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[libc++][bit] Improves rotate functions. (#98032)

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Investigating #96612 shows our implementation was different from the
Standard and could cause UB. Testing the codegen showed quite a bit of
assembly generated for these functions. The functions have been written
differently which allows Clang to optimize the code to use simple CPU
rotate instructions.

Fixes: https://github.com/llvm/llvm-project/issues/96612
This commit is contained in:
Mark de Wever 2024-08-03 11:19:00 +02:00 committed by Tobias Hieta
parent e2f25af711
commit 01a49d21c7
3 changed files with 33 additions and 12 deletions
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37
libcxx/include/__bit/rotate.h
View File

@ -20,24 +20,37 @@
_LIBCPP_BEGIN_NAMESPACE_STD
// Writing two full functions for rotl and rotr makes it easier for the compiler
// to optimize the code. On x86 this function becomes the ROL instruction and
// the rotr function becomes the ROR instruction.
template <class _Tp>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __rotr(_Tp __t, int __cnt) _NOEXCEPT {
static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__rotr requires an unsigned integer type");
const unsigned int __dig = numeric_limits<_Tp>::digits;
if ((__cnt % __dig) == 0)
return __t;
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __rotl(_Tp __x, int __s) _NOEXCEPT {
static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__rotl requires an unsigned integer type");
const int __N = numeric_limits<_Tp>::digits;
int __r = __s % __N;
if (__cnt < 0) {
__cnt *= -1;
return (__t << (__cnt % __dig)) | (__t >> (__dig - (__cnt % __dig))); // rotr with negative __cnt is similar to rotl
}
if (__r == 0)
return __x;
return (__t >> (__cnt % __dig)) | (__t << (__dig - (__cnt % __dig)));
if (__r > 0)
return (__x << __r) | (__x >> (__N - __r));
return (__x >> -__r) | (__x << (__N + __r));
}
template <class _Tp>
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __rotl(_Tp __t, int __cnt) _NOEXCEPT {
return std::__rotr(__t, -__cnt);
_LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp __rotr(_Tp __x, int __s) _NOEXCEPT {
static_assert(__libcpp_is_unsigned_integer<_Tp>::value, "__rotr requires an unsigned integer type");
const int __N = numeric_limits<_Tp>::digits;
int __r = __s % __N;
if (__r == 0)
return __x;
if (__r > 0)
return (__x >> __r) | (__x << (__N - __r));
return (__x << -__r) | (__x >> (__N + __r));
}
#if _LIBCPP_STD_VER >= 20

4
libcxx/test/std/numerics/bit/bitops.rot/rotl.pass.cpp
View File

@ -41,6 +41,8 @@ constexpr bool test()
assert(std::rotl(T(max - 1), 5) == T(max - 32));
assert(std::rotl(T(max - 1), 6) == T(max - 64));
assert(std::rotl(T(max - 1), 7) == T(max - 128));
assert(std::rotl(T(max - 1), std::numeric_limits<int>::max()) ==
std::rotl(T(max - 1), std::numeric_limits<int>::max() % std::numeric_limits<T>::digits));
assert(std::rotl(T(max - 1), -1) == T(max - highbit));
assert(std::rotl(T(max - 1), -2) == T(max - (highbit >> 1)));
@ -49,6 +51,8 @@ constexpr bool test()
assert(std::rotl(T(max - 1), -5) == T(max - (highbit >> 4)));
assert(std::rotl(T(max - 1), -6) == T(max - (highbit >> 5)));
assert(std::rotl(T(max - 1), -7) == T(max - (highbit >> 6)));
assert(std::rotl(T(max - 1), std::numeric_limits<int>::min()) ==
std::rotl(T(max - 1), std::numeric_limits<int>::min() % std::numeric_limits<T>::digits));
assert(std::rotl(T(1), 0) == T(1));
assert(std::rotl(T(1), 1) == T(2));

4
libcxx/test/std/numerics/bit/bitops.rot/rotr.pass.cpp
View File

@ -41,6 +41,8 @@ constexpr bool test()
assert(std::rotr(T(max - 1), 5) == T(max - (highbit >> 4)));
assert(std::rotr(T(max - 1), 6) == T(max - (highbit >> 5)));
assert(std::rotr(T(max - 1), 7) == T(max - (highbit >> 6)));
assert(std::rotr(T(max - 1), std::numeric_limits<int>::max()) ==
std::rotr(T(max - 1), std::numeric_limits<int>::max() % std::numeric_limits<T>::digits));
assert(std::rotr(T(max - 1), -1) == T(max - 2));
assert(std::rotr(T(max - 1), -2) == T(max - 4));
@ -49,6 +51,8 @@ constexpr bool test()
assert(std::rotr(T(max - 1), -5) == T(max - 32));
assert(std::rotr(T(max - 1), -6) == T(max - 64));
assert(std::rotr(T(max - 1), -7) == T(max - 128));
assert(std::rotr(T(max - 1), std::numeric_limits<int>::min()) ==
std::rotr(T(max - 1), std::numeric_limits<int>::min() % std::numeric_limits<T>::digits));
assert(std::rotr(T(128), 0) == T(128));
assert(std::rotr(T(128), 1) == T(64));