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Re: [v3 PATCH] Reduce the size of variant, it doesn't need an index of type size_t internally.

From: Jonathan Wakely <jwakely at redhat dot com>
To: Ville Voutilainen <ville dot voutilainen at gmail dot com>
Cc: "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>, libstdc++ <libstdc++ at gcc dot gnu dot org>
Date: Wed, 11 Jan 2017 10:45:50 +0000
Subject: Re: [v3 PATCH] Reduce the size of variant, it doesn't need an index of type size_t internally.
Authentication-results: sourceware.org; auth=none
References: <CAFk2RUZvtch1=XANuPeomv+vfL_5tt8ZXWA6jr_tfXwgY03EHA@mail.gmail.com> <CAFk2RUZD3r6nbWKJ-T8MUQnbFSxp6HtFXaaJDCvcg5tK+UqJrA@mail.gmail.com> <20170111102907.GQ13348@redhat.com>

On 11/01/17 10:29 +0000, Jonathan Wakely wrote:

On 11/01/17 00:19 +0200, Ville Voutilainen wrote:

@@ -1086,7 +1099,12 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     { return !this->_M_valid(); }

     constexpr size_t index() const noexcept
-      { return this->_M_index; }
+      {
+	if (this->_M_index ==
+	    typename _Base::_Storage::__index_type(variant_npos))
+	  return variant_npos;
+	return this->_M_index;


GCC doesn't seem to be smart enough to optimize the branch away here.


But that's only for 32-bit x86. It optimizes well for x86_64, so no
need to obfuscate it.

References:
- [v3 PATCH] Reduce the size of variant, it doesn't need an index of type size_t internally.
  - From: Ville Voutilainen
- Re: [v3 PATCH] Reduce the size of variant, it doesn't need an index of type size_t internally.
  - From: Ville Voutilainen
- Re: [v3 PATCH] Reduce the size of variant, it doesn't need an index of type size_t internally.
  - From: Jonathan Wakely

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