Rollup merge of #119365 - nbdd0121:asm-goto, r=Amanieu

Add asm goto support to `asm!`

Tracking issue: #119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``

1 files changed, 51 insertions, 0 deletions

diff --git a/tests/codegen/asm-goto.rs b/tests/codegen/asm-goto.rs
new file mode 100644
index 00000000000..e522d0da5b4
--- /dev/null
+++ b/tests/codegen/asm-goto.rs
@@ -0,0 +1,51 @@
+//@ compile-flags: -O
+//@ only-x86_64
+
+#![crate_type = "rlib"]
+#![feature(asm_goto)]
+
+use std::arch::asm;
+
+#[no_mangle]
+pub extern "C" fn panicky() {}
+
+struct Foo;
+
+impl Drop for Foo {
+    fn drop(&mut self) {
+        println!();
+    }
+}
+
+// CHECK-LABEL: @asm_goto
+#[no_mangle]
+pub unsafe fn asm_goto() {
+    // CHECK: callbr void asm sideeffect alignstack inteldialect "
+    // CHECK-NEXT: to label %[[FALLTHROUGHBB:[a-b0-9]+]] [label %[[JUMPBB:[a-b0-9]+]]]
+    asm!("jmp {}", label {});
+}
+
+// CHECK-LABEL: @asm_goto_with_outputs
+#[no_mangle]
+pub unsafe fn asm_goto_with_outputs() -> u64 {
+    let out: u64;
+    // CHECK: [[RES:%[0-9]+]] = callbr i64 asm sideeffect alignstack inteldialect "
+    // CHECK-NEXT: to label %[[FALLTHROUGHBB:[a-b0-9]+]] [label %[[JUMPBB:[a-b0-9]+]]]
+    asm!("{} /* {} */", out(reg) out, label { return 1; });
+    // CHECK: [[JUMPBB]]:
+    // CHECK-NEXT: [[RET:%.+]] = phi i64 [ [[RES]], %[[FALLTHROUGHBB]] ], [ 1, %start ]
+    // CHECK-NEXT: ret i64 [[RET]]
+    out
+}
+
+// CHECK-LABEL: @asm_goto_noreturn
+#[no_mangle]
+pub unsafe fn asm_goto_noreturn() -> u64 {
+    let out: u64;
+    // CHECK: callbr void asm sideeffect alignstack inteldialect "
+    // CHECK-NEXT: to label %unreachable [label %[[JUMPBB:[a-b0-9]+]]]
+    asm!("jmp {}", label { return 1; }, options(noreturn));
+    // CHECK: [[JUMPBB]]:
+    // CHECK-NEXT: ret i64 1
+    out
+}