Make the instruction after an MSR write a region leader

src/backend/llvm/register_state.cpp

@@ -1,4 +1,5 @@
 #include "backend/llvm/emitter.h"
+
 #include "cpu/cpu.h"
 
 #include <llvm/ADT/SmallVector.h>
@@ -138,6 +139,43 @@ void FunctionEmitter::scanRegionLeaders() {
       region_leaders_[i] = true;
     if (i + 1u < source_.block_count && term.kind != DOLIR_TERM_FALLTHROUGH)
       region_leaders_[i + 1u] = true;
+    // A block that writes MSR can return to the dispatcher and resume at the
+    // NEXT instruction, so that instruction has to be enterable. Terminators
+    // are already covered above; this catches the fallthrough case, which is
+    // mtmsr.
+    //
+    // Keyed on an MSR write rather than on MAY_EXIT generally. MAY_EXIT is
+    // carried by a large number of instructions -- the broader rule fired
+    // 160,525 times on Colosseum -- and every extra leader fragments a region,
+    // which costs optimisation. Measured: the broad rule cost plain llvm
+    // 5-16% on the three titles that never needed it, while only a handful of
+    // addresses are ever actually re-entered.
+    //
+    // Without this, re-entry lands on a non-leader address, the entry dispatch
+    // has no case for it, and the runtime interprets from there until it
+    // reaches the next leader. Colosseum calls OSRestoreInterrupts roughly
+    // every 139 guest cycles, and its last two instructions sit exactly in that
+    // gap: 166M interpreted instructions, 92% of all dispatches in that arm.
+    // The C backend never pays this because it labels every address.
+    //
+    // REQUIRES the runtime to deliver pending external interrupts at the
+    // post-mtmsr boundary. emitStateWrite side-exits to guest_pc+4 when MSR[EE]
+    // goes 0->1; before this rule the address was not enterable, so the runtime
+    // interpreted from there and delivered the interrupt as a side effect of
+    // that detour. Removing the detour without the runtime half leaves the
+    // guest spinning with EE set on an interrupt that never arrives -- Colosseum
+    // advances 19 frames in 20s instead of 1069. native_exc is the tell: 312
+    // while hung, 25,009 once delivery is restored.
+    if (i + 1u < source_.block_count) {
+      const DolIRBlock &body = source_.blocks[i];
+      for (u32 n = 0; n < body.instruction_count; n++) {
+        if (dolir_state_mask_test(body.instructions[n].state_defs,
+                                  DOLIR_STATE_MSR)) {
+          region_leaders_[i + 1u] = true;
+          break;
+        }
+      }
+    }
     u32 count = term.kind == DOLIR_TERM_COND_BRANCH ? 2u
                 : term.kind == DOLIR_TERM_BRANCH    ? 1u
                 : term.kind == DOLIR_TERM_INDIRECT  ? 2u