SpirvShader: Handle dead code branches.

src/Pipeline/SpirvShader.cpp

@@ -469,7 +469,27 @@ namespace sw
 			}
 		}
 
-		// Assign all Block::ins
+		MarkReachableBlocks(mainBlockId);
+		AssignBlockIns();
+	}
+
+	void SpirvShader::MarkReachableBlocks(Block::ID id)
+	{
+		auto it = blocks.find(id);
+		ASSERT_MSG(it != blocks.end(), "Unknown block %d", id.value());
+		auto &block = it->second;
+		if (!block.reachable)
+		{
+			block.reachable = true;
+			for (auto out : block.outs)
+			{
+				MarkReachableBlocks(out);
+			}
+		}
+	}
+
+	void SpirvShader::AssignBlockIns()
+	{
 		for (auto &it : blocks)
 		{
 			auto &blockId = it.first;
@@ -1183,6 +1203,13 @@ namespace sw
 
 	void SpirvShader::EmitBlock(Block::ID id, EmitState *state) const
 	{
+		auto &block = getBlock(id);
+
+		if (!block.reachable)
+		{
+			return;
+		}
+
 		if (state->visited.count(id) > 0)
 		{
 			return; // Already processed this block.
@@ -1190,8 +1217,6 @@ namespace sw
 
 		state->visited.emplace(id);
 
-		auto &block = getBlock(id);
-
 		switch (block.kind)
 		{
 			case Block::Simple:
@@ -1207,7 +1232,7 @@ namespace sw
 					for (auto in : block.ins)
 					{
 						EmitBlock(in, state);
-						auto inMask = state->getActiveLaneMaskEdge(in, id);
+						auto inMask = GetActiveLaneMaskEdge(state, in, id);
 						activeLaneMask.replace(0, activeLaneMask.Int(0) | inMask);
 					}
 					state->setActiveLaneMask(activeLaneMask.Int(0));
@@ -1257,7 +1282,7 @@ namespace sw
 			if (!existsPath(blockId, in)) // if not a loop back edge
 			{
 				EmitBlock(in, state);
-				loopActiveLaneMask |= state->getActiveLaneMaskEdge(in, blockId);
+				loopActiveLaneMask |= GetActiveLaneMaskEdge(state, in, blockId);
 			}
 		}
 
@@ -1310,7 +1335,7 @@ namespace sw
 						for (uint32_t i = 0; i < type.sizeInComponents; i++)
 						{
 							auto in = GenericValue(this, state->routine, varId);
-							auto mask = state->getActiveLaneMaskEdge(blockId, state->currentBlock);
+							auto mask = GetActiveLaneMaskEdge(state, blockId, state->currentBlock);
 							phi.storage[i] = phi.storage[i] | (in.Int(i) & mask);
 						}
 					}
@@ -1360,7 +1385,7 @@ namespace sw
 			if (existsPath(blockId, in))
 			{
 				EmitBlock(in, state);
-				loopActiveLaneMask |= state->getActiveLaneMaskEdge(in, blockId);
+				loopActiveLaneMask |= GetActiveLaneMaskEdge(state, in, blockId);
 			}
 		}
 
@@ -2956,7 +2981,7 @@ namespace sw
 			auto blockId = Block::ID(insn.word(w + 1));
 
 			auto in = GenericValue(this, routine, varId);
-			auto mask = state->getActiveLaneMaskEdge(blockId, state->currentBlock);
+			auto mask = GetActiveLaneMaskEdge(state, blockId, state->currentBlock);
 
 			for (uint32_t i = 0; i < type.sizeInComponents; i++)
 			{
@@ -3130,11 +3155,16 @@ namespace sw
 		}
 	}
 
-	RValue<SIMD::Int> SpirvShader::EmitState::getActiveLaneMaskEdge(Block::ID from, Block::ID to)
+	RValue<SIMD::Int> SpirvShader::GetActiveLaneMaskEdge(EmitState *state, Block::ID from, Block::ID to) const
+	{
+		if (!getBlock(from).reachable)
 		{
+			return SIMD::Int(0);
+		}
+
 		auto edge = Block::Edge{from, to};
-		auto it = edgeActiveLaneMasks.find(edge);
-		ASSERT_MSG(it != edgeActiveLaneMasks.end(), "Could not find edge %d -> %d", from.value(), to.value());
+		auto it = state->edgeActiveLaneMasks.find(edge);
+		ASSERT_MSG(it != state->edgeActiveLaneMasks.end(), "Could not find edge %d -> %d", from.value(), to.value());
 		return it->second;
 	}
 

src/Pipeline/SpirvShader.hpp

@@ -304,12 +304,13 @@ namespace sw
 			};
 
 			Kind kind;
-			InsnIterator mergeInstruction; // Merge instruction.
-			InsnIterator branchInstruction; //
+			InsnIterator mergeInstruction; // Structured control flow merge instruction.
+			InsnIterator branchInstruction; // Branch instruction.
 			ID mergeBlock; // Structured flow merge block.
 			ID continueTarget; // Loop continue block.
 			Set ins; // Blocks that branch into this block.
 			Set outs; // Blocks that this block branches to.
+			bool reachable = false;
 
 		private:
 			InsnIterator begin_;
@@ -480,6 +481,13 @@ namespace sw
 		HandleMap<Block> blocks;
 		Block::ID mainBlockId; // Block of the entry point function.
 
+		// Walks all reachable the blocks starting from id, and sets
+		// Block::reachable to true.
+		void MarkReachableBlocks(Block::ID id);
+
+		// Assigns Block::ins from Block::outs for every block.
+		void AssignBlockIns();
+
 		// DeclareType creates a Type for the given OpTypeX instruction, storing
 		// it into the types map. It is called from the analysis pass (constructor).
 		void DeclareType(InsnIterator insn);
@@ -559,10 +567,6 @@ namespace sw
 			// they will be ORed together.
 			void addActiveLaneMaskEdge(Block::ID from, Block::ID to, RValue<SIMD::Int> mask);
 
-			// Lookup the active lane mask for the edge from -> to.
-			// Asserts if the edge does not exist.
-			RValue<SIMD::Int> getActiveLaneMaskEdge(Block::ID from, Block::ID to);
-
 			SpirvRoutine *routine = nullptr; // The current routine being built.
 			rr::Value *activeLaneMaskValue = nullptr; // The current active lane mask.
 			Block::ID currentBlock; // The current block being built.
@@ -581,6 +585,11 @@ namespace sw
 		// the 'from' block to the 'to' block.
 		bool existsPath(Block::ID from, Block::ID to) const;
 
+		// Lookup the active lane mask for the edge from -> to.
+		// If from is unreachable, then a mask of all zeros is returned.
+		// Asserts if from is reachable and the edge does not exist.
+		RValue<SIMD::Int> GetActiveLaneMaskEdge(EmitState *state, Block::ID from, Block::ID to) const;
+
 		void EmitBlock(Block::ID id, EmitState *state) const;
 		void EmitInstructions(InsnIterator begin, InsnIterator end, EmitState *state) const;
 		void EmitLoop(EmitState *state) const;