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``

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -264,7 +264,8 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
             mir::UnwindAction::Unreachable => None,
         };
 
-        if let Some(cleanup) = unwind_target {
+        if operands.iter().any(|x| matches!(x, InlineAsmOperandRef::Label { .. })) {
+            assert!(unwind_target.is_none());
             let ret_llbb = if let Some(target) = destination {
                 fx.llbb(target)
             } else {
@@ -277,11 +278,29 @@ impl<'a, 'tcx> TerminatorCodegenHelper<'tcx> {
                 options,
                 line_spans,
                 instance,
-                Some((ret_llbb, cleanup, self.funclet(fx))),
+                Some(ret_llbb),
+                None,
+            );
+            MergingSucc::False
+        } else if let Some(cleanup) = unwind_target {
+            let ret_llbb = if let Some(target) = destination {
+                fx.llbb(target)
+            } else {
+                fx.unreachable_block()
+            };
+
+            bx.codegen_inline_asm(
+                template,
+                operands,
+                options,
+                line_spans,
+                instance,
+                Some(ret_llbb),
+                Some((cleanup, self.funclet(fx))),
             );
             MergingSucc::False
         } else {
-            bx.codegen_inline_asm(template, operands, options, line_spans, instance, None);
+            bx.codegen_inline_asm(template, operands, options, line_spans, instance, None, None);
 
             if let Some(target) = destination {
                 self.funclet_br(fx, bx, target, mergeable_succ)
@@ -1100,7 +1119,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         operands: &[mir::InlineAsmOperand<'tcx>],
         options: ast::InlineAsmOptions,
         line_spans: &[Span],
-        destination: Option<mir::BasicBlock>,
+        targets: &[mir::BasicBlock],
         unwind: mir::UnwindAction,
         instance: Instance<'_>,
         mergeable_succ: bool,
@@ -1152,6 +1171,9 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 mir::InlineAsmOperand::SymStatic { def_id } => {
                     InlineAsmOperandRef::SymStatic { def_id }
                 }
+                mir::InlineAsmOperand::Label { target_index } => {
+                    InlineAsmOperandRef::Label { label: self.llbb(targets[target_index]) }
+                }
             })
             .collect();
 
@@ -1162,7 +1184,11 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             &operands,
             options,
             line_spans,
-            destination,
+            if options.contains(InlineAsmOptions::NORETURN) {
+                None
+            } else {
+                targets.get(0).copied()
+            },
             unwind,
             instance,
             mergeable_succ,
@@ -1318,7 +1344,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 ref operands,
                 options,
                 line_spans,
-                destination,
+                ref targets,
                 unwind,
             } => self.codegen_asm_terminator(
                 helper,
@@ -1328,7 +1354,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 operands,
                 options,
                 line_spans,
-                destination,
+                targets,
                 unwind,
                 self.instance,
                 mergeable_succ(),

compiler/rustc_codegen_ssa/src/mono_item.rs

@@ -76,7 +76,8 @@ impl<'a, 'tcx: 'a> MonoItemExt<'a, 'tcx> for MonoItem<'tcx> {
                             hir::InlineAsmOperand::In { .. }
                             | hir::InlineAsmOperand::Out { .. }
                             | hir::InlineAsmOperand::InOut { .. }
-                            | hir::InlineAsmOperand::SplitInOut { .. } => {
+                            | hir::InlineAsmOperand::SplitInOut { .. }
+                            | hir::InlineAsmOperand::Label { .. } => {
                                 span_bug!(*op_sp, "invalid operand type for global_asm!")
                             }
                         })

compiler/rustc_codegen_ssa/src/traits/asm.rs

@@ -33,6 +33,9 @@ pub enum InlineAsmOperandRef<'tcx, B: BackendTypes + ?Sized> {
     SymStatic {
         def_id: DefId,
     },
+    Label {
+        label: B::BasicBlock,
+    },
 }
 
 #[derive(Debug)]
@@ -51,7 +54,8 @@ pub trait AsmBuilderMethods<'tcx>: BackendTypes {
         options: InlineAsmOptions,
         line_spans: &[Span],
         instance: Instance<'_>,
-        dest_catch_funclet: Option<(Self::BasicBlock, Self::BasicBlock, Option<&Self::Funclet>)>,
+        dest: Option<Self::BasicBlock>,
+        catch_funclet: Option<(Self::BasicBlock, Option<&Self::Funclet>)>,
     );
 }