Programming Model for Multi-Port Access in HBM - 2023.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2023.2 English

High Bandwidth Memory (or HBM) provides high bandwidth by splitting arrays into different banks/pseudo-channels in the design. This is a common practice in partitioning an array into different memory regions in high-performance computing. The software application interacting with the hardware allocates a single buffer, which will be spread across the pseudo-channels.

Enabling Dependence Analysis on Different Arguments

Vitis HLS considers different pointers to be independent channels, and removes any dependency analysis. But the software application allocates a single buffer for both pointers, and this lets the tool maintain dependency analysis through pragma HLS ALIAS. The ALIAS pragma informs data dependence analysis about the pointer distance. Refer to the ALIAS pragma or directive for more information.

In the following code example, the kernel arg0 is allocated in bank0 and kernel arg1 is allocated in bank1. The pointer distance should be specified in the distance option of the ALIAS pragma as shown below:
//Assume that the host code looks like this:
int *buf = clCreateBuffer(ctxt, CL_MEM_READ_ONLY, 2*bank_size, ...);
clSetKernelArg(kernel, 0, 0x20000000, buf);            // bank0
clSetKernelArg(kernel, 1, 0x20000000, buf+bank_size); // bank1

//The ALIAS pragma informs data dependence analysis about the pointer distance
void kernel(int *bank0, int *bank1, ...)
#pragma HLS alias ports=bank0,bank1 distance=bank_size

The ALIAS pragma can be specified using one of the following forms:

  • Constant distance:
    #pragma HLS alias ports=arr0,arr1,arr2,arr3 distance=1024
  • Variable distance:
    #pragma HLS alias ports=arr0,arr1,arr2,arr3 offset=0,512,1024,2048


  • The depths of all the ports in the interface pragma must be the same
  • All ports must be assigned to different bundles, bound to different HBM controllers
  • The number of ports specified in the second form must be the same as the number of offsets specified, one offset per port. #pragma HLS interface offset=off is not supported

  • Each port can only be used in one ALIAS pragma