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Workaround intel bug #3652

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merged 5 commits into from
Sep 21, 2023
Merged

Workaround intel bug #3652

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bbaac96
Workaround intel bug
gsjaardema Sep 20, 2023
1de436e
Finish the proposed change -- remove struct accessor
gsjaardema Sep 20, 2023
4c346e9
Refactor proposed intel fix.
gsjaardema Sep 20, 2023
7572bf1
Fix incorrect function name.
gsjaardema Sep 20, 2023
8e60c51
Add missing inline...
gsjaardema Sep 20, 2023
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41 changes: 17 additions & 24 deletions include/fmt/format.h
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Original file line number Diff line number Diff line change
Expand Up @@ -1740,28 +1740,6 @@ FMT_CONSTEXPR inline fp operator*(fp x, fp y) {
return {multiply(x.f, y.f), x.e + y.e + 64};
}

template <typename T = void> struct basic_data {
// For checking rounding thresholds.
// The kth entry is chosen to be the smallest integer such that the
// upper 32-bits of 10^(k+1) times it is strictly bigger than 5 * 10^k.
static constexpr uint32_t fractional_part_rounding_thresholds[8] = {
2576980378U, // ceil(2^31 + 2^32/10^1)
2190433321U, // ceil(2^31 + 2^32/10^2)
2151778616U, // ceil(2^31 + 2^32/10^3)
2147913145U, // ceil(2^31 + 2^32/10^4)
2147526598U, // ceil(2^31 + 2^32/10^5)
2147487943U, // ceil(2^31 + 2^32/10^6)
2147484078U, // ceil(2^31 + 2^32/10^7)
2147483691U // ceil(2^31 + 2^32/10^8)
};
};
// This is a struct rather than an alias to avoid shadowing warnings in gcc.
struct data : basic_data<> {};

#if FMT_CPLUSPLUS < 201703L
template <typename T>
constexpr uint32_t basic_data<T>::fractional_part_rounding_thresholds[];
#endif

template <typename T, bool doublish = num_bits<T>() == num_bits<double>()>
using convert_float_result =
Expand Down Expand Up @@ -3283,6 +3261,21 @@ FMT_CONSTEXPR20 void format_hexfloat(Float value, int precision,
template <typename Float>
FMT_CONSTEXPR20 auto format_float(Float value, int precision, float_specs specs,
buffer<char>& buf) -> int {

// For checking rounding thresholds.
// The kth entry is chosen to be the smallest integer such that the
// upper 32-bits of 10^(k+1) times it is strictly bigger than 5 * 10^k.
static constexpr uint32_t fractional_part_rounding_thresholds[8] = {
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You need to drop static here:

/Users/runner/work/fmt/fmt/include/fmt/format.h:3268:29: error: definition of a static variable in a constexpr function is a C++2b extension [-Werror,-Wc++2b-extensions]
  static constexpr uint32_t fractional_part_rounding_thresholds[8] = {
                            ^

Also please move to a separate function to avoid parametrization on Float, something like

FMT_CONSTEXPR uint32_t fractional_part_rounding_threshold(int index) {
  constexpr uint32_t thresholds[] = {...};
  return thresholds[index];
}

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@vitaut
Will a non-static constexpr array be optimized by the compiler?

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¯_(ツ)_/¯

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https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2022/p2647r1.html#workarounds
So, yes.
¯_(ツ)_/¯

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Done.

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@vitaut

It doesn't get optimized: https://godbolt.org/z/drn31x9zK

Maybe revert this PR?

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I don't think we should revert this PR but ideas/PRs to optimize it would be welcome.

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@vitaut
Solution for GCC, Clang: https://godbolt.org/z/ezj9GhnYW
But not for MSVC (current code: https://godbolt.org/z/5Pxn3zMfT)

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I have another idea: 06b2038.

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@vitaut Solution for GCC, Clang: https://godbolt.org/z/ezj9GhnYW But not for MSVC (current code: https://godbolt.org/z/5Pxn3zMfT)

That looks good. I had something similar but didn't think of the unreachable to quiet the warnings and keep the optimization.

2576980378U, // ceil(2^31 + 2^32/10^1)
2190433321U, // ceil(2^31 + 2^32/10^2)
2151778616U, // ceil(2^31 + 2^32/10^3)
2147913145U, // ceil(2^31 + 2^32/10^4)
2147526598U, // ceil(2^31 + 2^32/10^5)
2147487943U, // ceil(2^31 + 2^32/10^6)
2147484078U, // ceil(2^31 + 2^32/10^7)
2147483691U // ceil(2^31 + 2^32/10^8)
};

// float is passed as double to reduce the number of instantiations.
static_assert(!std::is_same<Float, float>::value, "");
FMT_ASSERT(value >= 0, "value is negative");
Expand Down Expand Up @@ -3485,7 +3478,7 @@ FMT_CONSTEXPR20 auto format_float(Float value, int precision, float_specs specs,
if (precision < 9) {
uint32_t fractional_part = static_cast<uint32_t>(prod);
should_round_up = fractional_part >=
data::fractional_part_rounding_thresholds
fractional_part_rounding_thresholds
[8 - number_of_digits_to_print] ||
((fractional_part >> 31) &
((digits & 1) | (second_third_subsegments != 0) |
Expand Down Expand Up @@ -3525,7 +3518,7 @@ FMT_CONSTEXPR20 auto format_float(Float value, int precision, float_specs specs,
// consisting of a genuine digit from the input.
uint32_t fractional_part = static_cast<uint32_t>(prod);
should_round_up = fractional_part >=
data::fractional_part_rounding_thresholds
fractional_part_rounding_thresholds
[8 - number_of_digits_to_print] ||
((fractional_part >> 31) &
((digits & 1) | (third_subsegment != 0) |
Expand Down
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