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Avoiding __builtin_strlen #4429

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Merged
vitaut merged 5 commits into from
Apr 27, 2025
Merged

Avoiding __builtin_strlen #4429

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5cfca8e
Avoiding __builtin_strlen
brevzin Apr 26, 2025
8b0d857
Only do this for C++17
brevzin Apr 26, 2025
4988b3a
Making test narrower
brevzin Apr 26, 2025
ce781e9
Wrong #if
brevzin Apr 26, 2025
f490963
Moving test
brevzin Apr 27, 2025
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2 changes: 1 addition & 1 deletion include/fmt/base.h
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Original file line number Diff line number Diff line change
Expand Up @@ -539,7 +539,7 @@ template <typename Char> class basic_string_view {
#endif
FMT_CONSTEXPR20 basic_string_view(const Char* s) : data_(s) {
#if FMT_HAS_BUILTIN(__builtin_strlen) || FMT_GCC_VERSION || FMT_CLANG_VERSION
if (std::is_same<Char, char>::value) {
if (std::is_same<Char, char>::value && !detail::is_constant_evaluated()) {
size_ = __builtin_strlen(detail::narrow(s));
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@mwinterb mwinterb Apr 26, 2025

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What's the advantage of still using __builtin_strlen? Will gcc or clang still end up producing a constant in some scenarios that they wouldn't if just strlen was used?

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@mwinterb mwinterb Apr 26, 2025

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Well... libstdc++ __builtin_strlen in char_traits, so I guess there's that.

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@vitaut vitaut Apr 27, 2025

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There was some benefit to the debug codegen at some point but probably no longer matters on newer compilers.

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@vitaut vitaut Apr 27, 2025

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More importantly, strlen is not constexpr.

return;
}
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23 changes: 23 additions & 0 deletions test/format-test.cc
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Expand Up @@ -2563,3 +2563,26 @@ auto fmt::formatter<incomplete_type>::format(const incomplete_type&,
-> fmt::appender {
return formatter<int>::format(42, ctx);
}

#if FMT_USE_CONSTEVAL
namespace {

template <size_t N>
struct fixed_string {
char data[N] = {};

constexpr fixed_string(char const (&m)[N]) {
for (size_t i = 0; i != N; ++i) {
data[i] = m[i];
}
}
};

}

TEST(base_test, from_constexpr_fixed_string) {
static constexpr auto fs = fixed_string<5>("x={}");
static constexpr auto fmt = fmt::string_view(fs.data);
EXPECT_EQ(fmt, "x={}");
}
#endif
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