[llvm-bugs] [Bug 35637] New: Really bad codegen for libc++ vector

via llvm-bugs llvm-bugs at lists.llvm.org
Mon Dec 11 21:00:37 PST 2017


            Bug ID: 35637
           Summary: Really bad codegen for libc++ vector
           Product: clang
           Version: trunk
          Hardware: Macintosh
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mclow.lists at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

Given the following code:

        #include <vector>
        #include <memory>

        auto f1()
                return std::vector<unsigned char>( 1920*1080*4 );

        auto f2()
                return new std::vector<unsigned char>( 1920*1080*4 );

        int main() {}

clang generates fine code for "f1", noticing that it has to zero 8+MB of data,
and calling memset to do so.

For "f2", it generates a loop. Clearing a byte? at a time, and recalculating
the pointer each time.  Reproduced on godbolt with "-std=c++1z -Wall -W -O3
-stdlib=libc++" with both clang 5.0 and clang trunk.

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