Improving performance of 2xi64 icmp slt/sgt/ult/ugt on SSE2 using 2xi64 sub

Here are links to LLVM IR snippets demonstrating the two alternatives for 2xi64 icmp sgt on SSE2:

• https://alive2.llvm.org/ce/z/gtqKGW
• https://godbolt.org/z/1fa5zczna

Here is a more efficient implementation (at least according to llvm-mca) of 2xi64 icmp slt on SSE2:

SSE2_2xI64_CompareGt_2:                 # @SSE2_2xI64_CompareGt_2
        movdqa  xmm2, xmm0
        pcmpgtd xmm2, xmm1
        # xmm2 == {(int32_t)a[0] > (int32_t)b[0],
        #          (int32_t)(a[0] >> 32) > (int32_t)(b[0] >> 32),
        #          (int32_t)a[1] > (int32_t)b[1],
        #          (int32_t)(a[1] >> 32) > (int32_t)(b[1] >> 32)}
        #          (as 4xi32 vector)

        movdqa  xmm3, xmm0
        pcmpeqd xmm3, xmm1
        # xmm3 == {(int32_t)a[0] == (int32_t)b[0],
        #          (int32_t)(a[0] >> 32) == (int32_t)(b[0] >> 32),
        #          (int32_t)a[1] == (int32_t)b[1],
        #          (int32_t)(a[1] >> 32) == (int32_t)(b[1] >> 32)}
        #          (as 4xi32 vector)

        psubq   xmm1, xmm0
        # xmm2 == {b[0] - a[0], b[1] - a[1]} (as 2xi64 vector)

        # If (a[0] >> 32) == (b[0] >> 32) && a[0] <= b[0] is true, then
        # (xmm1[0] >> 32) == 0 will be true (if a, b, and xmm1 are treated as
        # 2xi64 vectors).

        # If (a[0] >> 32) == (b[0] >> 32) && a[0] > b[0] is true, then
        # (xmm1[0] >> 32) == -1 will be true (if a, b, and xmm1 are treated as
        # 2xi64 vectors).

        pand    xmm1, xmm3
        # If xmm1, a, and b are treated as 2xi64 vectors:
        # (xmm1[0] >> 32) == (((a[0] >> 32) == (b[0] >> 32) && a[0] > b[0]) ?
        #                    -1 : 0)
        # (xmm1[1] >> 32) == (((a[1] >> 32) == (b[1] >> 32) && a[1] > b[1]) ?
        #                    -1 : 0)
        # (xmm2[0] >> 32) == (((a[0] >> 32) > (b[0] >> 32)) ? -1 : 0)
        # (xmm2[1] >> 32) == (((a[1] >> 32) > (b[1] >> 32)) ? -1 : 0)
        por     xmm1, xmm2

        # If xmm1, a, and b are treated as 2xi64 vectors:
        # (xmm1[0] >> 32) == ((a[0] > b[0]) ? -1 : 0)
        # (xmm1[1] >> 32) == ((a[1] > b[1]) ? -1 : 0)

        pshufd  xmm0, xmm1, 245                 # xmm0 = xmm1[1,1,3,3]

        # xmm0 == { a[0] > b[0] ? -1 : 0, a[1] > b[1] ? -1 : 0 }
        # (as 2xi64 vector)
        ret

If (a >> 32) == (b >> 32) is true, then -2**32 + 1 <= b - a <= 2**32 - 1 will be true, making (b - a) >> 32 equal to either -1 or 0 if (a >> 32) is equal to (b >> 32) (with right shifts being arithmetic right shifts and a and b being 64-bit signed integers).