[PATCH] Use __builtin_is_constant_evaluated in std::less etc. (PR tree-optimization/88775)

[Jakub Jelinek]

Hi!

In Marc's testcase, we generate terrible code for std::string assignment,
because the __builtin_constant_p is kept in the IL for way too long and the
optimizers (jump threading?) create way too many copies of the
memcpy/memmove calls that it is then hard to bring it back in sanitity.
On the testcase in the PR, GCC 7 emits on x86_64 with -O2 99 bytes long
function, GCC 9 unpatched 259 bytes long, with this patch it emits
139 bytes long, better but still not as good as before.  I guess we'll need
to improve GIMPLE optimizers too, but having twice as small IL for these
heavily used operators where e.g. _M_disjunct uses two of them and we wind
up with twice as many branches because of that is IMHO very useful.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?

1) I'm not really sure about proper formatting in libstdc++, I thought you
   don't use space before ( in function calls, but then why is there a space
   in __builtin_constant_p?
2) not really sure about that #if __cplusplus >= 201402L either, I think we
   don't really want to use __builtin_is_constant_evaluated at least in
   C++98 code, but even in C++11, if the operator isn't constexpr, is there
   any point trying to help it do the right thing in constexpr contexts?

2019-01-09  Jakub Jelinek  <ja...@redhat.com>

        PR tree-optimization/88775
        * include/bits/stl_function.h (greater<_Tp*>::operator(),
        less<_Tp*>::operator(), greater_equal<_Tp*>::operator(),
        less_equal<_Tp*>::operator()): Use __builtin_is_constant_evaluated
        instead of __builtin_constant_p if available.  Don't bother with
        the pointer comparison in C++11 and earlier.

--- libstdc++-v3/include/bits/stl_function.h.jj 2019-01-01 12:45:51.182541077 
+0100
+++ libstdc++-v3/include/bits/stl_function.h    2019-01-09 23:15:34.824800676 
+0100
@@ -413,8 +413,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX14_CONSTEXPR bool
       operator()(_Tp* __x, _Tp* __y) const _GLIBCXX_NOTHROW
       {
+#if __cplusplus >= 201402L
+#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+       if (__builtin_is_constant_evaluated())
+#else
        if (__builtin_constant_p (__x > __y))
+#endif
          return __x > __y;
+#endif
        return (__UINTPTR_TYPE__)__x > (__UINTPTR_TYPE__)__y;
       }
     };
@@ -426,8 +432,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX14_CONSTEXPR bool
       operator()(_Tp* __x, _Tp* __y) const _GLIBCXX_NOTHROW
       {
+#if __cplusplus >= 201402L
+#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+       if (__builtin_is_constant_evaluated())
+#else
        if (__builtin_constant_p (__x < __y))
+#endif
          return __x < __y;
+#endif
        return (__UINTPTR_TYPE__)__x < (__UINTPTR_TYPE__)__y;
       }
     };
@@ -439,8 +451,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX14_CONSTEXPR bool
       operator()(_Tp* __x, _Tp* __y) const _GLIBCXX_NOTHROW
       {
+#if __cplusplus >= 201402L
+#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+       if (__builtin_is_constant_evaluated())
+#else
        if (__builtin_constant_p (__x >= __y))
+#endif
          return __x >= __y;
+#endif
        return (__UINTPTR_TYPE__)__x >= (__UINTPTR_TYPE__)__y;
       }
     };
@@ -452,8 +470,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       _GLIBCXX14_CONSTEXPR bool
       operator()(_Tp* __x, _Tp* __y) const _GLIBCXX_NOTHROW
       {
+#if __cplusplus >= 201402L
+#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+       if (__builtin_is_constant_evaluated())
+#else
        if (__builtin_constant_p (__x <= __y))
+#endif
          return __x <= __y;
+#endif
        return (__UINTPTR_TYPE__)__x <= (__UINTPTR_TYPE__)__y;
       }
     };

        Jakub