[dpdk-dev] How to disable SVE auto vectorization while using GCC
Honnappa Nagarahalli
Honnappa.Nagarahalli at arm.com
Fri Apr 30 22:54:57 CEST 2021
<snip>
>
> 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.
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.
> >
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