From 199782edd16ca1fe08cee57ee2219c0a0c385fdc Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Alex=20R=C3=B8nne=20Petersen?= <alex@alexrp.com>
Date: Sat, 2 Nov 2024 08:59:14 +0100
Subject: [PATCH] riscv64: Handle writes to the zero register sensibly in
 result bookkeeping.

---
 src/arch/riscv64/CodeGen.zig | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/src/arch/riscv64/CodeGen.zig b/src/arch/riscv64/CodeGen.zig
index e54374682e..db02088271 100644
--- a/src/arch/riscv64/CodeGen.zig
+++ b/src/arch/riscv64/CodeGen.zig
@@ -1768,8 +1768,15 @@ fn finishAirBookkeeping(func: *Func) void {
 fn finishAirResult(func: *Func, inst: Air.Inst.Index, result: MCValue) void {
     if (func.liveness.isUnused(inst)) switch (result) {
         .none, .dead, .unreach => {},
-        else => unreachable, // Why didn't the result die?
+        // Why didn't the result die?
+        .register => |r| if (r != .zero) unreachable,
+        else => unreachable,
     } else {
+        switch (result) {
+            .register => |r| if (r == .zero) unreachable, // Why did we discard a used result?
+            else => {},
+        }
+
         tracking_log.debug("%{d} => {} (birth)", .{ inst, result });
         func.inst_tracking.putAssumeCapacityNoClobber(inst, InstTracking.init(result));
         // In some cases, an operand may be reused as the result.