> I just tested building our application with modified macros using GCC.
> With the changes below, GCC emitted slightly more efficient code (fewer
> instructions).
> It shows that using uintptr_t instead of normal pointer arithmetic confuses
> the compiler.
>
> /**
> * add a byte-value offset to a const pointer
> */
> -#define RTE_CONST_PTR_ADD(ptr, x) ((const void*)((uintptr_t)(ptr) + (x)))
> +#define RTE_CONST_PTR_ADD(ptr, x) ((const void*)((const char*)(ptr) + (x)))
>
> /**
> * subtract a byte-value offset from a const pointer
> */
> -#define RTE_CONST_PTR_SUB(ptr, x) ((const void*)((uintptr_t)(ptr) - (x)))
> +#define RTE_CONST_PTR_SUB(ptr, x) ((const void*)((const char*)(ptr) - (x)))
>
>
> Snippet of code emitted using const char*:
>
> if (likely((eth->ether_type == RTE_BE16(RTE_ETHER_TYPE_VLAN)) |
> 4dc47e: 41 0f b7 45 0c movzwl 0xc(%r13),%eax
> 4dc483: 89 c7 mov %eax,%edi
> 4dc485: 83 e7 ef and $0xffffffef,%edi
> 4dc488: 66 81 ff 81 00 cmp $0x81,%di
> 4dc48d: 74 0a je 4dc499
> <service_ingress_dedicated_management+0x1b9>
> 4dc48f: 66 3d 88 a8 cmp $0xa888,%ax
> 4dc493: 0f 85 3f 01 00 00 jne 4dc5d8
> <service_ingress_dedicated_management+0x2f8>
> if (vhdr->eth_proto == RTE_BE16(RTE_ETHER_TYPE_VLAN)) {
> 4dc499: 66 41 81 7d 10 81 00 cmpw $0x81,0x10(%r13)
> 4dc4a0: 0f 84 1a 01 00 00 je 4dc5c0
> <service_ingress_dedicated_management+0x2e0>
> eth = RTE_CONST_PTR_ADD(eth, sizeof(struct rte_vlan_hdr));
> 4dc4a6: 49 8d 7d 04 lea 0x4(%r13),%rdi
> packet_type = (union packet_type){
> 4dc4aa: b8 01 00 00 00 mov $0x1,%eax
> 4dc4af: 41 b8 12 00 00 00 mov $0x12,%r8d
> if (likely((*(const rte_be32_t *)RTE_CONST_PTR_ADD(eth, offsetof(struct
> rte_ether_hdr, ether_type)) & RTE_BE32(0xFFFFFF00)) ==
> 4dc4b5: 44 8b 4f 0c mov 0xc(%rdi),%r9d
>
> Snippet of code emitted using uintptr_t:
>
> if (likely((eth->ether_type == RTE_BE16(RTE_ETHER_TYPE_VLAN)) |
> 4dc47e: 41 0f b7 45 0c movzwl 0xc(%r13),%eax
> + eth = RTE_CONST_PTR_ADD(eth, 2 * sizeof(struct rte_vlan_hdr));
> + 4dc483: 4c 89 ef mov %r13,%rdi
> + if (likely((eth->ether_type == RTE_BE16(RTE_ETHER_TYPE_VLAN)) |
> 4dc486: 41 89 c0 mov %eax,%r8d
> 4dc489: 41 83 e0 ef and $0xffffffef,%r8d
> 4dc48d: 66 41 81 f8 81 00 cmp $0x81,%r8w
> 4dc493: 74 0a je 4dc49f
> <service_ingress_dedicated_management+0x1bf>
> 4dc495: 66 3d 88 a8 cmp $0xa888,%ax
> 4dc499: 0f 85 51 01 00 00 jne 4dc5f0
> <service_ingress_dedicated_management+0x310>
> if (vhdr->eth_proto == RTE_BE16(RTE_ETHER_TYPE_VLAN)) {
> 4dc49f: 66 41 81 7d 10 81 00 cmpw $0x81,0x10(%r13)
> 4dc4a6: 0f 84 24 01 00 00 je 4dc5d0
> <service_ingress_dedicated_management+0x2f0>
> eth = RTE_CONST_PTR_ADD(eth, sizeof(struct rte_vlan_hdr));
> 4dc4ac: 49 8d 7d 04 lea 0x4(%r13),%rdi
> packet_type = (union packet_type){
> 4dc4b0: b8 01 00 00 00 mov $0x1,%eax
> 4dc4b5: 41 b8 12 00 00 00 mov $0x12,%r8d
> + eth = RTE_CONST_PTR_ADD(eth, sizeof(struct rte_vlan_hdr));
> + 4dc4bb: 49 89 f9 mov %rdi,%r9
> if (likely((*(const rte_be32_t *)RTE_CONST_PTR_ADD(eth, offsetof(struct
> rte_ether_hdr, ether_type)) & RTE_BE32(0xFFFFFF00)) ==
> 4dc4be: 8b 7f 0c mov 0xc(%rdi),%edi
https://godbolt.org/z/5bc1bTrhe
In addition to less optimal assembly, it also prevents clang from
vectorizing and unrolling (even with constant length).
