[Debug info] Only the first four elements of a matrix loaded from a buffer get debug values

[hbystuff]

Version: dxcompiler.dll 1.9(5402-0d3ee6b5)(1.9.0.5402) - 1.9.0.5402 (0d3ee6b55-dirty)

Repro files: matrix_buffer_load.zip

dxc -T cs_6_0 -Zi -Qembed_debug -Od -Fc float4x4.ll float4x4.hlsl

RWStructuredBuffer<float4x4> buffer : register(u0);

[numthreads(1, 1, 1)]
void main()
{
    float4x4 value  = buffer[0];
    float4x4 result = value * 2;
    buffer[1] = result;
}

Actual

All 16 elements of value are live SSA values, but only the four produced by the first bufferLoad get a dbg.value:

  %1  = call %dx.types.ResRet.f32 @dx.op.bufferLoad.f32(i32 68, %dx.types.Handle %buffer_UAV_structbuf, i32 0, i32 0)
  %2  = extractvalue %dx.types.ResRet.f32 %1, 0
  %3  = extractvalue %dx.types.ResRet.f32 %1, 1
  %4  = extractvalue %dx.types.ResRet.f32 %1, 2
  %5  = extractvalue %dx.types.ResRet.f32 %1, 3
  %6  = call %dx.types.ResRet.f32 @dx.op.bufferLoad.f32(i32 68, %dx.types.Handle %buffer_UAV_structbuf, i32 0, i32 16)
  %7  = extractvalue %dx.types.ResRet.f32 %6, 0
  ...
  %20 = extractvalue %dx.types.ResRet.f32 %16, 3

  ; nothing for %7..%10, %12..%15, %17..%20
  call void @llvm.dbg.value(metadata float %2, ...)  ; var:"value" !DIExpression(DW_OP_bit_piece,  0, 32)
  call void @llvm.dbg.value(metadata float %3, ...)  ; var:"value" !DIExpression(DW_OP_bit_piece, 32, 32)
  call void @llvm.dbg.value(metadata float %4, ...)  ; var:"value" !DIExpression(DW_OP_bit_piece, 64, 32)
  call void @llvm.dbg.value(metadata float %5, ...)  ; var:"value" !DIExpression(DW_OP_bit_piece, 96, 32)

result on the other hand is computed rather than loaded, gets all 16 fragments.

repro	matrix	elements	fragments for value	fragments for result
float2x3.hlsl	float2x3	6	4	6
float2x4.hlsl	float2x4	8	4	8
float3x2.hlsl	float3x2	6	4	6
float3x3.hlsl	float3x3	9	4	9
float3x4.hlsl	float3x4	12	4	12
float4x2.hlsl	float4x2	8	4	8
float4x3.hlsl	float4x3	12	4	12
float4x4.hlsl	float4x4	16	4	16
int4x4.hlsl	int4x4	16	4	16
half4x4.hlsl	half4x4	16	4	16
double4x4.hlsl	double4x4	16	4	16

float2x2 is unaffected because it has exactly four elements.

Expected

value should get one dbg.value per element, like result does.