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

Add asm goto support to `asm!`

Tracking issue: rust-lang#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``

src/base.rs

@@ -445,7 +445,7 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
  template,
  operands,
  options,
- destination,
+ targets,
  line_spans: _,
  unwind: _,
  } => {
@@ -456,13 +456,25 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
  );
  }
 
+ let have_labels = if options.contains(InlineAsmOptions::NORETURN) {
+ !targets.is_empty()
+ } else {
+ targets.len() > 1
+ };
+ if have_labels {
+ fx.tcx.dcx().span_fatal(
+ source_info.span,
+ "cranelift doesn't support labels in inline assembly.",
+ );
+ }
+
  crate::inline_asm::codegen_inline_asm_terminator(
  fx,
  source_info.span,
  template,
  operands,
  *options,
- *destination,
+ targets.get(0).copied(),
  );
  }
  TerminatorKind::UnwindTerminate(reason) => {

src/global_asm.rs

@@ -78,7 +78,8 @@ pub(crate) fn codegen_global_asm_item(tcx: TyCtxt<'_>, global_asm: &mut String,
  InlineAsmOperand::In { .. }
  | InlineAsmOperand::Out { .. }
  | InlineAsmOperand::InOut { .. }
- | InlineAsmOperand::SplitInOut { .. } => {
+ | InlineAsmOperand::SplitInOut { .. }
+ | InlineAsmOperand::Label { .. } => {
  span_bug!(op_sp, "invalid operand type for global_asm!")
  }
  }

src/inline_asm.rs

@@ -129,6 +129,9 @@ pub(crate) fn codegen_inline_asm_terminator<'tcx>(
  let instance = Instance::mono(fx.tcx, def_id).polymorphize(fx.tcx);
  CInlineAsmOperand::Symbol { symbol: fx.tcx.symbol_name(instance).name.to_owned() }
  }
+ InlineAsmOperand::Label { .. } => {
+ span_bug!(span, "asm! label operands are not yet supported");
+ }
  })
  .collect::<Vec<_>>();