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/asm.rs

@@ -107,7 +107,7 @@ enum ConstraintOrRegister {
 
 
 impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
-    fn codegen_inline_asm(&mut self, template: &[InlineAsmTemplatePiece], rust_operands: &[InlineAsmOperandRef<'tcx, Self>], options: InlineAsmOptions, span: &[Span], instance: Instance<'_>, _dest_catch_funclet: Option<(Self::BasicBlock, Self::BasicBlock, Option<&Self::Funclet>)>) {
+    fn codegen_inline_asm(&mut self, template: &[InlineAsmTemplatePiece], rust_operands: &[InlineAsmOperandRef<'tcx, Self>], options: InlineAsmOptions, span: &[Span], instance: Instance<'_>, dest: Option<Self::BasicBlock>, _catch_funclet: Option<(Self::BasicBlock, Option<&Self::Funclet>)>) {
         if options.contains(InlineAsmOptions::MAY_UNWIND) {
             self.sess().dcx()
                 .create_err(UnwindingInlineAsm { span: span[0] })
@@ -126,6 +126,10 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
         // added to `outputs.len()`
         let mut inputs = vec![];
 
+        // GCC index of a label equals its position in the array added to
+        // `outputs.len() + inputs.len()`.
+        let mut labels = vec![];
+
         // Clobbers collected from `out("explicit register") _` and `inout("expl_reg") var => _`
         let mut clobbers = vec![];
 
@@ -269,6 +273,10 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                     // some targets to add a leading underscore (Mach-O).
                     constants_len += self.tcx.symbol_name(Instance::mono(self.tcx, def_id)).name.len();
                 }
+
+                InlineAsmOperandRef::Label { label } => {
+                    labels.push(label);
+                }
             }
         }
 
@@ -368,6 +376,10 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                 InlineAsmOperandRef::Const { .. } => {
                     // processed in the previous pass
                 }
+
+                InlineAsmOperandRef::Label { .. } => {
+                    // processed in the previous pass
+                }
             }
         }
 
@@ -454,6 +466,14 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
                         InlineAsmOperandRef::Const { ref string } => {
                             template_str.push_str(string);
                         }
+
+                        InlineAsmOperandRef::Label { label } => {
+                            let label_gcc_index = labels.iter()
+                                .position(|&l| l == label)
+                                .expect("wrong rust index");
+                            let gcc_index = label_gcc_index + outputs.len() + inputs.len();
+                            push_to_template(Some('l'), gcc_index);
+                        }
                     }
                 }
             }
@@ -466,7 +486,12 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
         // 4. Generate Extended Asm block
 
         let block = self.llbb();
-        let extended_asm = block.add_extended_asm(None, &template_str);
+        let extended_asm = if let Some(dest) = dest {
+            assert!(!labels.is_empty());
+            block.end_with_extended_asm_goto(None, &template_str, &labels, Some(dest))
+        } else {
+            block.add_extended_asm(None, &template_str)
+        };
 
         for op in &outputs {
             extended_asm.add_output_operand(None, &op.to_constraint(), op.tmp_var);
@@ -494,7 +519,7 @@ impl<'a, 'gcc, 'tcx> AsmBuilderMethods<'tcx> for Builder<'a, 'gcc, 'tcx> {
         if !options.contains(InlineAsmOptions::NOSTACK) {
             // TODO(@Commeownist): figure out how to align stack
         }
-        if options.contains(InlineAsmOptions::NORETURN) {
+        if dest.is_none() && options.contains(InlineAsmOptions::NORETURN) {
             let builtin_unreachable = self.context.get_builtin_function("__builtin_unreachable");
             let builtin_unreachable: RValue<'gcc> = unsafe { std::mem::transmute(builtin_unreachable) };
             self.call(self.type_void(), None, None, builtin_unreachable, &[], None);