From 3bbdcf084846b8da1e2c30e7bb215a1aecccad78 Mon Sep 17 00:00:00 2001
From: Alan Wu <XrXr@users.noreply.github.com>
Date: Fri, 18 Jul 2025 15:31:07 -0400
Subject: [PATCH] ZJIT: Remove no-op movs after register allocation

Previously `no_dead_mov_from_vreg` generated:

    0x0: ldur x0, [x0]
    0x4: mov x0, x0
    0x8: ret

Because of phase ordering. Split couldn't recognize that the no-op mov
because at that point it sees a `VReg`.
---
 zjit/src/backend/arm64/mod.rs | 14 ++++++++++++++
 zjit/src/backend/lir.rs       |  3 +++
 2 files changed, 17 insertions(+)

diff --git a/zjit/src/backend/arm64/mod.rs b/zjit/src/backend/arm64/mod.rs
index e97c06414d..dc5c7e6058 100644
--- a/zjit/src/backend/arm64/mod.rs
+++ b/zjit/src/backend/arm64/mod.rs
@@ -1424,6 +1424,20 @@ mod tests {
         ");
     }
 
+    #[test]
+    fn no_dead_mov_from_vreg() {
+        let (mut asm, mut cb) = setup_asm();
+
+        let ret_val = asm.load(Opnd::mem(64, C_RET_OPND, 0));
+        asm.cret(ret_val);
+
+        asm.compile_with_num_regs(&mut cb, 1);
+        assert_disasm!(cb, "000040f8c0035fd6", "
+            0x0: ldur x0, [x0]
+            0x4: ret
+        ");
+    }
+
     #[test]
     fn test_emit_add() {
         let (mut asm, mut cb) = setup_asm();
diff --git a/zjit/src/backend/lir.rs b/zjit/src/backend/lir.rs
index 94c53569b4..47ba5ddf70 100644
--- a/zjit/src/backend/lir.rs
+++ b/zjit/src/backend/lir.rs
@@ -1752,6 +1752,9 @@ impl Assembler
                         asm.push_insn(Insn::PosMarker(end_marker));
                     }
                 }
+                Insn::Mov { src, dest } | Insn::LoadInto { dest, opnd: src } if src == dest => {
+                    // Remove no-op move now that VReg are resolved to physical Reg
+                }
                 _ => asm.push_insn(insn),
             }