On Mon, 28 Jan 2013, Jakub Jelinek wrote:
> Hi!
>
> gimple_expand_builtin_pow last two optimizations rely on earlier
> optimizations in the same function to be performed, e.g.
> folding pow (x, c) for n = 2c into sqrt(x) * powi(x, n / 2) is only
> correct for c which isn't an integer (otherwise the sqrt(x) factor would
> need to be skipped), but they actually do not check this.
> E.g. the pow (x, n) where n is integer is optimized only if:
> && ((n >= -1 && n <= 2)
> || (flag_unsafe_math_optimizations
> && optimize_insn_for_speed_p ()
> && powi_cost (n) <= POWI_MAX_MULTS)))
> and as in the testcase the function is called, it isn't optimized and
> we fall through till the above mentioned optimization which blindly assumes
> that c isn't an integer.
>
> Fixed by both checking that c isn't an integer (and for the last
> optimization also that 2c isn't an integer), and also not doing the
> -> sqrt(x) * powi(x, n / 2) resp. 1.0 / sqrt(x) * powi(x, abs(n) / 2)
> optimization for -Os or cold functions, at least
> __attribute__((cold)) double
> foo (double x, double n)
> {
> return __builtin_pow (x, -1.5);
> }
> is smaller when expanded as pow call both on x86_64 and on powerpc (with
> -Os -ffast-math). Even just the c*_is_int tests alone could be enough
> to fix the bug, so if you say want to enable it for -Os even with c 1.5,
> but not for negative values which add another operation, it can be adjusted.
>
> Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Ok.
Thanks,
Richard.
> 2013-01-28 Jakub Jelinek <ja...@redhat.com>
>
> PR tree-optimization/56125
> * tree-ssa-math-opts.c (gimple_expand_builtin_pow): Don't optimize
> pow(x,c) into sqrt(x) * powi(x, n/2) or
> 1.0 / (sqrt(x) * powi(x, abs(n/2))) if c is an integer or when
> optimizing for size.
> Don't optimize pow(x,c) into powi(x, n/3) * powi(cbrt(x), n%3) or
> 1.0 / (powi(x, abs(n)/3) * powi(cbrt(x), abs(n)%3)) if 2c is an
> integer.
>
> * gcc.dg/pr56125.c: New test.
>
> --- gcc/tree-ssa-math-opts.c.jj 2013-01-11 09:02:48.000000000 +0100
> +++ gcc/tree-ssa-math-opts.c 2013-01-28 10:56:40.105950483 +0100
> @@ -1110,7 +1110,7 @@ gimple_expand_builtin_pow (gimple_stmt_i
> HOST_WIDE_INT n;
> tree type, sqrtfn, cbrtfn, sqrt_arg0, sqrt_sqrt, result, cbrt_x,
> powi_cbrt_x;
> enum machine_mode mode;
> - bool hw_sqrt_exists;
> + bool hw_sqrt_exists, c_is_int, c2_is_int;
>
> /* If the exponent isn't a constant, there's nothing of interest
> to be done. */
> @@ -1122,8 +1122,9 @@ gimple_expand_builtin_pow (gimple_stmt_i
> c = TREE_REAL_CST (arg1);
> n = real_to_integer (&c);
> real_from_integer (&cint, VOIDmode, n, n < 0 ? -1 : 0, 0);
> + c_is_int = real_identical (&c, &cint);
>
> - if (real_identical (&c, &cint)
> + if (c_is_int
> && ((n >= -1 && n <= 2)
> || (flag_unsafe_math_optimizations
> && optimize_insn_for_speed_p ()
> @@ -1221,7 +1222,8 @@ gimple_expand_builtin_pow (gimple_stmt_i
> return build_and_insert_call (gsi, loc, cbrtfn, sqrt_arg0);
> }
>
> - /* Optimize pow(x,c), where n = 2c for some nonzero integer n, into
> + /* Optimize pow(x,c), where n = 2c for some nonzero integer n
> + and c not an integer, into
>
> sqrt(x) * powi(x, n/2), n > 0;
> 1.0 / (sqrt(x) * powi(x, abs(n/2))), n < 0.
> @@ -1230,10 +1232,13 @@ gimple_expand_builtin_pow (gimple_stmt_i
> real_arithmetic (&c2, MULT_EXPR, &c, &dconst2);
> n = real_to_integer (&c2);
> real_from_integer (&cint, VOIDmode, n, n < 0 ? -1 : 0, 0);
> + c2_is_int = real_identical (&c2, &cint);
>
> if (flag_unsafe_math_optimizations
> && sqrtfn
> - && real_identical (&c2, &cint))
> + && c2_is_int
> + && !c_is_int
> + && optimize_function_for_speed_p (cfun))
> {
> tree powi_x_ndiv2 = NULL_TREE;
>
> @@ -1286,6 +1291,7 @@ gimple_expand_builtin_pow (gimple_stmt_i
> && cbrtfn
> && (gimple_val_nonnegative_real_p (arg0) || !HONOR_NANS (mode))
> && real_identical (&c2, &c)
> + && !c2_is_int
> && optimize_function_for_speed_p (cfun)
> && powi_cost (n / 3) <= POWI_MAX_MULTS)
> {
> --- gcc/testsuite/gcc.dg/pr56125.c.jj 2013-01-28 11:00:04.359814742 +0100
> +++ gcc/testsuite/gcc.dg/pr56125.c 2013-01-28 11:00:55.048532118 +0100
> @@ -0,0 +1,21 @@
> +/* PR tree-optimization/56125 */
> +/* { dg-do run } */
> +/* { dg-options "-O2 -ffast-math" } */
> +
> +extern void abort (void);
> +extern double fabs (double);
> +
> +__attribute__((cold)) double
> +foo (double x, double n)
> +{
> + double u = x / (n * n);
> + return u;
> +}
> +
> +int
> +main ()
> +{
> + if (fabs (foo (29, 2) - 7.25) > 0.001)
> + abort ();
> + return 0;
> +}
>
> Jakub
>
>
--
Richard Biener <rguent...@suse.de>
SUSE / SUSE Labs
SUSE LINUX Products GmbH - Nuernberg - AG Nuernberg - HRB 16746
GF: Jeff Hawn, Jennifer Guild, Felix Imend
