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Keenutssudonatalie
Keenuts
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sudonatalie
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opt: prevent meld to merge block with MaximalReconvergence (KhronosGroup#5557)
The extension SPV_KHR_maximal_reconvergence adds more constraints around the merge blocks, and how the control flow can be altered. The one we address here is explained in the following part of the spec: Note: This means that the instructions in a break block will execute as if they were still diverged according to the loop iteration. This restricts potential transformations an implementation may perform on the IR to match shader author expectations. Similarly, instructions in the loop construct cannot be moved into the continue construct unless it can be proven that invocations are always converged. Until the optimizer is clever enough to determine if the invocation have already converged, we shall not meld a block which branches to a merge block into it, as it might move some instructions outside of the convergence region. This behavior being only required with the extension, this commit behavior change is gated by the extension. This means using wave operations without the maximal reconvergence extension might lead to undefined behaviors. Co-authored-by: Natalie Chouinard <[email protected]>
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‎source/opt/block_merge_util.cpp

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@@ -98,6 +98,17 @@ bool CanMergeWithSuccessor(IRContext* context, BasicBlock* block) {
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return false;
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}
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// Note: This means that the instructions in a break block will execute as if
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// they were still diverged according to the loop iteration. This restricts
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// potential transformations an implementation may perform on the IR to match
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// shader author expectations. Similarly, instructions in the loop construct
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// cannot be moved into the continue construct unless it can be proven that
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// invocations are always converged.
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if (succ_is_merge && context->get_feature_mgr()->HasExtension(
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kSPV_KHR_maximal_reconvergence)) {
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return false;
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}
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if (pred_is_merge && IsContinue(context, lab_id)) {
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// Cannot merge a continue target with a merge block.
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return false;

‎test/opt/block_merge_test.cpp

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@@ -1320,6 +1320,148 @@ OpFunctionEnd
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SinglePassRunAndMatch<BlockMergePass>(text, true);
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}
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TEST_F(BlockMergeTest, MaximalReconvergenceNoMeldToMerge) {
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const std::string text = R"(
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OpCapability Shader
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OpCapability GroupNonUniformBallot
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OpCapability GroupNonUniformArithmetic
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OpExtension "SPV_KHR_maximal_reconvergence"
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OpMemoryModel Logical GLSL450
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OpEntryPoint GLCompute %main "main" %gl_GlobalInvocationID %output
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OpExecutionMode %main LocalSize 1 1 1
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OpExecutionMode %main MaximallyReconvergesKHR
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OpSource HLSL 660
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OpName %type_RWStructuredBuffer_uint "type.RWStructuredBuffer.uint"
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OpName %output "output"
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OpName %main "main"
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OpDecorate %gl_GlobalInvocationID BuiltIn GlobalInvocationId
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OpDecorate %output DescriptorSet 0
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OpDecorate %output Binding 0
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OpDecorate %_runtimearr_uint ArrayStride 4
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OpMemberDecorate %type_RWStructuredBuffer_uint 0 Offset 0
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OpDecorate %type_RWStructuredBuffer_uint Block
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%uint = OpTypeInt 32 0
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%bool = OpTypeBool
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%int = OpTypeInt 32 1
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%int_0 = OpConstant %int 0
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%int_1 = OpConstant %int 1
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%_runtimearr_uint = OpTypeRuntimeArray %uint
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%type_RWStructuredBuffer_uint = OpTypeStruct %_runtimearr_uint
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%_ptr_StorageBuffer_type_RWStructuredBuffer_uint = OpTypePointer StorageBuffer %type_RWStructuredBuffer_uint
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%v3uint = OpTypeVector %uint 3
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%_ptr_Input_v3uint = OpTypePointer Input %v3uint
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%void = OpTypeVoid
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%15 = OpTypeFunction %void
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%uint_3 = OpConstant %uint 3
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%_ptr_StorageBuffer_uint = OpTypePointer StorageBuffer %uint
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%output = OpVariable %_ptr_StorageBuffer_type_RWStructuredBuffer_uint StorageBuffer
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%gl_GlobalInvocationID = OpVariable %_ptr_Input_v3uint Input
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%main = OpFunction %void None %15
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%18 = OpLabel
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%19 = OpLoad %v3uint %gl_GlobalInvocationID
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OpBranch %20
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%20 = OpLabel
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OpLoopMerge %21 %22 None
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; CHECK: OpLoopMerge [[merge:%\w+]] [[continue:%\w+]]
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OpBranch %23
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%23 = OpLabel
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%24 = OpCompositeExtract %uint %19 0
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%25 = OpGroupNonUniformBroadcastFirst %uint %uint_3 %24
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%26 = OpIEqual %bool %24 %25
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OpSelectionMerge %27 None
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OpBranchConditional %26 %28 %27
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%28 = OpLabel
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%29 = OpGroupNonUniformIAdd %int %uint_3 Reduce %int_1
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%30 = OpBitcast %uint %29
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OpBranch %21
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; CHECK: [[t1:%\w+]] = OpGroupNonUniformIAdd %int %uint_3 Reduce %int_1
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; CHECK-NEXT: [[t2:%\w+]] = OpBitcast %uint [[t1]]
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; CHECK-NEXT: OpBranch [[merge]]
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%27 = OpLabel
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OpBranch %22
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%22 = OpLabel
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OpBranch %20
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%21 = OpLabel
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%31 = OpAccessChain %_ptr_StorageBuffer_uint %output %int_0 %24
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OpStore %31 %30
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OpReturn
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OpFunctionEnd
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)";
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SetTargetEnv(SPV_ENV_VULKAN_1_3);
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SinglePassRunAndMatch<BlockMergePass>(text, true);
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}
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TEST_F(BlockMergeTest, NoMaximalReconvergenceMeldToMerge) {
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const std::string text = R"(
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OpCapability Shader
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OpCapability GroupNonUniformBallot
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OpCapability GroupNonUniformArithmetic
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OpMemoryModel Logical GLSL450
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OpEntryPoint GLCompute %main "main" %gl_GlobalInvocationID %output
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OpExecutionMode %main LocalSize 1 1 1
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OpSource HLSL 660
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OpName %type_RWStructuredBuffer_uint "type.RWStructuredBuffer.uint"
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OpName %output "output"
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OpName %main "main"
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OpDecorate %gl_GlobalInvocationID BuiltIn GlobalInvocationId
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OpDecorate %output DescriptorSet 0
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OpDecorate %output Binding 0
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OpDecorate %_runtimearr_uint ArrayStride 4
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OpMemberDecorate %type_RWStructuredBuffer_uint 0 Offset 0
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OpDecorate %type_RWStructuredBuffer_uint Block
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%uint = OpTypeInt 32 0
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%bool = OpTypeBool
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%int = OpTypeInt 32 1
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%int_0 = OpConstant %int 0
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%int_1 = OpConstant %int 1
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%_runtimearr_uint = OpTypeRuntimeArray %uint
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%type_RWStructuredBuffer_uint = OpTypeStruct %_runtimearr_uint
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%_ptr_StorageBuffer_type_RWStructuredBuffer_uint = OpTypePointer StorageBuffer %type_RWStructuredBuffer_uint
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%v3uint = OpTypeVector %uint 3
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%_ptr_Input_v3uint = OpTypePointer Input %v3uint
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%void = OpTypeVoid
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%15 = OpTypeFunction %void
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%uint_3 = OpConstant %uint 3
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%_ptr_StorageBuffer_uint = OpTypePointer StorageBuffer %uint
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%output = OpVariable %_ptr_StorageBuffer_type_RWStructuredBuffer_uint StorageBuffer
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%gl_GlobalInvocationID = OpVariable %_ptr_Input_v3uint Input
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%main = OpFunction %void None %15
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%18 = OpLabel
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%19 = OpLoad %v3uint %gl_GlobalInvocationID
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OpBranch %20
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%20 = OpLabel
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OpLoopMerge %21 %22 None
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; CHECK: OpLoopMerge [[merge:%\w+]] [[continue:%\w+]]
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OpBranch %23
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%23 = OpLabel
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%24 = OpCompositeExtract %uint %19 0
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%25 = OpGroupNonUniformBroadcastFirst %uint %uint_3 %24
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%26 = OpIEqual %bool %24 %25
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OpSelectionMerge %27 None
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OpBranchConditional %26 %28 %27
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%28 = OpLabel
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%29 = OpGroupNonUniformIAdd %int %uint_3 Reduce %int_1
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%30 = OpBitcast %uint %29
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OpBranch %21
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; CHECK: [[merge]] = OpLabel
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; CHECK-NEXT: [[t1:%\w+]] = OpGroupNonUniformIAdd %int %uint_3 Reduce %int_1
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; CHECK-NEXT: [[t2:%\w+]] = OpBitcast %uint [[t1]]
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%27 = OpLabel
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OpBranch %22
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%22 = OpLabel
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OpBranch %20
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%21 = OpLabel
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%31 = OpAccessChain %_ptr_StorageBuffer_uint %output %int_0 %24
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OpStore %31 %30
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OpReturn
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OpFunctionEnd
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)";
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SetTargetEnv(SPV_ENV_VULKAN_1_3);
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SinglePassRunAndMatch<BlockMergePass>(text, true);
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}
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// TODO(greg-lunarg): Add tests to verify handling of these cases:
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//
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// More complex control flow

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