> > From: Konstantin Ananyev [mailto:[email protected]] > > Sent: Thursday, 23 October 2025 10.51 > > > > > -#define RTE_MBUF_DIRECT(mb) \ > > > - (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL))) > > > + * > > > + * Note: Macro optimized for code size. > > > + * > > > + * The plain macro would be: > > > + * #define RTE_MBUF_DIRECT(mb) \ > > > + * (!((mb)->ol_flags & (RTE_MBUF_F_INDIRECT | > > RTE_MBUF_F_EXTERNAL))) > > > + * > > > + * The flags RTE_MBUF_F_INDIRECT and RTE_MBUF_F_EXTERNAL are both in > > the > > > MSB (most significant > > > + * byte) of the 64-bit ol_flags field, so we only compare this one > > byte instead of all > > > 64 bits. > > > + * > > > + * E.g., GCC version 16.0.0 20251019 (experimental) generates the > > following code > > > for x86-64. > > > + * > > > + * With the plain macro, 17 bytes of instructions: > > > + * movabs rax,0x6000000000000000 // 10 bytes > > > + * and rax,QWORD PTR [rdi+0x18] // 4 bytes > > > + * sete al // 3 bytes > > > + * With this optimized macro, only 7 bytes of instructions: > > > + * test BYTE PTR [rdi+0x1f],0x60 // 4 bytes > > > + * sete al // 3 bytes > > > + */ > > > +#if RTE_BYTE_ORDER == RTE_LITTLE_ENDIAN > > > +/* On little endian architecture, the MSB of a 64-bit integer is at > > byte offset 7. */ > > > +#define RTE_MBUF_DIRECT(mb) !(((const char *)(&(mb)- > > >ol_flags))[7] & 0x60) > > > +#elif RTE_BYTE_ORDER == RTE_BIG_ENDIAN > > > +/* On big endian architecture, the MSB of a 64-bit integer is at > > byte offset 0. */ > > > +#define RTE_MBUF_DIRECT(mb) !(((const char *)(&(mb)- > > >ol_flags))[0] & 0x60) > > > > A stupid q: why then not simply do: > > (mb->ol_flags >> 56) & 0x60 > > then? > > Should help to all these LE/BE casts, etc. > > GCC is too stupid for that too. > > Playing around with Godbolt shows that > return !((char)(p[3] >> 56) & 0x60); > becomes > movzx eax,BYTE PTR [rdi+0x1f] // 4 bytes > test al,0x60 // 2 bytes > Instead of simply > test BYTE PTR [rdi+0x1f],0x60 // 4 bytes
And these 2 extra bytes in instructions, are that really that critical? My guess, we wouldn't notice any real diff here. But if it really is, can I ask you to create a new define for 0x60, to avoid hardcoded constants in the code? Might be something like #define RTE_MBUF_F_INDIRECT_EXTERNAL_1B ... or so. Konstantin > Good suggestion, though. > > > > > > +#endif > > > +/* Verify the optimization above. */ > > > +static_assert((RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) == > > > UINT64_C(0x60) << (7 * CHAR_BIT), > > > + "(RTE_MBUF_F_INDIRECT | RTE_MBUF_F_EXTERNAL) is not 0x60 at > > MSB"); > > > > > > /** Uninitialized or unspecified port. */ > > > #define RTE_MBUF_PORT_INVALID UINT16_MAX > > > -- > > > 2.43.0
