CFG: Handle degenerate selection constructs.

Apparently, it's legal to use a selection construct where both paths
branch to same location, but a different merge point is used.
This breaks many assumptions the variable scope analyzer makes.

The only logical way to generate code for this scenario is to treat the
selection construct as a trivial switch construct with only a default
case.

1 files changed, 43 insertions, 0 deletions

diff --git a/spirv_parser.cpp b/spirv_parser.cpp
index d50d2e84..58fcec10 100644
--- a/spirv_parser.cpp
+++ b/spirv_parser.cpp
@@ -961,6 +961,49 @@ void Parser::parse(const Instruction &instruction)
 		current_block->false_block = ops[2];
 
 		current_block->terminator = SPIRBlock::Select;
+
+		if (current_block->true_block == current_block->false_block)
+		{
+			// Bogus conditional, translate to a direct branch.
+			// Avoids some ugly edge cases later when analyzing CFGs.
+
+			// There are some super jank cases where the merge block is different from the true/false,
+			// and later branches can "break" out of the selection construct this way.
+			// This is complete nonsense, but CTS hits this case.
+			// In this scenario, we should see the selection construct as more of a Switch with one default case.
+			// The problem here is that this breaks any attempt to break out of outer switch statements,
+			// but it's theoretically solvable if this ever comes up using the ladder breaking system ...
+
+			if (current_block->true_block != current_block->next_block &&
+			    current_block->merge == SPIRBlock::MergeSelection)
+			{
+				uint32_t ids = ir.increase_bound_by(2);
+
+				SPIRType type;
+				type.basetype = SPIRType::Int;
+				type.width = 32;
+				set<SPIRType>(ids, type);
+				auto &c = set<SPIRConstant>(ids + 1, ids);
+
+				current_block->condition = c.self;
+				current_block->default_block = current_block->true_block;
+				current_block->terminator = SPIRBlock::MultiSelect;
+				ir.block_meta[current_block->next_block] &= ~ParsedIR::BLOCK_META_SELECTION_MERGE_BIT;
+				ir.block_meta[current_block->next_block] |= ParsedIR::BLOCK_META_MULTISELECT_MERGE_BIT;
+			}
+			else
+			{
+				ir.block_meta[current_block->next_block] &= ~ParsedIR::BLOCK_META_SELECTION_MERGE_BIT;
+				current_block->next_block = current_block->true_block;
+				current_block->condition = 0;
+				current_block->true_block = 0;
+				current_block->false_block = 0;
+				current_block->merge_block = 0;
+				current_block->merge = SPIRBlock::MergeNone;
+				current_block->terminator = SPIRBlock::Direct;
+			}
+		}
+
 		current_block = nullptr;
 		break;
 	}