For example, if I have a chip -mcpu=cortex-a7 , I should define -mfpu=neon-vfpv4 , not -mfpu=neon . However, I'm wondering what will happen if I define -mfpu=neon on Cortex-A7? Will it just ignore the flag and don't do the SIMD, or what will it do with the wrong flag like that?
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yugr
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Nguyễn Thanh Vũ
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A relevant question: https://stackoverflow.com/questions/7269946/cortex-a9-neon-vs-vfp-usage-confusion – yugr Nov 14 '17 at 12:11
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It will either complain or use the wrong instructions. – old_timer Nov 14 '17 at 13:58
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It will use an older set of NEON instructions (-mfpu=neon is for selecting the NEON instructions that are available on the Cortex-A8 core). For example, it will not include the VFMA instructions.
Note that from GCC 8 (still in development) you will be able to just use -mfpu=auto or leave out the -mfpu entirely and have the compiler pick the optimal FPU setting for the -mcpu option you selected
Kyrill
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If you do this compiler won't use VFPv4 instructions and potentially generate suboptimal code.
yugr
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