Why is the AVX-256 VMOVAPS Instruction only copying four single precision floats instead of 8?

Question

I am trying to familiarize myself with the 256-bit AVX instructions available on some of the newer Intel processors. I have already verified that my i7-4720HQ supports 256-bit AVX instructions. The problem I am having is that the VMOVAPS instruction, which should copy 8 single precision floating point values, is only copying 4.

dot PROC
    VMOVAPS YMM1, ymmword ptr [RCX]                
    VDPPS   YMM2, YMM1, ymmword ptr [RDX], 255      
    VMOVAPS ymmword ptr [RCX], YMM2                 
    MOVSS   XMM0, DWORD PTR [RCX]                  
    RET
dot ENDP

In case you aren't familiar with the calling convention, Visual C++ 2015 expects the return of this function (since it is a float) to be in XMM0 upon return.

In addition to this, the standard is for the first argument to be passed in RCX and the second argument to be passed in RDX.

Here is the C code that calls this function.

_declspec(align(32)) float d1[] = { 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f, 1.0f };
_declspec(align(32)) float d2[] = { 2.0f, 2.0f, 2.0f, 2.0f, 2.0f, 2.0f, 2.0f, 2.0f };
printf("Dot Product Test: %f\n", dot(d1, d2));

The return value of the dot function is always 8.0. In addition to this, I have debugged the function and found that after the first assembly instruction, only four values get copied into YMM1. The rest of YMM1 remains zeroed.

Am I doing something wrong here? I've looked through the Intel documentation and some third party documentation. As far as I can tell I'm doing everything right. Am I using the wrong instruction? By the way, if you are here to tell me to use the Intel compiler intrinsics, don't bother.

You could get the same (non-working) behaviour in fewer instructions by using ymm0 as the destination for vdpps. Then you wouldn't need to store or reload, just return. If the __vectorcall ABI is like the SysV ABI, you're allowed to leave non-zero garbage in parts of the register outside the bits that hold the return value. (e.g. high elements of a vector reg). For such a tiny function, writing it by hand in asm instead of something that can inline means the function-call overhead can be significant. Esp. since you pass args by ref, rather than in vector regs. — Peter Cordes
I just wrote that code in two minutes to test the assembly instruction. I'm well aware that it can be optimized . — A. Robinson

Jester Jester · Accepted Answer · 2016-04-22T16:16:45

You forgot to read the instruction set reference page for VDPPS. It mentions that the result is produced in two halves:

VDPPS (VEX.256 encoded version)
DEST[127:0] ← DP_Primitive(SRC1[127:0], SRC2[127:0]);
DEST[255:128] ← DP_Primitive(SRC1[255:128], SRC2[255:128]);

It's not the VMOVAPS that's wrong.

Why is the AVX-256 VMOVAPS Instruction only copying four single precision floats instead of 8?

2 Answers