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?
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
