feat: enable //go:linkname pragma for globals

Author: mikesmitty

compiler/symbol.go

@@ -630,7 +630,7 @@ func (c *compilerContext) addStandardAttributes(llvmFn llvm.Value) {
 // linkName is equal to .RelString(nil) on a global and extern is false, but for
 // some symbols this is different (due to //go:extern for example).
 type globalInfo struct {
-	linkName string // go:extern
+	linkName string // go:extern, go:linkname
 	extern   bool   // go:extern
 	align    int    // go:align
 	section  string // go:section
@@ -702,14 +702,6 @@ func (c *compilerContext) getGlobal(g *ssa.Global) llvm.Value {
 			})
 			llvmGlobal.AddMetadata(0, diglobal)
 		}
-
-		if info.linkName == "machine.rttSerialInstance" {
-			// Create the symbol alias and make sure neither gets optimized away.
-			alias := llvm.AddAlias(c.mod, llvmType, 0, llvmGlobal, "_SEGGER_RTT")
-			alias.SetVisibility(llvm.HiddenVisibility)
-			llvmutil.AppendToGlobal(c.mod, "llvm.used", alias)
-			llvmutil.AppendToGlobal(c.mod, "llvm.used", llvmGlobal)
-		}
 	}
 	return llvmGlobal
 }
@@ -723,14 +715,14 @@ func (c *compilerContext) getGlobalInfo(g *ssa.Global) globalInfo {
 	// Check for //go: pragmas, which may change the link name (among others).
 	doc := c.astComments[info.linkName]
 	if doc != nil {
-		info.parsePragmas(doc)
+		info.parsePragmas(doc, c, g)
 	}
 	return info
 }
 
 // Parse //go: pragma comments from the source. In particular, it parses the
-// //go:extern pragma on globals.
-func (info *globalInfo) parsePragmas(doc *ast.CommentGroup) {
+// //go:extern and //go:linkname pragmas on globals.
+func (info *globalInfo) parsePragmas(doc *ast.CommentGroup, c *compilerContext, g *ssa.Global) {
 	for _, comment := range doc.List {
 		if !strings.HasPrefix(comment.Text, "//go:") {
 			continue
@@ -751,6 +743,17 @@ func (info *globalInfo) parsePragmas(doc *ast.CommentGroup) {
 			if len(parts) == 2 {
 				info.section = parts[1]
 			}
+		case "//go:linkname":
+			if len(parts) != 3 || parts[1] != g.Name() {
+				continue
+			}
+			// Only enable go:linkname when the package imports "unsafe".
+			// This is a slightly looser requirement than what gc uses: gc
+			// requires the file to import "unsafe", not the package as a
+			// whole.
+			if hasUnsafeImport(g.Pkg.Pkg) {
+				info.linkName = parts[2]
+			}
 		}
 	}
 }

monitor.go

@@ -35,8 +35,8 @@ func Monitor(executable, port string, config *compileopts.Config) error {
 		// Use the RTT interface, which is documented (in part) here:
 		// https://wiki.segger.com/RTT
 
-		// Try to find the "machine.rttSerialInstance" symbol, which is the RTT
-		// control block.
+		// Try to find the "_SEGGER_RTT" symbol (machine.rttSerialInstance)
+		// symbol, which is the RTT control block.
 		file, err := elf.Open(executable)
 		if err != nil {
 			return fmt.Errorf("could not open ELF file to determine RTT control block: %w", err)
@@ -48,7 +48,7 @@ func Monitor(executable, port string, config *compileopts.Config) error {
 		}
 		var address uint64
 		for _, symbol := range symbols {
-			if symbol.Name == "machine.rttSerialInstance" {
+			if symbol.Name == "_SEGGER_RTT" {
 				address = symbol.Value
 				break
 			}

src/machine/serial-rtt.go

@@ -16,6 +16,8 @@ import (
 )
 
 // This symbol name is known by the compiler, see monitor.go.
+//
+//go:linkname rttSerialInstance _SEGGER_RTT
 var rttSerialInstance rttSerial
 
 var Serial = &rttSerialInstance