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charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Subject: Re: [dpdk-dev] rte_ether_addr_copy() strange comment X-BeenThere: dev@dpdk.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: DPDK patches and discussions List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: dev-bounces@dpdk.org Sender: "dev" On 6/25/2020 4:45 PM, Morten Brørup wrote: > The function rte_ether_addr_copy() checks for __INTEL_COMPILER and has a comment about "a strange gcc warning". It says: > > static inline void rte_ether_addr_copy(const struct rte_ether_addr *ea_from, > struct rte_ether_addr *ea_to) > { > #ifdef __INTEL_COMPILER > uint16_t *from_words = (uint16_t *)(ea_from->addr_bytes); > uint16_t *to_words = (uint16_t *)(ea_to->addr_bytes); > > to_words[0] = from_words[0]; > to_words[1] = from_words[1]; > to_words[2] = from_words[2]; > #else > /* > * Use the common way, because of a strange gcc warning. > */ > *ea_to = *ea_from; > #endif > } > > I can see that from_words discards the const qualifier. Is that the "strange" gcc warning the comment is referring to? > > This goes back to before the first public release of DPDK in 2013, ref. https://git.dpdk.org/dpdk/log/lib/librte_ether/rte_ether.h > > > It should be fixed as follows: > > - uint16_t *from_words = (uint16_t *)(ea_from->addr_bytes); > - uint16_t *to_words = (uint16_t *)(ea_to->addr_bytes); > + const uint16_t *from_words = (const uint16_t *)ea_from; > + uint16_t *to_words = (uint16_t *)ea_to; > > And the consequential question: Is copying the three shorts faster than copying the struct? In other words: Should we get rid of the #ifdef and use the first method only? I tried to investigate this in godbolt: https://godbolt.org/z/YSmvDn First I don't see the "strange" gcc warning with various gcc versions there. Related to the struct copy vs word copy, struct copy seems with less instruction [1],[2], my vote to remove ifdef and keep struct copy. [1] copy as individual function [1a] gcc 10.1, struct copy: copy: movdqa (%rsi), %xmm0 movaps %xmm0, (%rdi) ret [1b] gcc 10.1, word copy: copy: movzwl (%rsi), %eax movw %ax, (%rdi) movzwl 2(%rsi), %eax movw %ax, 2(%rdi) movzwl 4(%rsi), %eax movw %ax, 4(%rdi) ret [1c] icc 19.0.1, struct copy copy: movups (%rsi), %xmm0 #19.13 movups %xmm0, (%rdi) #19.13 ret [2] gcc 10.1, copy as inline function that knows the data, both seems similar // .addr = {1, 1, 1, 1, 1, 1}, [2a] struct copy: ... movl $257, %eax movw %ax, 4(%rsp) leaq 16(%rsp), %rdi movl $16843009, (%rsp) movdqa (%rsp), %xmm0 movaps %xmm0, 16(%rsp) ... [2b] word copy: movl $257, %eax movq %rsp, %rdi movw %ax, 4(%rsp) movl $16843009, (%rsp) > > PS: I will provide a patch which improves rte_is_broadcast_ether_addr() too. The magic formula here is: return (w[0] & w[1] & w[2]) == 0xFFFF; > > > Med venlig hilsen / kind regards > - Morten Brørup > >