> From: Morten Brørup
> Sent: Friday, 9 January 2026 16.58
>
> > From: Scott Mitchell [mailto:[email protected]]
> > Sent: Friday, 9 January 2026 16.27
> >
> > On Fri, Jan 9, 2026 at 4:26 AM Morten Brørup
> <[email protected]>
> > wrote:
> > >
> > > > Changes in v8:
> > > > - __rte_raw_cksum: use native pointer arithmetic instead of
> > RTE_PTR_ADD
> > > > to avoid incorrect results with -O3 for UDP checksums. Also
> > improves
> > > > performance due to less assembly generated with Clang.
> > >
> > > Personally, I also have observed GCC's optimizer behave as if it
> > loses some contextual information when using RTE_PTR_ADD, and thus
> > emitting less optimal code.
> > > I didn't look further into it, and thus have no data or examples to
> > back up the claim. Which is why I haven't started a discussion about
> > discouraging the use of RTE_PTR_ADD.
> > > In other words: I support this change.
> >
> > Sounds good! I observed ~600 (dpdk ptr macros) vs ~500 (native c ptr
> > operations) TSC cycles/block in cksum_perf_autotest.
>
> That is a significant performance degradation caused by the
> RTE_PTR_ADD() macros. We really should look into that - some day. ;-)
> Our application code base has RTE_CONST_PTR_ADD/SUB() for type
> consistency reasons (not for performance reasons). But I haven't gotten
> around to submitting them to the DPDK project yet.
> I wonder if the implicit stripping of "const" when using the
> RTE_PTR_ADD() macros makes the difference, or if the difference stems
> from other optimizer context getting lost.
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
