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[libc++] Add constant folding for optimized std::find variants #96491

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[libc++] Add constant folding for optimized std::find variants #96491

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13 changes: 13 additions & 0 deletions libcxx/include/__string/constexpr_c_functions.h
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Original file line number Diff line number Diff line change
Expand Up @@ -132,6 +132,19 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __constexpr_memchr(_Tp*
static_assert(sizeof(_Tp) == 1 && __libcpp_is_trivially_equality_comparable<_Tp, _Up>::value,
"Calling memchr on non-trivially equality comparable types is unsafe.");

if (__builtin_constant_p(__count) && __builtin_constant_p(__value)) {
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@ldionne ldionne Jul 4, 2024

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Let's add some kind of comment explaining that this is to aid constant-folding and it also handles evaluation inside constant expressions, since that is not super obvious.

for (; __count; --__count) {
if (!__builtin_constant_p(*__str))
break;
if (*__str == __value)
return __str;
++__str;
}
}

if (__builtin_constant_p(__count) && __count == 0)
return nullptr;

if (__libcpp_is_constant_evaluated()) {
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@ldionne ldionne Jul 4, 2024

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I think we can get rid of this branch since __builtin_constant_p is always true inside a constant expression. IOW, the code path above is always taken when __libcpp_is_constant_evaluated() is true, so the branch below is basically dead now.

// use __builtin_char_memchr to optimize constexpr evaluation if we can
#if _LIBCPP_STD_VER >= 17 && __has_builtin(__builtin_char_memchr)
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@ldionne ldionne Jul 4, 2024

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Can we file a bug report against Clang to complain that __builtin_memchr doesn't constant-fold? It seems like an obvious QOI issue since Clang can already constant-evaluate that function.

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13 changes: 13 additions & 0 deletions libcxx/include/cwchar
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Expand Up @@ -231,6 +231,19 @@ _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR_SINCE_CXX14 _Tp* __constexpr_wmemchr(_Tp
__libcpp_is_trivially_equality_comparable<_Tp, _Tp>::value,
"Calling wmemchr on non-trivially equality comparable types is unsafe.");

if (__builtin_constant_p(__count) && __builtin_constant_p(__value)) {
for (; __count; --__count) {
if (!__builtin_constant_p(*__str))
break;
if (*__str == __value)
return __str;
++__str;
}
}

if (__builtin_constant_p(__count) && __count == 0)
return nullptr;

#if __has_builtin(__builtin_wmemchr)
if (!__libcpp_is_constant_evaluated()) {
wchar_t __value_buffer = 0;
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