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From: "wouter.vermaelen at scarlet dot be" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/50385] New: missed-optimization: jump to __builtin_unreachable() not removed
Date: Tue, 13 Sep 2011 15:21:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50385

             Bug #: 50385
           Summary: missed-optimization: jump to __builtin_unreachable()
                    not removed
    Classification: Unclassified
           Product: gcc
           Version: 4.7.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: wouter.vermaelen@scarlet.be


I'm not sure, but this issue might be the same as bug 49054 (if so, feel free
to delete this one).


#include <vector>

struct S { int a, b; };
std::vector<S> v;

int search_1(int a) {
    for (auto it = v.begin(); /**/; ++it)
        if (it->a == a) return it->b;
}
int search_2(int a) {
    for (auto& e : v)
        if (e.a == a) return e.b;
    __builtin_unreachable();
}


I expected to see the same generated code for both functions. Instead the 2nd
one still contains some useless comparisons and jumps past the end of the
function. Since such a (conditional) jump is anyway undefined behavior it can
as well be removed (including the instructions required to calculate the
condition).

Tested with SVN revision 178775 (20110912) on Linux x86_64.