Ping.
Thanks,
Kyrill
> On 17 Jul 2026, at 15:25, Kyrylo Tkachov <[email protected]> wrote:
>
> From: Kyrylo Tkachov <[email protected]>
>
> profile_probability::apply uses RDIV, which adds half the denominator
> before division. This does not round negative values correctly.
> Multiplying the signed input by the fixed-point probability can also
> overflow for large inputs.
>
> Scale the unsigned magnitude with safe_scale_64bit, then restore the sign.
> Return the input directly for a unit probability so that the minimum
> gcov_type value remains representable. Preserve the existing truncation
> for uninitialized probabilities.
>
> Add selftests for signed rounding, zero and unit probabilities,
> uninitialized probabilities, and the minimum and maximum gcov_type values.
>
> Bootstrapped and tested on aarch64-linux-gnu and x86_64-linux.
> Ok for trunk?
>
> Thanks,
> Kyrill
>
> gcc/ChangeLog:
>
> * profile-count.h (profile_probability::apply): Scale an unsigned
> magnitude with safe_scale_64bit.
> * profile-count.cc (test_profile_probability_apply): New.
> (profile_count_cc_tests): New.
> * selftest.h (profile_count_cc_tests): Declare.
> * selftest-run-tests.cc (selftest::run_tests): Run it.
>
> Signed-off-by: Kyrylo Tkachov <[email protected]>
> ---
> gcc/profile-count.cc | 47 +++++++++++++++++++++++++++++++++++++++
> gcc/profile-count.h | 26 +++++++++++++++++++++-
> gcc/selftest-run-tests.cc | 1 +
> gcc/selftest.h | 1 +
> 4 files changed, 74 insertions(+), 1 deletion(-)
>
> diff --git a/gcc/profile-count.cc b/gcc/profile-count.cc
> index 2cce9caa772..9be209efda2 100644
> --- a/gcc/profile-count.cc
> +++ b/gcc/profile-count.cc
> @@ -33,6 +33,7 @@ along with GCC; see the file COPYING3. If not see
> #include "wide-int.h"
> #include "sreal.h"
> #include "profile.h"
> +#include "selftest.h"
>
> /* Names from profile_quality enum values. */
>
> @@ -595,3 +596,49 @@ profile_count::force_nonzero () const
> }
> return ret;
> }
> +
> +#if CHECKING_P
> +
> +namespace selftest {
> +
> +/* Verify profile_probability::apply. */
> +
> +static void
> +test_profile_probability_apply ()
> +{
> + const gcov_type min = INTTYPE_MINIMUM (gcov_type);
> + const gcov_type max = INTTYPE_MAXIMUM (gcov_type);
> + profile_probability quarter = profile_probability::guessed_always () / 4;
> + profile_probability even = profile_probability::even ();
> +
> + ASSERT_EQ (1, quarter.apply (3));
> + ASSERT_EQ (-1, quarter.apply (-3));
> + ASSERT_EQ (2, even.apply (3));
> + ASSERT_EQ (-2, even.apply (-3));
> + ASSERT_EQ (0, even.apply (0));
> +
> + ASSERT_EQ (0, profile_probability::never ().apply (min));
> + ASSERT_EQ (min, profile_probability::always ().apply (min));
> + ASSERT_EQ (max, profile_probability::always ().apply (max));
> + ASSERT_EQ (min, profile_probability::guessed_always ().apply (min));
> + ASSERT_EQ (min / 2, even.apply (min));
> + ASSERT_EQ (max / 2 + 1, even.apply (max));
> +
> + profile_probability uninitialized = profile_probability::uninitialized ();
> + ASSERT_EQ (1, uninitialized.apply (3));
> + ASSERT_EQ (-1, uninitialized.apply (-3));
> + ASSERT_EQ (min / 2, uninitialized.apply (min));
> + ASSERT_EQ (max / 2, uninitialized.apply (max));
> +}
> +
> +/* Run all of the selftests within this file. */
> +
> +void
> +profile_count_cc_tests ()
> +{
> + test_profile_probability_apply ();
> +}
> +
> +} // namespace selftest
> +
> +#endif
> diff --git a/gcc/profile-count.h b/gcc/profile-count.h
> index b424ecba3ea..76325d0fdfa 100644
> --- a/gcc/profile-count.h
> +++ b/gcc/profile-count.h
> @@ -513,11 +513,35 @@ public:
> return ret;
> }
>
> + /* Return VAL scaled by this probability. Round initialized probabilities
> + to the nearest integer, with halfway values away from zero. Treat an
> + uninitialized probability as one half and truncate toward zero. */
> gcov_type apply (gcov_type val) const
> {
> if (*this == uninitialized ())
> return val / 2;
> - return RDIV (val * m_val, max_probability);
> +
> + /* A unit probability leaves VAL unchanged. Return it directly because
> + the magnitude of the minimum gcov_type value is one greater than the
> + maximum gcov_type value. */
> + if (m_val == max_probability)
> + return val;
> +
> + /* Convert to unsigned before negating so that the minimum gcov_type
> + value has a representable magnitude. Scale the magnitude with
> + overflow-safe arithmetic, then restore the sign. */
> + gcov_type_unsigned magnitude
> + = val < 0 ? -(gcov_type_unsigned) val : (gcov_type_unsigned) val;
> + uint64_t scaled;
> + bool scaled_p
> + = safe_scale_64bit (magnitude, m_val, max_probability, &scaled);
> + /* The scaled result fits in uint64_t. With the unit case handled
> above,
> + it also fits in the nonnegative range of gcov_type. */
> + gcc_checking_assert (scaled_p);
> + gcc_checking_assert
> + (scaled <= (gcov_type_unsigned) INTTYPE_MAXIMUM (gcov_type));
> +
> + return val < 0 ? -(gcov_type) scaled : (gcov_type) scaled;
> }
>
> /* Return 1-*THIS. */
> diff --git a/gcc/selftest-run-tests.cc b/gcc/selftest-run-tests.cc
> index e39a94f8688..a281a239b10 100644
> --- a/gcc/selftest-run-tests.cc
> +++ b/gcc/selftest-run-tests.cc
> @@ -72,6 +72,7 @@ selftest::run_tests ()
> wide_int_cc_tests ();
> ggc_tests_cc_tests ();
> sreal_cc_tests ();
> + profile_count_cc_tests ();
> fibonacci_heap_cc_tests ();
> typed_splay_tree_cc_tests ();
> opt_suggestions_cc_tests ();
> diff --git a/gcc/selftest.h b/gcc/selftest.h
> index 8891d0b7b6f..f9dacd69af7 100644
> --- a/gcc/selftest.h
> +++ b/gcc/selftest.h
> @@ -248,6 +248,7 @@ extern void path_coverage_cc_tests ();
> extern void predict_cc_tests ();
> extern void pretty_print_cc_tests ();
> extern void pretty_print_token_buffer_cc_tests ();
> +extern void profile_count_cc_tests ();
> extern void pub_sub_cc_tests ();
> extern void range_op_tests ();
> extern void range_tests ();
> --
> 2.50.1 (Apple Git-155)
>
