[LLVMbugs] [Bug 12762] New: missed bswap.

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12762

             Bug #: 12762
           Summary: missed bswap.
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: pluto at agmk.net
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


here's a tricky example of loop which should emit bswap
for some types.

#include <algorithm>
#include <iterator>

template < typename T >
void __attribute__((noinline)) reverse( T& t )
{
        unsigned char* bytes = reinterpret_cast< unsigned char* >( &t );
        std::swap_ranges( bytes, bytes + sizeof( T ) / 2,
                std::reverse_iterator< unsigned char* >( bytes + sizeof( T ) )
);
}

template void reverse( std::size_t& );

int main()
{
        std::size_t x = 0x0011223344556677ull;
        reverse( x );
        return ( x == 0x7766554433221100ull ? 0 : 1 );
}

currently it's transformed to:

_Z7reverseImEvRT_:
        movb    (%rdi), %al
        movb    7(%rdi), %cl
        movb    %cl, (%rdi)
        movb    %al, 7(%rdi)
        movb    1(%rdi), %al
        movb    6(%rdi), %cl
        movb    %cl, 1(%rdi)
        movb    %al, 6(%rdi)
        movb    2(%rdi), %al
        movb    5(%rdi), %cl
        movb    %cl, 2(%rdi)
        movb    %al, 5(%rdi)
        movb    3(%rdi), %al
        movb    4(%rdi), %cl
        movb    %cl, 3(%rdi)
        movb    %al, 4(%rdi)
        ret

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