VFP introduction on i.MX6

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VFP introduction on i.MX6

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VFP introduction on i.MX6

Recently, I was asked about software/hardware floating point support on i.MX6. There are some great articles on the freescale community already but lacks of introduction. This document shares some basic knowledge on it.

VFP is ARM's "Vector Floating Point" unit. SIMD operations can be better performed on several FPU extensions provided by ARM (NEON as in Cortex-A8 and Cortex-A9) [1]. To test if hardware floating support on freescale's toolchain, I used a simple application below:

$ cat haha.c

#include <stdio.h>;

int main()

{

        float a = 0.3f, b=1.2f;

        printf("%f\n", a * b);

        return 0;

}

Compile it as below, and got the hardware floating point enabled.

$ arm-linux-gcc -march=armv7-a -mfpu=neon -mfloat-abi=hard -o haha haha.c 

This can be checked by readelf. If Tag_ABI_VFP_args[2] shows VFP, it is hard floating. Otherwise, soft floating.

$ arm-linux-readelf -A haha

Attribute Section: aeabi

File Attributes

  Tag_CPU_name: "7-A"

  Tag_CPU_arch: v7

  Tag_CPU_arch_profile: Application

  Tag_ARM_ISA_use: Yes

  Tag_THUMB_ISA_use: Thumb-2

  Tag_FP_arch: VFPv3

  Tag_ABI_PCS_wchar_t: 4

  Tag_ABI_FP_denormal: Needed

  Tag_ABI_FP_exceptions: Needed

  Tag_ABI_FP_number_model: IEEE 754

  Tag_ABI_align_needed: 8-byte

  Tag_ABI_align_preserved: 8-byte, except leaf SP

  Tag_ABI_enum_size: int

  Tag_ABI_HardFP_use: SP and DP

  Tag_ABI_VFP_args: VFP registers

  Tag_DIV_use: Not allowed

Compared to the one by not specifying floating, compiler use soft floating by default,

$ arm-linux-gcc -o haha_soft haha.c 

And readelf won't have Tag_ABI_VFP_args.

$ arm-linux-readelf -A haha_soft

Attribute Section: aeabi

File Attributes

  Tag_CPU_name: "ARM10TDMI"

  Tag_CPU_arch: v5T

  Tag_ARM_ISA_use: Yes

  Tag_THUMB_ISA_use: Thumb-1

  Tag_ABI_PCS_wchar_t: 4

  Tag_ABI_FP_denormal: Needed

  Tag_ABI_FP_exceptions: Needed

  Tag_ABI_FP_number_model: IEEE 754

  Tag_ABI_align8_needed: Yes

  Tag_ABI_align8_preserved: Yes, except leaf SP

  Tag_ABI_enum_size: int

  Tag_unknown_44: 1 (0x1)

[1]: https://wiki.debian.org/ArmHardFloatPort/VfpComparison

[2]: For more detail on the Tag expression, check http://infocenter.arm.com/help/topic/com.arm.doc.ihi0045d/IHI0045D_ABI_addenda.pdf

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Last update:
‎09-10-2020 02:30 AM
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