Route fallback blocks through per-edge trampolines so the phi matches the CFG

src/backend/llvm/branch_targets.cpp

@@ -112,9 +112,7 @@ BasicBlock *FunctionEmitter::exitDestination(u32 pc) {
 
 BasicBlock *
 FunctionEmitter::fallbackDestination(const DolIRTerminator &terminator) {
-  BasicBlock *source = builder_.GetInsertBlock();
-  fallback_pc_->addIncoming(builder_.getInt32(terminator.guest_pc), source);
-  return fallback_block_;
+  return fallbackEdge(terminator.guest_pc);
 }
 
 BasicBlock *FunctionEmitter::fallbackEdge(u32 pc) {

src/backend/llvm/emitter.cpp

@@ -80,7 +80,7 @@ bool FunctionEmitter::emit(raw_ostream &diagnostics) {
   BasicBlock *region = nullptr;
   for (u32 i = 0; i < source_.block_count; i++) {
     if (source_.blocks[i].terminator.kind == DOLIR_TERM_FALLBACK) {
-      blocks_[i] = fallback_block_;
+      blocks_[i] = fallbackEdge(source_.blocks[i].guest_address);
       region = nullptr;
       continue;
     }

tests/llvm_fixture.cpp

@@ -42,6 +42,10 @@ static u32 psq_dform(bool store, u8 reg, u8 base, u16 displacement) {
          (displacement & 0x0fffu);
 }
 
+static u32 branch(bool link, u32 delta) {
+  return 0x48000000u | (delta & 0x03fffffcu) | (link ? 1u : 0u);
+}
+
 static bool add_chunk(DolIRModule *module, const u32 *words, u32 count,
                       u32 address) {
   PPCInst *instructions = new PPCInst[count];
@@ -339,6 +343,17 @@ int main(int argc, char **argv) {
   CHECK(add_chunk(&module, return_continuation_words, 3, 0x80003B08u));
   CHECK(add_chunk(&module, return_callee_words, 2, 0x80003C00u));
 
+  // Both indirect returns can dispatch to the linked branch's fallback
+  // continuation. The mtctr/bctr path also carries modified state into it.
+  const u32 indirect_fallback_words[] = {
+      branch(true, 0x10u),
+      0x00000000u,
+      mtspr(3, 9),
+      0x4E800420u,
+      0x4E800020u,
+  };
+  CHECK(add_chunk(&module, indirect_fallback_words, 5, 0x80003D00u));
+
   CHECK(dolir_verify(&module, stderr));
   DolLLVMOptions options{};
   options.optimization_level = 2;