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

[Jerin Jacob]

On Fri, Apr 30, 2021 at 5:27 PM fengchengwen <fengchengwen at 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.


>   3. So we build the DPDK program with -march=armv8-a+sve+crc (GCC 10.2).
>      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?
>
>   BTW: we already test -fno-tree-vectorize (as link below) but found no effect.
>   https://stackoverflow.com/questions/7778174/how-can-i-disable-vectorization-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.
>