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spirv-opt: Demote r/w output variables to private
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Fix #5516

Pass: eliminate dead output stores

Stores to write-only output variables are safe to eliminate; however,
stores to read-write output variables may be required for functional
correctness. If a read-write output variable is detected, then it will
be demoted to a private variable and any associated stores will not be
eliminated.
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@jeremy-lunarg
jeremy-lunarg committed May 16, 2024
1 parent ccf3e3c commit 55b6cb8
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Showing 3 changed files with 145 additions and 1 deletion.
73 changes: 72 additions & 1 deletion source/opt/eliminate_dead_output_stores_pass.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,8 @@ constexpr uint32_t kOpDecorateBuiltInLiteralInIdx = 2;
constexpr uint32_t kOpDecorateMemberBuiltInLiteralInIdx = 3;
constexpr uint32_t kOpAccessChainIdx0InIdx = 1;
constexpr uint32_t kOpConstantValueInIdx = 0;
constexpr uint32_t kVariableStorageClassInIdx = 0;
constexpr uint32_t kSpvTypePointerTypeIdInIdx = 1;
} // namespace

Pass::Status EliminateDeadOutputStoresPass::Process() {
Expand All @@ -45,6 +47,64 @@ bool EliminateDeadOutputStoresPass::IsLiveBuiltin(uint32_t bi) {
return live_builtins_->find(bi) != live_builtins_->end();
}

bool EliminateDeadOutputStoresPass::IsVariableRead(
Instruction const* const variable) {
analysis::DefUseManager* def_use_mgr = context()->get_def_use_mgr();

bool is_read = false;
def_use_mgr->ForEachUser(variable, [&is_read](Instruction* user) {
if (user->opcode() == spv::Op::OpLoad) is_read = true;
});

return is_read;
}

bool EliminateDeadOutputStoresPass::DemoteToPrivate(
Instruction* const variable) {
assert(spv::StorageClass(variable->GetSingleWordInOperand(
kVariableStorageClassInIdx)) == spv::StorageClass::Output);

auto type_mgr = context()->get_type_mgr();

// Set the variable's storage class to private.
variable->SetInOperand(kVariableStorageClassInIdx,
{uint32_t(spv::StorageClass::Private)});

// Get the pointee type of the variable.
auto const res_type = get_def_use_mgr()->GetDef(variable->type_id());
auto pointee_type_id =
res_type->GetSingleWordInOperand(kSpvTypePointerTypeIdInIdx);

// Find a type that matches the pointee type and uses private storage.
pointee_type_id =
type_mgr->FindPointerToType(pointee_type_id, spv::StorageClass::Private);
if (pointee_type_id == 0) {
return false;
}
context()->UpdateDefUse(get_def_use_mgr()->GetDef(pointee_type_id));

// Remove variable decorations that are not valid for private storage.
get_def_use_mgr()->ForEachUser(variable, [this](Instruction* user) {
if (user->opcode() == spv::Op::OpDecorate) {
auto const decoration = spv::Decoration(user->GetSingleWordInOperand(1));
if (decoration == spv::Decoration::Location ||
decoration == spv::Decoration::Component) {
kill_list_.push_back(user);
}
}
});

// Update the variable's result type to match the pointee type.
variable->SetResultType(pointee_type_id);
context()->UpdateDefUse(variable);

// Move the variable after the result type.
variable->RemoveFromList();
variable->InsertAfter(get_def_use_mgr()->GetDef(pointee_type_id));

return true;
}

bool EliminateDeadOutputStoresPass::AnyLocsAreLive(uint32_t start,
uint32_t count) {
auto finish = start + count;
Expand Down Expand Up @@ -194,6 +254,17 @@ Pass::Status EliminateDeadOutputStoresPass::DoDeadOutputStoreElimination() {
if (ptr_type->storage_class() != spv::StorageClass::Output) {
continue;
}

// Stores to write-only output variables are safe to eliminate; however,
// stores to read-write output variables may be required for functional
// correctness. If a read-write output variable is detected, then it will
// be demoted to a private variable and any associated stores will not be
// eliminated.
if (IsVariableRead(&var)) {
if (!DemoteToPrivate(&var)) return Status::Failure;
continue;
}

// If builtin decoration on variable, process as builtin.
auto var_id = var.result_id();
bool is_builtin = false;
Expand All @@ -217,7 +288,7 @@ Pass::Status EliminateDeadOutputStoresPass::DoDeadOutputStoreElimination() {
// locations are dead, kill store or all access chain's stores
def_use_mgr->ForEachUser(
var_id, [this, &var, is_builtin](Instruction* user) {
auto op = user->opcode();
auto const op = user->opcode();
if (op == spv::Op::OpEntryPoint || op == spv::Op::OpName ||
op == spv::Op::OpDecorate || user->IsNonSemanticInstruction())
return;
Expand Down
6 changes: 6 additions & 0 deletions source/opt/eliminate_dead_output_stores_pass.h
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Original file line number Diff line number Diff line change
Expand Up @@ -69,6 +69,12 @@ class EliminateDeadOutputStoresPass : public Pass {
// Return true if builtin |bi| is live.
bool IsLiveBuiltin(uint32_t bi);

// Return true if an OpLoad is using this variable.
bool IsVariableRead(Instruction const* const var);

// Demote the variable's storage class from output to private.
bool DemoteToPrivate(Instruction* const var);

std::unordered_set<uint32_t>* live_locs_;
std::unordered_set<uint32_t>* live_builtins_;

Expand Down
67 changes: 67 additions & 0 deletions test/opt/eliminate_dead_output_stores_test.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -946,6 +946,73 @@ TEST_F(ElimDeadOutputStoresTest, VertMultipleLocationsF16) {
&live_builtins);
}

TEST_F(ElimDeadOutputStoresTest, DemoteOutputVariableToPrivate) {
//
// #version 450
//
// layout(location=0) out vec4 pos;
// void main()
// {
// pos = vec4(0.0);
// gl_Position = pos;
// }

const std::string text = R"(OpCapability Shader
%1 = OpExtInstImport "GLSL.std.450"
OpMemoryModel Logical GLSL450
OpEntryPoint Vertex %main "main" %pos %_
OpSource GLSL 450
OpName %main "main"
OpName %pos "pos"
OpName %gl_PerVertex "gl_PerVertex"
OpMemberName %gl_PerVertex 0 "gl_Position"
OpMemberName %gl_PerVertex 1 "gl_PointSize"
OpMemberName %gl_PerVertex 2 "gl_ClipDistance"
OpMemberName %gl_PerVertex 3 "gl_CullDistance"
OpName %_ ""
OpDecorate %pos Location 0
OpMemberDecorate %gl_PerVertex 0 BuiltIn Position
OpMemberDecorate %gl_PerVertex 1 BuiltIn PointSize
OpMemberDecorate %gl_PerVertex 2 BuiltIn ClipDistance
OpMemberDecorate %gl_PerVertex 3 BuiltIn CullDistance
OpDecorate %gl_PerVertex Block
%void = OpTypeVoid
%3 = OpTypeFunction %void
%float = OpTypeFloat 32
%v4float = OpTypeVector %float 4
%_ptr_Output_v4float = OpTypePointer Output %v4float
;CHECK: %_ptr_Private_v4float = OpTypePointer Private %v4float
%pos = OpVariable %_ptr_Output_v4float Output
;CHECK: %pos = OpVariable %_ptr_Private_v4float Private
%float_0 = OpConstant %float 0
%11 = OpConstantComposite %v4float %float_0 %float_0 %float_0 %float_0
%uint = OpTypeInt 32 0
%uint_1 = OpConstant %uint 1
%_arr_float_uint_1 = OpTypeArray %float %uint_1
%gl_PerVertex = OpTypeStruct %v4float %float %_arr_float_uint_1 %_arr_float_uint_1
%_ptr_Output_gl_PerVertex = OpTypePointer Output %gl_PerVertex
%_ = OpVariable %_ptr_Output_gl_PerVertex Output
%int = OpTypeInt 32 1
%int_0 = OpConstant %int 0
%main = OpFunction %void None %3
%5 = OpLabel
OpStore %pos %11
;CHECK: OpStore %pos %11
%20 = OpLoad %v4float %pos
%21 = OpAccessChain %_ptr_Output_v4float %_ %int_0
OpStore %21 %20
OpReturn
OpFunctionEnd)";

SetTargetEnv(SPV_ENV_VULKAN_1_3);
SetAssembleOptions(SPV_TEXT_TO_BINARY_OPTION_PRESERVE_NUMERIC_IDS);

std::unordered_set<uint32_t> live_inputs{};
std::unordered_set<uint32_t> live_builtins{};
SinglePassRunAndMatch<EliminateDeadOutputStoresPass>(text, true, &live_inputs,
&live_builtins);
}

} // namespace
} // namespace opt
} // namespace spvtools

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