Read current sample from an attachment in vulkan - vulkan

Question

I have a full screen effect pass that is multisampled.
If I read from an input attachement (which is also multisampled). I seem to only ever read the same sample.
The pipeline sample settings:
VkPipelineMultisampleStateCreateInfo multisampling_info = {};
multisampling_info.sType = VK_STRUCTURE_TYPE_PIPELINE_MULTISAMPLE_STATE_CREATE_INFO;
multisampling_info.rasterizationSamples = static_cast<VkSampleCountFlagBits>(samples);
multisampling_info.sampleShadingEnable = samples > 1 ? VK_TRUE : VK_FALSE;
multisampling_info.minSampleShading = 1.0;
multisampling_info.pSampleMask = nullptr;
multisampling_info.alphaToCoverageEnable = VK_FALSE;
multisampling_info.alphaToOneEnable = VK_FALSE;
Inside the shader I use an image type of:
%28 = OpTypeImage %float SubpassData 0 0 1 2 Unknown
And read from it with coordinates of (0, 0)
%34 = OpImageRead %v4float %30 %36
The shader seems to be called for every sample, as I'm getting the SampleId correctly.
The Spir-v spec says:
When the Image Dim operand is SubpassData, Coordinate is relative to the current fragment location. That is, the integer value (rounded down) of the current fragment’s window-relative (x, y) coordinate is added to (u, v).
If I understand this correctly, with coordinates of (0, 0) I should always get the corresponding sample?
Am I misunderstanding something or is this not possible?
This is the complete spirv shader i'm using:
OpCapability Shader
OpCapability InputAttachment
OpCapability SampleRateShading
OpMemoryModel Logical GLSL450
OpEntryPoint Fragment %44 "main" %41 %gl_SampleID
OpExecutionMode %44 OriginUpperLeft
OpDecorate %_struct_3 Block
OpMemberDecorate %_struct_3 0 Offset 0
OpDecorate %gl_SampleID BuiltIn SampleId
OpDecorate %32 DescriptorSet 0
OpDecorate %32 Binding 0
OpDecorate %32 InputAttachmentIndex 0
OpDecorate %41 Location 0
%int = OpTypeInt 32 1
%_struct_3 = OpTypeStruct %int
%_ptr_PushConstant__struct_3 = OpTypePointer PushConstant %_struct_3
%5 = OpVariable %_ptr_PushConstant__struct_3 PushConstant
%_ptr_PushConstant_int = OpTypePointer PushConstant %int
%bool = OpTypeBool
%_ptr_Input_int = OpTypePointer Input %int
%gl_SampleID = OpVariable %_ptr_Input_int Input
%float = OpTypeFloat 32
%28 = OpTypeImage %float SubpassData 0 0 1 2 Unknown
%_ptr_UniformConstant_28 = OpTypePointer UniformConstant %28
%32 = OpVariable %_ptr_UniformConstant_28 UniformConstant
%v4float = OpTypeVector %float 4
%v2int = OpTypeVector %int 2
%_ptr_Output_v4float = OpTypePointer Output %v4float
%41 = OpVariable %_ptr_Output_v4float Output
%void = OpTypeVoid
%42 = OpTypeFunction %void
%int_0 = OpConstant %int 0
%int_3 = OpConstant %int 3
%int_1 = OpConstant %int 1
%int_5 = OpConstant %int 5
%int_16 = OpConstant %int 16
%36 = OpConstantComposite %v2int %int_16 %int_0
%39 = OpConstantNull %v4float
%44 = OpFunction %void None %42
%1 = OpLabel
%7 = OpAccessChain %_ptr_PushConstant_int %5 %int_0
%9 = OpLoad %int %7
%10 = OpShiftRightArithmetic %int %9 %int_3
%13 = OpSLessThan %bool %10 %int_0
%14 = OpSelect %int %13 %int_1 %int_0
%16 = OpBitwiseAnd %int %9 %int_5
%18 = OpINotEqual %bool %16 %int_0
%19 = OpSelect %int %18 %int_1 %int_0
%20 = OpBitwiseOr %int %19 %14
%21 = OpIAdd %int %10 %20
%22 = OpLoad %int %gl_SampleID
%25 = OpSLessThan %bool %22 %21
OpSelectionMerge %26 None
OpBranchConditional %25 %27 %26
%27 = OpLabel
%30 = OpLoad %28 %32
%34 = OpImageRead %v4float %30 %36
OpBranch %26
%26 = OpLabel
%38 = OpPhi %v4float %34 %27 %39 %1
OpStore %41 %38
OpReturn
OpFunctionEnd

Answer 1

In the end it is super simple. The OpImageRead may have Optional Image Operands, one of them can be Sample, which takes the sample it'll read.
Feeding the builtin variable SampleId (the current shaded sample) into it fixed it.