fix: popcount compiling windows clang-cl#114
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jamierpond wants to merge 1 commit into
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fix: popcount compiling windows clang-cl#114jamierpond wants to merge 1 commit into
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…ilers
PopcountLogic<LogarithmOfGroupSize, T>::CombiningMask computed
`T(1 << HalvedGroupSize) - 1`, which performs the shift at `int`
precision because `1` is an `int`. For `LogarithmOfGroupSize = 6`,
`HalvedGroupSize` is 32 — so the expression is `1 << 32`, which is UB
per [expr.shift] (shift count not less than width of the promoted type).
GCC happens to accept this in a constant-expression context; Clang and
MSVC /std:c++20 /permissive- correctly reject it ("non-type template
argument is not a constant expression", "shift count 32 >= width of
type 'int' (32 bits)"). The result is that any translation unit that
includes <zoo/swar/SWAR.h> under MSVC with conformance mode fails to
compile, because SWAR.h's unconditional static_asserts on line 589-596
force instantiation of PopcountLogic<6, uint64_t> via logarithmFloor.
Fix: do the shift at `T`'s precision — `T(T(1) << HalvedGroupSize) - 1`.
Add a regression test that directly instantiates PopcountLogic<6, u64>.
popcount<LogNBits>() would have caught this, but on non-MSVC platforms
it routes through PopcountIntrinsic (which uses __builtin_popcountll)
and never touches PopcountLogic<6>, which is why the existing popcount
tests at <1>, <2>, <4>, <5> all passed.
thecppzoo
requested changes
Jun 4, 2026
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| static_assert(meta::PopcountLogic<6, u64>::CombiningMask == 0x0000'0000'FFFF'FFFFull); | ||
| static_assert(meta::PopcountLogic<6, u64>::HalvedGroupSize == 32); | ||
| static_assert(meta::PopcountLogic<6, u64>::GroupSize == 64); | ||
| static_assert(0 == meta::PopcountLogic<6, u64>::execute(0ull)); | ||
| static_assert(1 == meta::PopcountLogic<6, u64>::execute(1ull)); | ||
| static_assert(1 == meta::PopcountLogic<6, u64>::execute(1ull << 32)); | ||
| static_assert(1 == meta::PopcountLogic<6, u64>::execute(1ull << 63)); | ||
| static_assert(32 == meta::PopcountLogic<6, u64>::execute(0xFFFFFFFFull)); | ||
| static_assert(32 == meta::PopcountLogic<6, u64>::execute(0xFFFFFFFF00000000ull)); | ||
| static_assert(32 == meta::PopcountLogic<6, u64>::execute(0xAAAAAAAAAAAAAAAAull)); | ||
| static_assert(32 == meta::PopcountLogic<6, u64>::execute(0x0123456789ABCDEFull)); | ||
| static_assert(64 == meta::PopcountLogic<6, u64>::execute(0xFFFFFFFFFFFFFFFFull)); |
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This is not the type of tests that are useful: they dump a haystack on the needle of wisdom
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Fixes Windows clang-cl compilation.