Hi Everyone,
I'm looking at the features of a BeagleBone Black. Its /proc/cpuinfo is below.
I think vfpd32 cpu flag means I have 32 D-registers. The cpu flags neon and vfpv3 flags means I want something more than -mfpu=neon-fp16, but I'm not sure what that is.
My question is, what GCC ARM option is used when we encounter the neon, vfpv3 and vfpd32 flags?
Thanks in advance.
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$ cat /proc/cpuinfo processor : 0 model name : ARMv7 Processor rev 2 (v7l) BogoMIPS : 996.14 Features : half thumb fastmult vfp edsp thumbee neon vfpv3 tls vfpd32 CPU implementer : 0x41 CPU architecture: 7 CPU variant : 0x3 CPU part : 0xc08 CPU revision : 2
Hardware : Generic AM33XX (Flattened Device Tree) Revision : 0000 Serial : 0000000000000000
On Thu, Jul 21, 2016 at 6:33 PM, Jeffrey Walton noloader@gmail.com wrote:
I think vfpd32 cpu flag means I have 32 D-registers. The cpu flags neon and vfpv3 flags means I want something more than -mfpu=neon-fp16, but I'm not sure what that is.
neon implies vfvp3 and 32 D-registers and asimd/neon support, so that part is correct. it isn't obvious to me if you have the half-precision float support. The "half" printed by the kernel means that half-word loads are supported, which is only false for some obsolete parts I think. The kernel doesn't appear to be checking to see if the hardware has half-precision float support or not, so you can't determine that from /proc/cpuinfo.
Jim
On Thu, Jul 21, 2016 at 11:30 PM, Jim Wilson jim.wilson@linaro.org wrote:
On Thu, Jul 21, 2016 at 6:33 PM, Jeffrey Walton noloader@gmail.com wrote:
I think vfpd32 cpu flag means I have 32 D-registers. The cpu flags neon and vfpv3 flags means I want something more than -mfpu=neon-fp16, but I'm not sure what that is.
neon implies vfvp3 and 32 D-registers and asimd/neon support, so that part is correct. it isn't obvious to me if you have the half-precision float support. The "half" printed by the kernel means that half-word loads are supported, which is only false for some obsolete parts I think. The kernel doesn't appear to be checking to see if the hardware has half-precision float support or not, so you can't determine that from /proc/cpuinfo.
Thanks Jim.
Is there an arm-msr-tools or similar that has setuid so we can access the MSRs?
My thinking is, I can tell people to install arm-msr-tools so we can query for the features directly. I want to avoid telling people to run a test script as root.
Jeff
On Thu, Jul 21, 2016 at 8:48 PM, Jeffrey Walton noloader@gmail.com wrote:
Is there an arm-msr-tools or similar that has setuid so we can access the MSRs?
I'm not familiar with any such tool, but I haven't looked for one before. I found an x86 msr-tools project at github with a web search. It seems to be a standard part of debian/ubuntu x86 distros. I don't see an obvious arm equivalent.
You could ask the kernel developers to add a hardware capability (hwcap) check for half-precision fp and emit that info into the /proc/cpuinfo file, though it would take a while for that to be implemented and propagate to your users.
Jim
On Thu, Jul 21, 2016 at 11:30 PM, Jim Wilson jim.wilson@linaro.org wrote:
On Thu, Jul 21, 2016 at 6:33 PM, Jeffrey Walton noloader@gmail.com wrote:
I think vfpd32 cpu flag means I have 32 D-registers. The cpu flags neon and vfpv3 flags means I want something more than -mfpu=neon-fp16, but I'm not sure what that is.
neon implies vfvp3 and 32 D-registers and asimd/neon support, so that part is correct. it isn't obvious to me if you have the half-precision float support. The "half" printed by the kernel means that half-word loads are supported, which is only false for some obsolete parts I think. The kernel doesn't appear to be checking to see if the hardware has half-precision float support or not, so you can't determine that from /proc/cpuinfo.
OK, so looking at http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dht0002a/ch01s03..., it appears the "minimum" of the -mfpu option is VFPv3-D16. Since I have 32 D-regs, I can use the one VFPv3-D32, which should equate to -mfpu=neon-vfp3 (which does not seem to exist).
I can't use -mfpu=neon-vfpv4 because vfpv4 is not signaled, and it could be missing the half word and fma extensions implied with vfpv4.
So I guess the question is, what do I use for -mfpu=neon-vfp3 (or -mfpu=neon-vfp3-d32)? Is -mfpu=neon enough?
Thanks again for the help with this.
On Thu, Jul 21, 2016 at 9:13 PM, Jeffrey Walton noloader@gmail.com wrote:
So I guess the question is, what do I use for -mfpu=neon-vfp3 (or -mfpu=neon-vfp3-d32)? Is -mfpu=neon enough?
The -mfpu=neon option is enough. neon implies vfpv3 and 32 D registers.
Jim
On Fri, Jul 22, 2016 at 12:19 AM, Jim Wilson jim.wilson@linaro.org wrote:
On Thu, Jul 21, 2016 at 9:13 PM, Jeffrey Walton noloader@gmail.com wrote:
So I guess the question is, what do I use for -mfpu=neon-vfp3 (or -mfpu=neon-vfp3-d32)? Is -mfpu=neon enough?
The -mfpu=neon option is enough. neon implies vfpv3 and 32 D registers.
Perfect, thanks.
Jeff
On 22/07/16 05:21, Jeffrey Walton wrote:
On Fri, Jul 22, 2016 at 12:19 AM, Jim Wilson jim.wilson@linaro.org wrote:
On Thu, Jul 21, 2016 at 9:13 PM, Jeffrey Walton noloader@gmail.com wrote:
So I guess the question is, what do I use for -mfpu=neon-vfp3 (or -mfpu=neon-vfp3-d32)? Is -mfpu=neon enough?
The -mfpu=neon option is enough. neon implies vfpv3 and 32 D registers.
Perfect, thanks.
Jeff _______________________________________________ linaro-toolchain mailing list linaro-toolchain@lists.linaro.org https://lists.linaro.org/mailman/listinfo/linaro-toolchain
According to https://beagleboard.org/black, this board contains a Cortex-A8. So -mfpu=neon is correct.
https://community.arm.com/groups/tools/blog/2013/04/15/arm-cortex-a-processo...
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