Re: [PATCH/RFA] libstdc++: provide conversion from day, month to unsigned long, PR99301

[Jonathan Wakely <jwakely@redhat.com>]

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On 27/02/21 12:42 +0100, Hans-Peter Nilsson via Libstdc++ wrote:
>Since 97d6161f6a7fa712 / r11-7370 "libstdc++: More efficient
>days from date" I see an additional 81 testsuite-errors for
>cris-elf, with this in g++.log for one randomly picked
>regressing test:
>
>FAIL: g++.dg/cpp1y/pr57640.C  -std=c++2a (test for excess errors)
>Excess errors:
>/x/gccobj/cris-elf/libstdc++-v3/include/chrono:2483:25: error: invalid 'static_cast' from type 'const std::chrono::month' to type 'uint32_t' {aka 'long unsigned int'}
>/x/gccobj/cris-elf/libstdc++-v3/include/chrono:2484:25: error: invalid 'static_cast' from type 'const std::chrono::day' to type 'uint32_t' {aka 'long unsigned int'}
>/x/gccobj/cris-elf/libstdc++-v3/include/chrono:2496:69: error: no matching function for call to 'std::chrono::duration<long long int, std::ratio<86400> >::duration(<brace-enclosed initializer list>)'
>
>The commit shows conversions to uint32_t, which for
>e.g. x86_64-linux is "unsigned int", and there are explicit
>conversions to unsigned int for month and day (see patch
>context).
>
>But, "newlib ILP32 targets" have an uint32_t that is
>effectively typedef'd "long unsigned int" (see
>newlib-stdint.h UINT32_TYPE).
>
>Better provide an unsigned long conversion aside the
>unsigned ones.

No, the allowed conversions are specific by the standard. The right
fix is to convert to unsigned explicitly.

I've pushed the attached patch after testing on x86_64-linux. This
should work for newlib ILP32 targets too.


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commit 699672d4dccfb5579dbe48977bda86f6836225a0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Sat Feb 27 12:50:53 2021

    libstdc++: Fix conversions from date types to integers [PR 99301]
    
    The conversions to integer types are explicit, so need to use the
    correct type. Converting to uint32_t only works if that is the same type
    as unsigned.
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/99301
            * include/std/chrono (year_month_day::_M_days_since_epoch()):
            Convert chrono::month and chrono::day to unsigned before
            converting to uint32_t.

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index fcdaee7328e..11729aae708 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -2496,8 +2496,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       auto constexpr __r2_e3 = static_cast<uint32_t>(536895458);
 
       const auto __y1 = static_cast<uint32_t>(static_cast<int>(_M_y)) - __z2;
-      const auto __m1 = static_cast<uint32_t>(_M_m);
-      const auto __d1 = static_cast<uint32_t>(_M_d);
+      const auto __m1 = static_cast<uint32_t>(static_cast<unsigned>(_M_m));
+      const auto __d1 = static_cast<uint32_t>(static_cast<unsigned>(_M_d));
 
       const auto __j  = static_cast<uint32_t>(__m1 < 3);
       const auto __y0 = __y1 - __j;