Add a test case

tests/assembly/x86_64-bigint-sub.rs

@@ -0,0 +1,33 @@
+//@ only-x86_64
+//@ assembly-output: emit-asm
+//@ compile-flags: --crate-type=lib -O -C target-cpu=x86-64-v4
+//@ compile-flags: -C llvm-args=-x86-asm-syntax=intel
+
+#![no_std]
+#![feature(bigint_helper_methods)]
+
+// This checks that the `borrowing_sub` implementation successfully chains, to catch
+// issues like <https://github.com/rust-lang/rust/issues/85532#issuecomment-2495119815>
+
+// This forces the ABI to avoid the windows-vs-linux ABI differences.
+
+// CHECK-LABEL: bigint_chain_borrowing_sub:
+#[no_mangle]
+pub unsafe extern "sysv64" fn bigint_chain_borrowing_sub(
+    dest: *mut u64,
+    src1: *const u64,
+    src2: *const u64,
+    n: usize,
+    mut carry: bool,
+) -> bool {
+    // CHECK: mov [[TEMP:r..]], qword ptr [rsi + 8*[[IND:r..]] + 8]
+    // CHECK: sbb [[TEMP]], qword ptr [rdx + 8*[[IND]] + 8]
+    // CHECK: mov qword ptr [rdi + 8*[[IND]] + 8], [[TEMP]]
+    // CHECK: mov [[TEMP]], qword ptr [rsi + 8*[[IND]] + 16]
+    // CHECK: sbb [[TEMP]], qword ptr [rdx + 8*[[IND]] + 16]
+    // CHECK: mov qword ptr [rdi + 8*[[IND]] + 16], [[TEMP]]
+    for i in 0..n {
+        (*dest.add(i), carry) = u64::borrowing_sub(*src1.add(i), *src2.add(i), carry);
+    }
+    carry
+}