I am using the FFTW3 library on Beagleboard xM in a C application to perform r2c FFTs of floats. I read on this page that FFTW3 includes support for Neon, which is part of the xM architecture.
Is there a way to tell if the Neon coprocessor is actually being used?
For example, can I lists symbols from the object files and parse for some special Neon symbols? Alternatively, can I look through gcc -S assembler output for any Neon instructions? What instruction(s) would I look for? (I'm not familiar with what Neon assembly looks like).
Look at the disassembly. NEON instructions that operate on float data have a .f32 suffix and the NEON registers have names of the form dN or qN (where N is an integer). So if you see instructions that look like:
vadd.f32 q0, q1, q2
then NEON is being used.
Related
I'm looking for the intrinsic corresponding to the operation 'SMLAL2 Vd.8H,Vn.16B,Vm.16B', which according to ARM's own documentation (ARM Neon Intrinsics Ref) should be something like
int16x8_t vmlal_high_s8 (int16x8_t a,int8x16_t b,int8x16_t c)
however in the arm_neon.h that is included in ARM's GNU Toolchain doesn't have anything corresponding to it. So my question would be if I have to just include something else or otherwise can somehow circumvent this problem.
Thanks in advance!
For anyone else hitting this problem: I had chosen the ARM embedded tool chain instead of the linaro one, which is suitable for aarch64
i use memcpy() in my implementation on ARM Cortex a8,
it is my first code to develop on ARM Processors.
i read in the following link that i can optimize performance through some strategies.
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.faqs/ka13544.html
my code says
..
memcpy(myvar1, myvar2, myvar_size * sizeof(var_complex));
..
how can i optimize this code for ARM Cortex-a8 using Eclipse GCC Toolchain.
my code contains both C and Assembly codes.
is that affects using some registers ?
i searched for some examples and didn't found.
What is the difference between NEON SIMD and NEON SIMD version 2 as in Cortex A15?
It adds SIMD FMA instruction (VFMA.F32) and also mandates NEON half precision extension. NEONv2 is supported in ARM Cortex-A7, ARM Cortex-A15, and Qualcomm Krait (not sure about ARM Cortex-A5).
It is not that much of a difference, from ARM ARM:
(in reverse order of definitions)
Advanced SIMDv2 is an OPTIONAL extension to the ARMv7-A and ARMv7-R profiles.
Advanced SIMDv2 adds both the Half-precision Extension and the fused
multiply-add instructions to the features of Advanced SIMDv1.
...
Advanced SIMDv1 can be extended by the OPTIONAL Half-precision Extension,
that provides conversion functions in both directions between half-precision
floating-point and single-precision floating-point.
...
The Advanced SIMD architecture extension, its associated implementations, and supporting software, are
commonly referred to as NEON™
technology.
I want to write some C code such that gcc using the -msse4.1 flag can optimize it. Basically I want to check whether or not the compiler is taking advantage of SSE4.1 instructions.
There are many SSE4.1 instructions (http://en.wikipedia.org/wiki/SSE4#New_instructions) but I am not able to write a fragment of C Code which is using any of those instructions in the generated assembly code.
Thanks in advance.
From what I've seen, compilers rarely ever generate SSE4.1 instructions. I've seen a few cases where it will use the insert/extract instructions to pack data.
But for the most part, if you want to use the SSE4.1 instructions, you need to do them explicitly using intrinsics:
http://software.intel.com/sites/products/documentation/studio/composer/en-us/2011/compiler_c/intref_cls/common/intref_bk_sse41.htm
I doubt GCC would emit SSE4.1 instructions that easily. But you could have a look at Intel SPMD Program Compiler:
Under the SPMD model, the programmer writes a program that mostly
appears to be a regular serial program, though the execution model is
actually that a number of program instances execute in parallel on the
hardware. (See a more detailed example that illustrates this concept.)
ispc compiles a C-based SPMD programming language to run on the SIMD
units of CPUs; it frequently provides a 3x or more speedup on CPUs
with 4-wide SSE units, without any of the difficulty of writing
intrinsics code.
I'm using Cortex-A8 processor and I'm not understanding how to use the -mfpu flag.
On the Cortex-A8 there are both vfpv3 and neon co-processors. Previously I was not knowing how to use neon so I was only using
gcc -marm -mfloat-abi=softfp -mfpu=vfpv3
Now I have understood how SIMD processors run and I have written certain code using NEON intrinsics. To use neon co-processor now my -mfpu flag has to change to -mfpu=neon, so my compiler command line looks like this
gcc -marm -mfloat-abi=softfp -mfpu=neon
Now, does this mean that my vfpv3 is not used any more? I have lots of code which is not making use of NEON, do those parts not make use of vfpv3.
If both neon and vfpv3 are still used then I have no issues, but if only one of them is used how can I make use of both?
NEON implies having the traditional VFP support too. VFP can be used for "normal" (non-vector) floating-point calculations. Also, NEON does not support double-precision FP so only VFP instructions can be used for that.
What you can do is add -S to gcc's command line and check the assembly. Instructions starting with V (e.g. vld1.32, vmla.f32) are NEON instructions, and those starting with F (fldd, fmacd) are VFP. (Although ARM docs now prefer using the V prefix even for VFP instructions, GCC does not do that.)