Comment 14 for bug 2001932

There are

L(loop_cross_page_2_vec):
        /* The first VEC_SIZE * 2 bytes match or are ignored. */
        vmovdqu (VEC_SIZE * 2)(%rax, %r10), %ymm2
        vmovdqu (VEC_SIZE * 3)(%rax, %r10), %ymm3
        VPCMPEQ (VEC_SIZE * 2)(%rdx, %r10), %ymm2, %ymm5
        VPMINU %ymm2, %ymm5, %ymm5
        VPCMPEQ (VEC_SIZE * 3)(%rdx, %r10), %ymm3, %ymm6
        VPCMPEQ %ymm7, %ymm5, %ymm5
        VPMINU %ymm3, %ymm6, %ymm6
        VPCMPEQ %ymm7, %ymm6, %ymm6

        vpmovmskb %ymm5, %edi
        vpmovmskb %ymm6, %esi

        salq $32, %rsi
        xorq %rsi, %rdi

        xorl %r8d, %r8d
        /* If ECX > VEC_SIZE * 2, skip ECX - (VEC_SIZE * 2) bytes. */
        subl $(VEC_SIZE * 2), %ecx
        jle 1f
        /* Skip ECX bytes. */
        shrq %cl, %rdi
        /* R8 has number of bytes skipped. */
        movl %ecx, %r8d
1:
        /* Before jumping back to the loop, set ESI to the number of
           VEC_SIZE * 4 blocks before page crossing. */
        movl $(PAGE_SIZE / (VEC_SIZE * 4) - 1), %esi

        testq %rdi, %rdi
        je L(back_to_loop)

When this branch is taken, there are (VEC_SIZE * 4) + %r10 matching bytes
starting at %rax, which may be >= the maximum offset.