Re: AArch32 gcc mode

[James Greenhalgh <james.greenhalgh@arm.com>]

On Wed, Oct 29, 2014 at 05:48:10AM +0000, Vyacheslav Barinov wrote:
> James Greenhalgh <james.greenhalgh@arm.com> writes:
> > On Tue, Oct 28, 2014 at 05:29:15AM +0000, Slava Barinov wrote:
> >>   I work now with ARM64 CPUs and wanted to check AArch32 mode, I can see
> >>   several mentions in GCC source code but can't find flag which switches on
> >>   AArch32 code generation.
> >> 
> >>   Is it possible to make current GCC generate it?
> >
> > AArch32 is the 32-bit execution state in the ARMv8 architecture. It is
> > closely related to the ARMv7 architecture, which in GCC is implemented as
> > the "arm" target. Code generation for the AArch64 execution state of the
> > ARMv8 architecture is implemented as the "aarch64" target.
> >
> > GCC has a model whereby the compiler can be built for only one target at
> > any time, so there is no flag which will generate AArch32 code from an
> > AArch64 compiler. You will need a compiler built to target "arm".
> >
> > For your use case that will mean installing two cross-compilers, one for
> > (for example) arm-none-linux-gnueabi, and one for aarch64-none-linux-gnu.
> 
>   That was the first thing I tried and it works partially.
>   But my initial idea was to use armv8 hardware extensions like accelerated
>   crypto engine.
> 
>   Will setting CPU model to armv8 in arm-none-linux-gnueabi toolchain make it
>   possible to use all its features in 32bit code?

Initial support in the ARM port for ARMv8-A was added in GCC 4.8. Support
for the ARMv8-A Crypto extension, and for the CRC instructions, was added to
the ARM port in GCC 4.9 [1].

To enable support for the CRC intrinsics you should use:

  -march=armv8-a+crc

To enable support for the Crypto intrinsics you should use:

  -mfpu=crypto-neon-fp-armv8

You can look at: https://gcc.gnu.org/onlinedocs/gcc/ARM-Options.html for
more details on the available options for the ARM target.

Thanks,
James

---
[1]: GCC 4.9 Release Series Changes, New Features, and Fixes
     https://gcc.gnu.org/gcc-4.9/changes.html