  mult_stmt = gimple_build_assign_with_ops (MULT_EXPR, target, op0, op1);
+  SSA_NAME_DEF_STMT (target) = mult_stmt;
+  gsi_insert_before (gsi, mult_stmt, GSI_SAME_STMT);
+
+  return target;
+}
+
+/* Convert ARG0**N to a tree of multiplications of ARG0 with itself.
+   This function needs to be kept in sync with powi_cost in builtins.c.  */
+
+static tree
+powi_as_mults (gimple_stmt_iterator *gsi, location_t loc,
+	       tree arg0, HOST_WIDE_INT n)
+{
+  tree cache[POWI_TABLE_SIZE], result, type = TREE_TYPE (arg0);
+
+  if (n == 0)
+    return omit_one_operand_loc (loc, type, build_real (type, dconst1), arg0);
+
+  memset (cache, 0,  sizeof (cache));
+  cache[1] = arg0;
+
+  result = powi_as_mults_1 (gsi, loc, type, (n < 0) ? -n : n, cache);
+
+  /* If the original exponent was negative, reciprocate the result.  */
+  if (n < 0)
+    result = build2_loc (loc, RDIV_EXPR, type,
+			 build_real (type, dconst1), result);
+  return result;
+}
+
+/* ARGS are the two arguments to a powi builtin in GSI with location info
+   LOC.  If the arguments are appropriate, create an equivalent set of
+   statements prior to GSI using an optimal number of multiplications,
+   and return an expession holding the result.  */
+
+static tree
+tree_expand_builtin_powi (gimple_stmt_iterator *gsi, location_t loc, tree *args)
+{
+  HOST_WIDE_INT n = TREE_INT_CST_LOW (args[1]);
+  HOST_WIDE_INT n_hi = TREE_INT_CST_HIGH (args[1]);
+
+  if ((n_hi == 0 || n_hi == -1)
+      /* Avoid largest negative number.  */
+      && (n != -n)
+      && ((n >= -1 && n <= 2)
+	  || (optimize_function_for_speed_p (cfun)
+	      && powi_cost (n) <= POWI_MAX_MULTS)))
+    return powi_as_mults (gsi, loc, args[0], n);
+
+  return NULL_TREE;
+}
+
 /* Go through all calls to sin, cos and cexpi and call execute_cse_sincos_1
-   on the SSA_NAME argument of each of them.  */
+   on the SSA_NAME argument of each of them.  Also expand powi(x,n) into
+   an optimal number of multiplies, when n is a constant.  */
 
 static unsigned int
 execute_cse_sincos (void)
@@ -821,7 +1052,7 @@  execute_cse_sincos (void)
 	      && (fndecl = gimple_call_fndecl (stmt))
 	      && DECL_BUILT_IN_CLASS (fndecl) == BUILT_IN_NORMAL)
 	    {
-	      tree arg;
+	      tree arg, *args;
 
 	      switch (DECL_FUNCTION_CODE (fndecl))
 		{
@@ -833,6 +1064,23 @@  execute_cse_sincos (void)
 		    cfg_changed |= execute_cse_sincos_1 (arg);
 		  break;
 
+		CASE_FLT_FN (BUILT_IN_POWI):
+		  args = gimple_call_arg_ptr (stmt, 0);
+		  if (host_integerp (args[1], 0) && !TREE_OVERFLOW (args[1]))
+		    {
+		      tree result = NULL_TREE;
+		      location_t loc = gimple_location (stmt);
+		      result = tree_expand_builtin_powi (&gsi, loc, args);
+		      if (result)
+			{
+			  tree lhs = gimple_get_lhs (stmt);
+			  gimple new_stmt = gimple_build_assign (lhs, result);
+			  gimple_set_location (new_stmt, loc);
+			  gsi_replace (&gsi, new_stmt, true);
+			}
+		    }
+		  break;
+
 		default:;
 		}
 	    }
@@ -849,10 +1097,9 @@  execute_cse_sincos (void)
 static bool
 gate_cse_sincos (void)
 {
-  /* Make sure we have either sincos or cexp.  */
-  return (TARGET_HAS_SINCOS
-	  || TARGET_C99_FUNCTIONS)
-	 && optimize;
+  /* We no longer require either sincos or cexp, since powi expansion
+     piggybacks on this pass.  */
+  return optimize;
 }
 
 struct gimple_opt_pass pass_cse_sincos =