Fix a miscompile that occurs when reading a matrix contained in a struct with no sibling fields

[aclysma]

Fix a miscompile that occurs when reading a matrix contained in a struct with no sibling fields. The expected behavior is that each load instruction is at a 4 byte offset, but observed behavior is that Loads are always at 0 offset.

This was introduced in commit 909c552 (merged 2025-03-17), and it's observable regressed between the releases v1.8.2502 and v1.8.2505. The regression was tested as still live in v1.10.2605.24 and current HEAD.

This change adds a test that reproduces the problem. ClangHLSLTests shows 2 failures with this test pre-patch and 0 failures after the patch.

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[aclysma]

@microsoft-github-policy-service agree

[aclysma]

Just FYI, I'm really not familiar with the code in DXC. I have high confidence in the added test, and that this was a legitimate bug and when the regression occurred. I don't have confidence that this is the "best" fix and am very happy for someone who is familiar with this area to determine the right way to fix it. (But it does resolve the added test without breaking other tests.)

[aclysma]

I put a godbolt repro here: https://godbolt.org/z/bG5reh3rh

[llvm-beanz]

This change looks correct to me.

Can you add one more test to verify the IR->IR transformation of the pass you're changing?

This should be a good test, you can just drop it into tools/clang/test/CodeGenDXIL/hlsl/intrinsics/:

; RUN: %dxopt %s -hlsl-passes-resume -dxilgen -S | FileCheck %s

target datalayout = "e-m:e-p:32:32-i1:32-i8:32-i16:32-i32:32-i64:64-f16:32-f32:32-f64:64-n8:16:32:64"
target triple = "dxil-ms-dx"

%struct.ByteAddressBuffer  = type { i32 }
%struct.RWByteAddressBuffer = type { i32 }
%struct.Box                = type { %class.matrix.float.4.4 }
%class.matrix.float.4.4   = type { [4 x <4 x float>] }
%dx.types.Handle           = type { i8* }
%dx.types.ResourceProperties = type { i32, i32 }

@"\01?src@@3UByteAddressBuffer@@A"   = external global %struct.ByteAddressBuffer,   align 4
@"\01?dst@@3URWByteAddressBuffer@@A" = external global %struct.RWByteAddressBuffer, align 4

; Function Attrs: nounwind
define void @main() #0 {
  %src = load %struct.ByteAddressBuffer, %struct.ByteAddressBuffer* @"\01?src@@3UByteAddressBuffer@@A"

  ; CHECK: [[HDL:%.*]] = call %dx.types.Handle @dx.op.createHandleForLib.struct.ByteAddressBuffer(i32 160, %struct.ByteAddressBuffer
  ; CHECK: [[ANHDL:%.*]] = call %dx.types.Handle @dx.op.annotateHandle(i32 216, %dx.types.Handle [[HDL]], %dx.types.ResourceProperties { i32 11, i32 0 })
  ; Column 0 of a column-major float4x4: elements at byte offsets 0, 4, 8, 12.
  ; The element-offset operand (4th arg) must be undef for raw-buffer loads.
  ; CHECK: @dx.op.rawBufferLoad.f32(i32 139, %dx.types.Handle [[ANHDL]], i32 0,  i32 undef, i8 1, i32 4)
  ; CHECK: @dx.op.rawBufferLoad.f32(i32 139, %dx.types.Handle [[ANHDL]], i32 4,  i32 undef, i8 1, i32 4)
  ; CHECK: @dx.op.rawBufferLoad.f32(i32 139, %dx.types.Handle [[ANHDL]], i32 8,  i32 undef, i8 1, i32 4)
  ; CHECK: @dx.op.rawBufferLoad.f32(i32 139, %dx.types.Handle [[ANHDL]], i32 12, i32 undef, i8 1, i32 4)

  %src.hdl = call %dx.types.Handle @"dx.hl.createhandle..%dx.types.Handle (i32, %struct.ByteAddressBuffer)"(i32 0, %struct.ByteAddressBuffer %src)
  %src.ann = call %dx.types.Handle @"dx.hl.annotatehandle..%dx.types.Handle (i32, %dx.types.Handle, %dx.types.ResourceProperties, %struct.ByteAddressBuffer)"(i32 14, %dx.types.Handle %src.hdl, %dx.types.ResourceProperties { i32 11, i32 0 }, %struct.ByteAddressBuffer zeroinitializer)

  ; Load<Box>(0): returns a virtual pointer at raw-buffer byte offset 0.
  %box.ptr = call %struct.Box* @"dx.hl.op.ro.%struct.Box* (i32, %dx.types.Handle, i32)"(i32 231, %dx.types.Handle %src.ann, i32 0)

  ; GEP to field 0 (the matrix m) inside Box -- byte offset within the struct = 0.
  %mat.ptr = getelementptr inbounds %struct.Box, %struct.Box* %box.ptr, i32 0, i32 0

  ; ColMatSubscript (opcode 1): extract rows 0-3 of column 0 from the 4x4 matrix.
  ; Row indices 0-3 map to flat element indices 0-3 (column-major storage),
  ; i.e. byte offsets 0, 4, 8, 12 relative to the matrix base.
  %col0.ptr = call <4 x float>* @"dx.hl.subscript.colMajor[].rn.<4 x float>* (i32, %class.matrix.float.4.4*, i32, i32, i32, i32)"(i32 1, %class.matrix.float.4.4* %mat.ptr, i32 0, i32 1, i32 2, i32 3)
  %col0 = load <4 x float>, <4 x float>* %col0.ptr

  ; Write the result to a UAV so the loads have an observable use.
  %dst = load %struct.RWByteAddressBuffer, %struct.RWByteAddressBuffer* @"\01?dst@@3URWByteAddressBuffer@@A"
  %dst.hdl = call %dx.types.Handle @"dx.hl.createhandle..%dx.types.Handle (i32, %struct.RWByteAddressBuffer)"(i32 0, %struct.RWByteAddressBuffer %dst)
  %dst.ann = call %dx.types.Handle @"dx.hl.annotatehandle..%dx.types.Handle (i32, %dx.types.Handle, %dx.types.ResourceProperties, %struct.RWByteAddressBuffer)"(i32 14, %dx.types.Handle %dst.hdl, %dx.types.ResourceProperties { i32 4107, i32 0 }, %struct.RWByteAddressBuffer zeroinitializer)
  %col0.x = extractelement <4 x float> %col0, i32 0
  %col0.y = extractelement <4 x float> %col0, i32 1
  %col0.z = extractelement <4 x float> %col0, i32 2
  %col0.w = extractelement <4 x float> %col0, i32 3
  call void @"dx.hl.op..void (i32, %dx.types.Handle, i32, float)"(i32 277, %dx.types.Handle %dst.ann, i32 0,  float %col0.x)
  call void @"dx.hl.op..void (i32, %dx.types.Handle, i32, float)"(i32 277, %dx.types.Handle %dst.ann, i32 4,  float %col0.y)
  call void @"dx.hl.op..void (i32, %dx.types.Handle, i32, float)"(i32 277, %dx.types.Handle %dst.ann, i32 8,  float %col0.z)
  call void @"dx.hl.op..void (i32, %dx.types.Handle, i32, float)"(i32 277, %dx.types.Handle %dst.ann, i32 12, float %col0.w)

  ret void
}

declare %dx.types.Handle @"dx.hl.createhandle..%dx.types.Handle (i32, %struct.ByteAddressBuffer)"(i32, %struct.ByteAddressBuffer) #1
declare %dx.types.Handle @"dx.hl.annotatehandle..%dx.types.Handle (i32, %dx.types.Handle, %dx.types.ResourceProperties, %struct.ByteAddressBuffer)"(i32, %dx.types.Handle, %dx.types.ResourceProperties, %struct.ByteAddressBuffer) #1
declare %struct.Box* @"dx.hl.op.ro.%struct.Box* (i32, %dx.types.Handle, i32)"(i32, %dx.types.Handle, i32) #2
declare <4 x float>* @"dx.hl.subscript.colMajor[].rn.<4 x float>* (i32, %class.matrix.float.4.4*, i32, i32, i32, i32)"(i32, %class.matrix.float.4.4*, i32, i32, i32, i32) #1
declare %dx.types.Handle @"dx.hl.createhandle..%dx.types.Handle (i32, %struct.RWByteAddressBuffer)"(i32, %struct.RWByteAddressBuffer) #1
declare %dx.types.Handle @"dx.hl.annotatehandle..%dx.types.Handle (i32, %dx.types.Handle, %dx.types.ResourceProperties, %struct.RWByteAddressBuffer)"(i32, %dx.types.Handle, %dx.types.ResourceProperties, %struct.RWByteAddressBuffer) #1
declare void @"dx.hl.op..void (i32, %dx.types.Handle, i32, float)"(i32, %dx.types.Handle, i32, float) #0

attributes #0 = { nounwind }
attributes #1 = { nounwind readnone }
attributes #2 = { nounwind readonly }

!pauseresume        = !{!1}
!dx.version         = !{!0}
!dx.valver          = !{!2}
!dx.shaderModel     = !{!3}
!dx.typeAnnotations = !{!4}
!dx.entryPoints     = !{!8}
!dx.fnprops         = !{!15}
!dx.options         = !{!16, !17}

!0  = !{i32 1, i32 6}
!1  = !{!"hlsl-hlemit", !"hlsl-hlensure"}
!2  = !{i32 1, i32 9}
!3  = !{!"cs", i32 6, i32 6}
; dx.typeAnnotations: function annotations only (no user struct needed for this pass).
!4  = !{i32 1, void ()* @main, !5}
!5  = !{!6}
!6  = !{i32 1, !7, !7}
!7  = !{}
; dx.entryPoints: entry = main, no signatures, resources = {SRV=!10, UAV=!12}.
!8  = !{void ()* @main, !"main", null, !9, null}
!9  = !{!10, !12, null, null}
!10 = !{!11}
!11 = !{i32 0, %struct.ByteAddressBuffer* @"\01?src@@3UByteAddressBuffer@@A", !"src", i32 0, i32 0, i32 1, i32 11, i32 0, null}
!12 = !{!13}
!13 = !{i32 0, %struct.RWByteAddressBuffer* @"\01?dst@@3URWByteAddressBuffer@@A", !"dst", i32 0, i32 0, i32 1, i32 11, i1 false, i1 false, i1 false, null}
; dx.fnprops: CS kind=5, numthreads(1,1,1).
!15 = !{void ()* @main, i32 5, i32 1, i32 1, i32 1}
!16 = !{i32 64}
!17 = !{i32 -1}

[aclysma]

I've added the additional test, verified:

• Test fails when the fix in this branch is NOT applied
• Test passes when fix is applied