Re: ARM VFP + hard-float ABI support in git HEAD

  • From: François Perrad <francois.perrad@xxxxxxxx>
  • To: luajit@xxxxxxxxxxxxx
  • Date: Thu, 9 Aug 2012 16:08:39 +0200

2012/8/9 Mike Pall <mike-1208@xxxxxxxxxx>:
> The extensions to the LuaJIT ARM port for VFP and hard-float support
> are now available from LuaJIT git HEAD.
>
> The ARM port of LuaJIT can now be built for three different
> CPU/ABI combinations:
>
> * ARMv5+, soft-float EABI, soft-float FP operations
> * ARMv6+, soft-float EABI, VFPv2+ FP operations
> * ARMv6+, hard-float EABI, VFPv2+ FP operations (e.g. Debian armhf)
>
> The JIT compiler can take advantage of extra features in ARMv7 and
> VFPv3, too. However you need to properly compile LuaJIT for your
> target CPU, since runtime feature detection is problematic on ARM.
>
> Most ARM toolchains are either generic or underspecify the minimum
> CPU or ARM architecture required. To unlock the full performance
> of your target CPU, you may need to give extra compiler options:
>
>   -mcpu=...           the (minimum) target CPU, alternatively
>   -march=...          the (minimum) ARM architecture version
>   -mfloat-abi=soft    soft-float ABI, soft-float computations
>   -mfloat-abi=softfp  soft-float ABI, VFP FPU
>   -mfloat-abi=hard    hard-float ABI, VFP FPU
>
> Add these to TARGET_CFLAGS on the make invocation.
>
> Here are some cross-compilation examples. More details can be
> found in: doc/install.html#cross
>
> # ARM soft-float
> make HOST_CC="gcc -m32" CROSS=arm-linux-gnueabi- \
>      TARGET_CFLAGS="-mfloat-abi=soft"
>
> # ARM soft-float ABI with VFP (example for Cortex-a8)
> make HOST_CC="gcc -m32" CROSS=arm-linux-gnueabi- \
>      TARGET_CFLAGS="-mcpu=cortex-a8 -mfloat-abi=softfp"
>
> # ARM hard-float ABI with VFP (armhf, requires recent toolchain)
> make HOST_CC="gcc -m32" CROSS=arm-linux-gnueabihf-
>
> Note that the hard-float ABI is incompatible with the soft-float
> ABI (with or without VFP) and requires a rebuild of the whole
> system with a relatively recent GCC etc. Debian armhf or Raspbian
> provide such an environment. Note this does *not* give you a
> substantial speedup with LuaJIT itself, compared to soft-float ABI
> plus VFP. The major speed difference that people see with armhf
> installs is due to the newer GCC versions with different default
> settings that enable ARMv7, Thumb-2 and VFP. The modified ABI
> itself has only a minor influence.
>
> If you already have a soft-float ABI system (e.g. Debian armel),
> then it's much easier to keep this environment and build LuaJIT
> with the proper CPU/arch and float-abi settings.
>
> On a related note, this update should also fix the issues with
> XCode for iOS. See: doc/install.html#ios
>
>
> I cannot possibly test all combinations of ARM CPUs and OS
> versions that are out there. Please report any issues you may
> find. Thank you!
>

looks good on Genesi Efika / i.MX515 (Cortex A8) / ARMv7 - VFPv3

François

> --Mike
>

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