Reverse Engineering challenge #16.

Now this is getting harder. Clang did a lot of optimization tricks and this code is heavily optimized for SSE2. Nevertheless, the original function is tiny and simple. What does it do?

Optimizing clang 3.4, LLVM 3.4, Intel syntax:

f:                                      # @f
# BB#0:
        xor     eax, eax
        test    rsi, rsi
        je      .LBB0_8
# BB#1:                                 #
        xor     ecx, ecx
        mov     rax, rsi
        and     rax, -4
        pxor    xmm0, xmm0
        pxor    xmm2, xmm2
        je      .LBB0_5
# BB#2:                                 # %vector.body.preheader
        pxor    xmm0, xmm0
        xor     ecx, ecx
        movdqa  xmm1, xmmword ptr [rip + .LCPI0_0]
        pxor    xmm2, xmm2
.LBB0_3:                                # %vector.body
                                        # =>This Inner Loop Header: Depth=1
        movdqa  xmm3, xmm2
        movdqa  xmm4, xmm0
        movzx   edx, word ptr [rdi + rcx]
        movd    xmm0, edx
        pinsrw  xmm0, edx, 0
        movzx   edx, dh
        pinsrw  xmm0, edx, 4
        movzx   edx, word ptr [rdi + rcx + 2]
        movd    xmm2, edx
        pinsrw  xmm2, edx, 0
        movzx   edx, dh
        pinsrw  xmm2, edx, 4
        pand    xmm0, xmm1
        pand    xmm2, xmm1
        paddq   xmm0, xmm4
        paddq   xmm2, xmm3
        add     rcx, 4
        cmp     rax, rcx
        jne     .LBB0_3
# BB#4:
        mov     rcx, rax
.LBB0_5:                                # %middle.block
        paddq   xmm0, xmm2
        movdqa  xmm1, xmm0
        punpckhqdq      xmm1, xmm1      # xmm1 = xmm1[1,1]
        paddq   xmm1, xmm0
        movq    rax, xmm1
        cmp     rcx, rsi
        je      .LBB0_8
# BB#6:                                 #
        add     rdi, rcx
        sub     rsi, rcx
.LBB0_7:                                #
                                        # =>This Inner Loop Header: Depth=1
        movzx   ecx, byte ptr [rdi]
        add     rax, rcx
        inc     rdi
        dec     rsi
        jne     .LBB0_7
.LBB0_8:                                # %._crit_edge

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