shader: treat call-target registers with several reaching definitions as GPU-selected

src/graphics/shader/recompiler/ShaderRecompiler.cpp

@@ -504,8 +504,11 @@ void PrepareCallTargets(ShaderSource& source, const CompileOptions& options) {
 	};
 	Register = [&](uint32_t before, uint32_t code) -> Value {
 		std::vector<uint32_t> crossed_calls;
-		const auto index = CFG::FindScalarDefinition(source.decoded, graph, before, code,
-		                                             source.wave_size, crossed_calls);
+		const auto definition = CFG::FindScalarDefinition(source.decoded, graph, before, code,
+		                                                  source.wave_size, crossed_calls);
+		// Several reaching definitions select the value on the GPU, so no SRT walk evaluates it.
+		if (!definition) return Value(&block.AppendNewInst(ValueOpcode::UndefU32));
+		const auto index = *definition;
 		for (const auto crossed: crossed_calls) {
 			std::ranges::find(source.calls, crossed, &ShaderSource::Call::instruction)->preserved_registers.set(code);
 		}

src/graphics/shader/recompiler/frontend/cfg/ShaderCFG.cpp

@@ -1720,9 +1720,9 @@ bool MayWriteScalarRegister(const Decoder::Instruction& inst, uint32_t code, uin
 	return InstructionWritesScalarCode(inst, code);
 }
 
-uint32_t FindScalarDefinition(const Decoder::Program& program, const Graph& graph,
-                              uint32_t before, uint32_t code, uint32_t wave_size,
-                              std::vector<uint32_t>& crossed_calls) {
+std::optional<uint32_t> FindScalarDefinition(const Decoder::Program& program, const Graph& graph,
+                                             uint32_t before, uint32_t code, uint32_t wave_size,
+                                             std::vector<uint32_t>& crossed_calls) {
 	EXIT_IF(before >= program.instructions.size());
 	const auto found = std::ranges::find_if(graph.blocks, [before](const auto& block) {
 		return block.inst_begin <= before && before < block.inst_end;
@@ -1748,7 +1748,7 @@ uint32_t FindScalarDefinition(const Decoder::Program& program, const Graph& grap
 			if (MayWriteScalarRegister(inst, code, wave_size)) { index = i; break; }
 		}
 		if (index != UINT32_MAX || block->id == graph.entry_block) {
-			EXIT_IF(definition && *definition != index);
+			if (definition && *definition != index) return std::nullopt;
 			definition = index;
 			continue;
 		}
@@ -1758,7 +1758,7 @@ uint32_t FindScalarDefinition(const Decoder::Program& program, const Graph& grap
 		}
 	}
 	EXIT_IF(!definition);
-	return *definition;
+	return definition;
 }
 
 std::string BranchConditionToString(BranchCondition condition) {

src/graphics/shader/recompiler/frontend/cfg/ShaderCFG.h

@@ -5,6 +5,7 @@
 #include "common/stringUtils.h"
 #include "graphics/shader/recompiler/frontend/decode/ShaderDecoder.h"
 
+#include <optional>
 #include <vector>
 
 namespace Libs::Graphics::ShaderRecompiler::CFG {
@@ -114,9 +115,11 @@ struct Graph {
 
 Graph       BuildGraph(const Decoder::Program& program);
 bool MayWriteScalarRegister(const Decoder::Instruction& inst, uint32_t code, uint32_t wave_size);
-uint32_t FindScalarDefinition(const Decoder::Program& program, const Graph& graph,
-                              uint32_t before, uint32_t code, uint32_t wave_size,
-                              std::vector<uint32_t>& crossed_calls);
+// Returns the one instruction defining `code` that reaches `before` (UINT32_MAX: the entry value),
+// or nullopt if several do.
+std::optional<uint32_t> FindScalarDefinition(const Decoder::Program& program, const Graph& graph,
+                                             uint32_t before, uint32_t code, uint32_t wave_size,
+                                             std::vector<uint32_t>& crossed_calls);
 // Returns structured control flow or failure diagnostics without changing the native graph.
 // On failure, failure_block is an original block ID or UINT32_MAX.
 Graph       Structurize(const Graph& graph);

tests/shaderCfgTests.cpp

@@ -11125,6 +11125,14 @@ void TestMemoryFedScalarLeafCall() {
   const auto skipped = PrepareShaderSource(gpu_callable, options);
   Check(skipped.unsupported_call_pc == 12u && skipped.calls.empty(),
         "GPU-buffer callable did not report its unsupported call site");
+  // The table index s20 is loop-carried, so two definitions reach the call.
+  const uint32_t loop_callable[] = {EncodeSmem0(0x08, 20, 0), 0xfa000000u,
+      EncodeSmem0(0x09, 14, 0), 20u << 25u, EncodeSop1(0x21, 14, 14),
+      EncodeSmem0(0x08, 20, 0), 20u << 25u, EncodeSopc(0x07, 20, 128),
+      EncodeSopp(0x05, 0xfff9u), EncodeSopp(0x01)};
+  const auto loop_skipped = PrepareShaderSource(loop_callable, options);
+  Check(loop_skipped.unsupported_call_pc == 16u && loop_skipped.calls.empty(),
+        "loop-selected GPU-buffer callable did not report its unsupported call site");
   auto source = PrepareShaderSource(shader, options);
   Check(source.calls.size() == 1 && source.decoded.has_swap_pc &&
             source.decoded.instructions[source.calls.front().instruction].branch_target == UINT32_MAX,