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shader_recompiler/dead_code_elimination: Add DeadBranchElimination pass 

This adds a pass to eliminate if(false) branches within the shader code
This commit is contained in:
ameerj 2022-03-22 01:22:21 -04:00
parent 75046a3351
commit f10d40a0a2
1 changed files with 62 additions and 9 deletions
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61
src/shader_recompiler/ir_opt/dead_code_elimination_pass.cpp
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@ -7,19 +7,72 @@
#include "shader_recompiler/ir_opt/passes.h"
namespace Shader::Optimization {
void DeadCodeEliminationPass(IR::Program& program) {
namespace {
template <bool TEST_USES>
void DeadInstElimination(IR::Block* const block) {
// We iterate over the instructions in reverse order.
// This is because removing an instruction reduces the number of uses for earlier instructions.
for (IR::Block* const block : program.post_order_blocks) {
auto it{block->end()};
while (it != block->begin()) {
--it;
if (!it->HasUses() && !it->MayHaveSideEffects()) {
if constexpr (TEST_USES) {
if (it->HasUses() || it->MayHaveSideEffects()) {
continue;
}
}
it->Invalidate();
it = block->Instructions().erase(it);
}
}
void DeadBranchElimination(IR::Program& program) {
const auto begin_it{program.syntax_list.begin()};
for (auto node_it = begin_it; node_it != program.syntax_list.end(); ++node_it) {
if (node_it->type != IR::AbstractSyntaxNode::Type::If) {
continue;
}
IR::Inst* const cond_ref{node_it->data.if_node.cond.Inst()};
const IR::U1 cond{cond_ref->Arg(0)};
if (!cond.IsImmediate()) {
continue;
}
if (cond.U1()) {
continue;
}
// False immediate condition. Remove condition ref, erase the entire branch.
cond_ref->Invalidate();
// Account for nested if-statements within the if(false) branch
u32 nested_ifs{1u};
while (node_it->type != IR::AbstractSyntaxNode::Type::EndIf || nested_ifs > 0) {
node_it = program.syntax_list.erase(node_it);
switch (node_it->type) {
case IR::AbstractSyntaxNode::Type::If:
++nested_ifs;
break;
case IR::AbstractSyntaxNode::Type::EndIf:
--nested_ifs;
break;
case IR::AbstractSyntaxNode::Type::Block: {
IR::Block* const block{node_it->data.block};
DeadInstElimination<false>(block);
break;
}
default:
break;
}
}
// Erase EndIf node of the if(false) branch
node_it = program.syntax_list.erase(node_it);
// Account for loop increment
--node_it;
}
}
} // namespace
void DeadCodeEliminationPass(IR::Program& program) {
DeadBranchElimination(program);
for (IR::Block* const block : program.post_order_blocks) {
DeadInstElimination<true>(block);
}
}