2 files changed, 9 insertions, 2 deletions
diff --git a/yjit/src/asm/x86_64/mod.rs b/yjit/src/asm/x86_64/mod.rs
index fbbfa714d8..0ef5e92117 100644
--- a/yjit/src/asm/x86_64/mod.rs
+++ b/yjit/src/asm/x86_64/mod.rs
@@ -1027,7 +1027,10 @@ pub fn mov(cb: &mut CodeBlock, dst: X86Opnd, src: X86Opnd) {
             }
 
             let output_num_bits:u32 = if mem.num_bits > 32 { 32 } else { mem.num_bits.into() };
-            assert!(imm_num_bits(imm.value) <= (output_num_bits as u8));
+            assert!(
+                mem.num_bits < 64 || imm_num_bits(imm.value) <= (output_num_bits as u8),
+                "immediate value should be small enough to survive sign extension"
+            );
             cb.write_int(imm.value as u64, output_num_bits);
         },
         // M + UImm
@@ -1042,7 +1045,10 @@ pub fn mov(cb: &mut CodeBlock, dst: X86Opnd, src: X86Opnd) {
             }
 
             let output_num_bits = if mem.num_bits > 32 { 32 } else { mem.num_bits.into() };
-            assert!(imm_num_bits(uimm.value as i64) <= (output_num_bits as u8));
+            assert!(
+                mem.num_bits < 64 || imm_num_bits(uimm.value as i64) <= (output_num_bits as u8),
+                "immediate value should be small enough to survive sign extension"
+            );
             cb.write_int(uimm.value, output_num_bits);
         },
         // * + Imm/UImm
diff --git a/yjit/src/asm/x86_64/tests.rs b/yjit/src/asm/x86_64/tests.rs
index 5ae983270f..eefcbfd52e 100644
--- a/yjit/src/asm/x86_64/tests.rs
+++ b/yjit/src/asm/x86_64/tests.rs
@@ -193,6 +193,7 @@ fn test_mov() {
     check_bytes("48c7470801000000", |cb| mov(cb, mem_opnd(64, RDI, 8), imm_opnd(1)));
     //check_bytes("67c7400411000000", |cb| mov(cb, mem_opnd(32, EAX, 4), imm_opnd(0x34))); // We don't distinguish between EAX and RAX here - that's probably fine?
     check_bytes("c7400411000000", |cb| mov(cb, mem_opnd(32, RAX, 4), imm_opnd(17)));
+    check_bytes("c7400401000080", |cb| mov(cb, mem_opnd(32, RAX, 4), uimm_opnd(0x80000001)));
     check_bytes("41895814", |cb| mov(cb, mem_opnd(32, R8, 20), EBX));
     check_bytes("4d8913", |cb| mov(cb, mem_opnd(64, R11, 0), R10));
     check_bytes("48c742f8f4ffffff", |cb| mov(cb, mem_opnd(64, RDX, -8), imm_opnd(-12)));