https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86590
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> --- I see differences in EH as well (comparing .original dumps of gnu++14 vs. gnu++17): @@ -126,7 +126,7 @@ ;; enabled by -tree-original -<<< Unknown tree: eh_spec_block +<<< Unknown tree: must_not_throw_expr <<cleanup_point return <retval> = __builtin_constant_p (__c) == 0 || (unsigned int) __c > 127 ? __btowc_alias (__c) : (wint_t) __c>> >>>; __builtin_unreachable (); and -;; Function static void std::char_traits<char>::assign(std::char_traits<char>::char_type&, const char_type&) (null) +;; Function static constexpr void std::char_traits<char>::assign(std::char_traits<char>::char_type&, const char_type&) (null) But the biggest difference seems to be the introduction of ;; Function constexpr bool std::__constant_string_p(const _CharT*) [with _CharT = char] (null) ;; enabled by -tree-original while (1) { if (__builtin_constant_p ((char) *__s) == 0 || *__s == 0) goto <D.10182>; <<cleanup_point <<< Unknown tree: expr_stmt (void) __s++ >>>>>; } <D.10182>:; return <retval> = __builtin_constant_p ((char) *__s) != 0; which inlined survices as <bb 2> [local count: 1073741826]: MEM[(struct &)&a] ={v} {CLOBBER}; _59 = &a.D.22134._M_local_buf; MEM[(struct &)&a] ={v} {CLOBBER}; MEM[(struct _Alloc_hider *)&a]._M_p = _59; <bb 3> [local count: 10037315480]: # __s_63 = PHI <"Hello world"(2), __s_66(5)> _64 = *__s_63; _65 = __builtin_constant_p (_64); if (_65 == 0) goto <bb 6>; [5.50%] else goto <bb 4>; [94.50%] which we are not able to simplify and which confuses us in optimizing. Only the first DOM pass forcefully drops that to false. This seems to be a quite stupid loop structure btw resulting from std::char_traits<char>::length (const char_type * __s) { bool retval.1; size_t D.33082; retval.1 = std::__constant_string_p<char> (__s); if (retval.1 != 0) goto <D.33080>; else goto <D.33081>; <D.33080>: D.33082 = __gnu_cxx::char_traits<char>::length (__s); return D.33082; <D.33081>: D.33082 = __builtin_strlen (__s); return D.33082; } and std::__constant_string_p<char> (const char * __s) { bool D.33088; <bb 2> : _1 = *__s; _2 = __builtin_constant_p (_1); if (_2 == 0) goto <bb 5>; [INV] else goto <bb 3>; [INV] <bb 3> : _3 = *__s; if (_3 == 0) goto <bb 5>; [INV] else goto <bb 4>; [INV] <bb 4> : __s = __s + 1; goto <bb 2>; [INV] <bb 5> : _4 = *__s; _5 = __builtin_constant_p (_4); D.33088 = _5 != 0; return D.33088; } /** * @brief Determine whether the characters of a NULL-terminated * string are known at compile time. * @param __s The string. * * Assumes that _CharT is a built-in character type. */ template<typename _CharT> static _GLIBCXX_ALWAYS_INLINE constexpr bool __constant_string_p(const _CharT* __s) { while (__builtin_constant_p(*__s) && *__s) __s++; return __builtin_constant_p(*__s); } WTF?