Constant out of range with NEON intrinsics

Question

Im compiling the following ARM NEON intrinsics test code (in Eclipse with Android NDK):

void foo(uint64_t* Res)
{
    uint64_t x = 0xff12aa8902acf78dULL;
    uint64x1_t a,b;

    a = vld1_u64 (&x);
    b = vext_u64 (a, a, 3);
    vst1_u64 (Res,b);
}

but I keep getting the following compiler error:

/home/developer/adt/ndk/ndk-build all 
Install        : libneon_test.so => libs/armeabi/libneon_test.so
Compile thumb  : neon_test <= neon.c
jni/neon.c: In function 'foo':
jni/neon.c:17:1: error: constant out of range
make: *** [obj/local/armeabi-v7a/objs/neon_test/neon.o] Error 1`

Line 17 is the last } brace of foo().

Does anyone have an idea what might be the cause of this strange error?

what constant is it talking about? Line 17 of the code you posted is the } brace. Is it the value 3? — ThePosey, Mar 09 '13 at 20:20
Thats the problem, I dont know either. The compiler always points this error to the last } brace which makes no sense. I think this might be a GCC bug with NEON so Im looking for some workaround maybe. — NumberFour, Mar 09 '13 at 20:26
I think ThePosey's deleted answer is on the right track. What are you expecting `vext_u64(a, a, 3)` to do? — tc., Mar 09 '13 at 23:31

auselen · Accepted Answer · 2013-03-10T00:56:04.153

Problem is in this line

b = vext_u64 (a, a, 3);

You can't have 3 as shift for vext_u64, it isn't allowed.

uint64x1_t vext_u64(uint64x1_t a, uint64x1_t b, __constrange(0,0) int c);  // VEXT.64 d0,d0,d0,#0

It looks like gcc's implementation (definition?) of neon intrinsics isn't as good as armcc's in this aspect.

A trick to understand such behavior is to pass -S to gcc to convert code to assembly then try to compile that assembly file. That way gcc will tell you exactly which line is giving the problem.

$ ~/bin/android-ndk-r8d/toolchains/arm-linux-androideabi-4.7/prebuilt/linux-x86/bin/arm-linux-androideabi-gcc --sysroot=/home/auselen/bin/android-ndk-r8d/platforms/android-14/arch-arm -std=c99 -S neon.c -mfloat-abi=softfp -mfpu=neon -O2
neon.c: In function 'foo':
neon.c:11:1: error: constant out of range
$ ~/bin/android-ndk-r8d/toolchains/arm-linux-androideabi-4.7/prebuilt/linux-x86/bin/arm-linux-androideabi-gcc --sysroot=/home/auselen/bin/android-ndk-r8d/platforms/android-14/arch-arm -std=c99 -c neon.s -mfloat-abi=softfp -mfpu=neon -O2
neon.s: Assembler messages:
neon.s:28: Error: shift out of range -- `vext.64 d16,d16,d16,#3'

I have the same problem when using `vecTmp = vrshrn_n_u32(vec128b, 15+8);`, the constant cannot be larger than 16, but in fact the constraint should be 0-31. I think is the bug of gcc tool chain of android? — QZHua, Sep 24 '17 at 14:29

Constant out of range with NEON intrinsics

1 Answers1