public inbox for gcc-bugs@sourceware.org help / color / mirror / Atom feed
From: "vincenzo.innocente at cern dot ch" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/53792] New: [C++11][constexpr] improving compiler-time constexpr evaluation Date: Thu, 28 Jun 2012 06:59:00 -0000 [thread overview] Message-ID: <bug-53792-4@http.gcc.gnu.org/bugzilla/> (raw) http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53792 Bug #: 53792 Summary: [C++11][constexpr] improving compiler-time constexpr evaluation Classification: Unclassified Product: gcc Version: 4.8.0 Status: UNCONFIRMED Severity: enhancement Priority: P3 Component: c++ AssignedTo: unassigned@gcc.gnu.org ReportedBy: vincenzo.innocente@cern.ch Filed under c++ even if it is most probably an optimization issue. At the moment it looks like that constexpr are evaluated at compile time only if explicitly assigned to a constexpr constant. There are cases though where the compiler can infer that the expression can still be evaluated at compile time even if used in a run-time context. Take the following quite realistic example of a consexpr "indexing table" used to access a non-const array using string literals though an inline function. In principle foo and bar are equivalent. At the moment gcc evaluates "getIndex" at compile time for bar (where the marco expansion explicitly instantiates a constexpr int, while it generates runtime code for foo that uses the inlined function getV. Would the compiler be able to transform getV in something like the code in the macro? constexpr entry theMap[] = { {"a", 0}, {"b", 1}, {nullptr,2} }; // filled at run time double v[3]; constexpr bool same(char const *x, char const *y) { return !*x && !*y ? true : (*x == *y && same(x+1, y+1)); } constexpr int getIndex(char const *label, entry const *entries) { return !entries->label ? entries->index : same(entries->label, label) ? entries->index : getIndex(label, entries+1); } inline double __attribute__((always_inline)) getV(const char * name ) { return v[getIndex(name,theMap)]; } #define SetV(X,NAME) \ constexpr int i_##X = getIndex(NAME, theMap);\ const double X = v[i_##X] int foo() { const double a = getV("a"); const double b = getV("b"); if (a==b) return 1; return 0; } int bar() { SetV(a,"a"); SetV(b,"b"); if (a==b) return 1; return 0; }
next reply other threads:[~2012-06-28 6:59 UTC|newest] Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top 2012-06-28 6:59 vincenzo.innocente at cern dot ch [this message] 2012-06-28 9:34 ` [Bug c++/53792] " rguenth at gcc dot gnu.org 2012-06-28 11:08 ` vincenzo.innocente at cern dot ch 2012-07-19 14:28 ` jason at gcc dot gnu.org 2012-07-27 10:48 ` vincenzo.innocente at cern dot ch 2012-07-27 15:06 ` jason at redhat dot com 2012-08-08 11:18 ` giulio.eulisse at cern dot ch 2012-08-08 18:19 ` paolo.carlini at oracle dot com 2015-05-06 15:03 ` [Bug c++/53792] [C++11] " balakrishnan.erode at gmail dot com
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=bug-53792-4@http.gcc.gnu.org/bugzilla/ \ --to=gcc-bugzilla@gcc.gnu.org \ --cc=gcc-bugs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).