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Date: Wed, 5 Jan 2022 11:08:54 +0100 From: Greg KH <gregkh@...uxfoundation.org> To: cgel.zte@...il.com Cc: arnd@...db.de, chi.minghao@....com.cn, fkassabri@...ana.ai, linux-kernel@...r.kernel.org, obitton@...ana.ai, ogabbay@...nel.org, osharabi@...ana.ai, ynudelman@...ana.ai, zealci@....com.cn Subject: Re: [PATCH misc-next] drivers/misc: remove redundant rc variable On Wed, Jan 05, 2022 at 09:22:29AM +0000, cgel.zte@...il.com wrote: > I found that there are indeed some optimizations > after removing the redundancy. > > For example: > > int foo(int a, int b) > { > int ret; > ret = a>b ? a: b; > return ret; > } > int main() > { > int val; > val = foo(7,2); > return 0; > } > > and > > int foo(int a, int b) > { > return a>b ? a: b; > } > int main() > { > int val; > val = foo(7,2); > return 0; > } > > The corresponding disassembly code is as follows: > > 1129: f3 0f 1e fa endbr64 > 112d: 55 push %rbp > 112e: 48 89 e5 mov %rsp,%rbp > 1131: 89 7d fc mov %edi,-0x4(%rbp) > 1134: 89 75 f8 mov %esi,-0x8(%rbp) > 1137: 8b 45 fc mov -0x4(%rbp),%eax > 113a: 39 45 f8 cmp %eax,-0x8(%rbp) > 113d: 0f 4d 45 f8 cmovge -0x8(%rbp),%eax > 1141: 5d pop %rbp > 1142: c3 retq > > and > > 1129: f3 0f 1e fa endbr64 > 112d: 55 push %rbp > 112e: 48 89 e5 mov %rsp,%rbp > 1131: 89 7d ec mov %edi,-0x14(%rbp) > 1134: 89 75 e8 mov %esi,-0x18(%rbp) > 1137: 8b 45 ec mov -0x14(%rbp),%eax > 113a: 39 45 e8 cmp %eax,-0x18(%rbp) > 113d: 0f 4d 45 e8 cmovge -0x18(%rbp),%eax > 1141: 89 45 fc mov %eax,-0x4(%rbp) > 1144: 8b 45 fc mov -0x4(%rbp),%eax > 1147: 5d pop %rbp > 1148: c3 retq > > After removing the redundancy, the compiler does > have some optimizations Please look at the actual output of the kernel files, the build options that the kernel enforces should not cause a difference here. thanks, greg k-h
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