Re: [dpdk-dev] How to disable SVE auto vectorization while using GCC

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> On Fri, Apr 30, 2021 at 5:27 PM fengchengwen
> <fengchengwen@huawei.com> wrote:
> >
> > Hi, ALL
> > We have a question for your help:
> >   1. We have two platforms, both of which are ARM64, one of which
> supports
> >      both NEON and SVE, the other only support NEON.
> >   2. We want to run on both platforms with a single binary file, and use the
> >      highest vector capability of the corresponding platform whenever
> possible.
> 
> I see VPP has a similar feature. IMO, it is not present in DPDK.
> Basically, In order to do this.
> - Compile slow-path code(90% of DPDK) with minimal CPU instruction set
> support
> - Have fastpath function compile with different CPU instruction set levels -In
> slowpath, Attach the fastpath function pointer-based on CPU instruction-
> level support.
Agree.

> 
> 
> >   3. So we build the DPDK program with -march=armv8-a+sve+crc (GCC
> 10.2).
This defines the minimum capabilities of the target machine.

> >      However, it is found that invalid instructions occur when the program
> >      runs on a machine that does not support SVE (pls see below).
> >   4. The problem is caused by the introduction of SVE in GCC automatic
> vector
> >      optimization.
> >
> >   So Is there a way to disable GCC automatic vector optimization or use only
> >   NEON to perform automatic vector optimization?
I do not think this is safe. Once SVE is enabled, compiler is allowed to use the SVE instructions wherever it finds it fit.

> >
> >   BTW: we already test -fno-tree-vectorize (as link below) but found no
> effect.
> >
> > https://stackoverflow.com/questions/7778174/how-can-i-disable-vectoriz
> > ation-while-using-gcc
> >
> >
> > The GDB output:
> >      EAL: Detected 128 lcore(s)
> >      EAL: Detected 4 NUMA nodes
> >      Option -w, --pci-whitelist is deprecated, use -a, --allow option
> > instead
> >
> >      Program received signal SIGILL, Illegal instruction.
> >      0x0000000000671b88 in eal_adjust_config ()
> >      (gdb)
> >      (gdb) where
> >      #0  0x0000000000671b88 in eal_adjust_config ()
> >      #1  0x0000000000682840 in rte_eal_init ()
> >      #2  0x000000000051c870 in main ()
> >      (gdb)
> >
> > The disassembly output of eal_adjust_config:
> >      671b7c:       f8237a81        str     x1, [x20, x3, lsl #3]
> >      671b80:       f110001f        cmp     x0, #0x400
> >      671b84:       54ffff21        b.ne    671b68 <eal_adjust_config+0x1f4>  //
> b.any
> >      671b88:       043357f5        addvl   x21, x19, #-1
> >      671b8c:       043457e1        addvl   x1, x20, #-1
> >      671b90:       910562b5        add     x21, x21, #0x158
> >      671b94:       04e0e3e0        cntd    x0
> >      671b98:       914012b5        add     x21, x21, #0x4, lsl #12
> >      671b9c:       52800218        mov     w24, #0x10                      // #16
> >      671ba0:       25d8e3e1        ptrue   p1.d
> >      671ba4:       25f80fe0        whilelo p0.d, wzr, w24
> >      671ba8:       a5e04020        ld1d    {z0.d}, p0/z, [x1, x0, lsl #3]
> >
> >
> > Best regards.
> >