Vitis deletes LLVM code?

[SerenaC94]

Hi,

I am observing unexpected behavior when passing an LLVM IR to Vitis. The synthesis process arrives to the end with no errors, but in the reports everything is zero (0 cycles, 0 DSP, etc). So I traced back in the log to the first pass that is applied to the custom LLVM input, and found this:

INFO: [HLS 200-1022] Running custom LLVM hook 'HLS_HOOKS::opt'
//// get_config commands
INFO-FLOW: Doing LTO.
Execute       ap_eval exec -ignorestderr /opt/Xilinx/Vitis_HLS/2021.1/lnx64/tools/clang-3.9-csynth/bin/clang 
//// rest of the options
-o proj/solution1/.autopilot/db/a.g.lto.bc

Then I disassembled a.g.lto.bc, and it looks like all the input code has disappeared (it was >1800 lines in the LLVM IR):

; ModuleID = 'a.g.lto.bc'
source_filename = "a.g.lto.bc"
target datalayout = "e-m:e-i64:64-i128:128-i256:256-i512:512-i1024:1024-i2048:2048-i4096:4096-n8:16:32:64-S128-v16:16-v24:32-v32:32-v48:64-v96:128-v192:256-v256:256-v512:512-v1024:1024"
target triple = "fpga64-xilinx-none"

@0 = private unnamed_addr constant [8 x i8] c"ap_auto\00"
@1 = private unnamed_addr constant [1 x i8] zeroinitializer
@2 = private unnamed_addr constant [15 x i8] c"atax_16_kernel\00"

define void @atax_16_kernel(i32* noalias nocapture %arg_2, i32* noalias nocapture %arg_3, i32* noalias %arg_4, i32* noalias nocapture %arg_5) {
bb_0:
  call void (...) @_ssdm_op_SpecTopModule([15 x i8]* @2)
  call void (...) @_ssdm_op_SpecBitsMap(i32* %arg_2), !map !2
  call void (...) @_ssdm_op_SpecInterface(i32* %arg_2, [8 x i8]* @0, i32 0, i32 0, [1 x i8]* @1, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1, [1 x i8]* @1, i32 0, i32 0, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1)
  call void (...) @_ssdm_op_SpecBitsMap(i32* %arg_3), !map !2
  call void (...) @_ssdm_op_SpecInterface(i32* %arg_3, [8 x i8]* @0, i32 0, i32 0, [1 x i8]* @1, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1, [1 x i8]* @1, i32 0, i32 0, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1)
  call void (...) @_ssdm_op_SpecBitsMap(i32* %arg_4), !map !2
  call void (...) @_ssdm_op_SpecInterface(i32* %arg_4, [8 x i8]* @0, i32 0, i32 0, [1 x i8]* @1, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1, [1 x i8]* @1, i32 0, i32 0, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1)
  call void (...) @_ssdm_op_SpecBitsMap(i32* %arg_5), !map !2
  call void (...) @_ssdm_op_SpecInterface(i32* %arg_5, [8 x i8]* @0, i32 0, i32 0, [1 x i8]* @1, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1, [1 x i8]* @1, i32 0, i32 0, i32 0, i32 0, [1 x i8]* @1, [1 x i8]* @1)
  store i32 0, i32* %arg_3, align 4
  store i32 0, i32* %arg_2, align 4
  ret void
}

declare void @_ssdm_op_SpecBitsMap(...)

declare void @_ssdm_op_SpecInterface(...)

declare void @_ssdm_op_SpecTopModule(...)

!blackbox_cfg = !{!0}
!llvm.module.flags = !{!1}

!0 = !{}
!1 = !{i32 7, !"reflow.full.lowering", i32 1}
!2 = !{!0}

I am attaching all the code needed to reproduce the issue in atax_issue.zip. I have no clues as to what is happening here, any help is appreciated.

[SerenaC94]

I am using Vitis HLS 2021.1 (both the frontend from this repo and the backend) on Ubuntu 20.04. This error happens on various different input codes, and what they have in common is that I unrolled some loops in the LLVM IR before passing it to Vitis.

[yu810226]

Hello @SerenaC94 , it's probably auto optimized out depends on the interface setting. I'll suggest to try with assigning array compatible interface in your IR to see if it brings your expected IR in mind.

[SerenaC94]

I think I have solved the issue by making sure that instead of pointers I can see arrays in the input IR. So moving from something like this

define void @atax_16_kernel(float* %0, float* %1, float* %2, float* %3)

to something like this

define void @atax_16_kernel([16 x float]* %arg_5, [16 x float]* %arg_6, [16 x [16 x float]]* %arg_7, [16 x float]* %arg_8)

Thanks for the pointer!