fix(opt): defensive panic on unmapped vreg instead of silent R0 fallback

crates/synth-synthesis/src/optimizer_bridge.rs

@@ -1431,6 +1431,17 @@ impl OptimizerBridge {
         // Helper to get ARM reg from virtual reg.
         // Also checks spill slots — if a vreg was spilled, returns R12 (IP scratch).
         // Callers should also call `reload_spill` to emit the actual load instruction.
+        //
+        // PANICS if the vreg is neither mapped nor spilled. The previous behavior was
+        // a silent `Reg::R0` fallback, which produced miscompilation: a downstream
+        // instruction reading the "unknown" vreg would silently consume whatever
+        // R0 happens to hold (often a live caller param or memset's dest pointer).
+        // Issue #93 was exactly this — `wasm_to_ir` had no handler for
+        // `I64ExtendI32U`/`I64ExtendI32S`/`I32WrapI64`, so the IR they should have
+        // produced never got mapped to ARM regs, and downstream i64 shifts read R0
+        // as their `rm_lo`/`rm_hi`, destroying the loop counter on real silicon.
+        // A loud panic here is strictly better than a quiet miscompilation —
+        // crash the compiler, not the firmware.
         let get_arm_reg =
             |vreg: &OptReg, map: &HashMap<u32, Reg>, spills: &HashMap<u32, i32>| -> Reg {
                 if let Some(&r) = map.get(&vreg.0) {
@@ -1439,7 +1450,12 @@ impl OptimizerBridge {
                     // Will be reloaded into R12 by reload_spill
                     Reg::R12
                 } else {
-                    Reg::R0
+                    panic!(
+                        "synth internal compiler error: vreg v{} has no assigned \
+                         ARM register and no spill slot. This is a wasm_to_ir bug — \
+                         likely a wasm op whose result is unmapped (see issue #93).",
+                        vreg.0
+                    );
                 }
             };