compiler: add //go:linkname support for globals

We previously used our own //go:extern but //go:linkname is probably the
better choice since it's used in the math/bits package.

compiler/symbol.go

@@ -688,20 +688,25 @@ 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, 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) {
+func (info *globalInfo) parsePragmas(doc *ast.CommentGroup, g *ssa.Global) {
 	for _, comment := range doc.List {
 		if !strings.HasPrefix(comment.Text, "//go:") {
 			continue
 		}
 		parts := strings.Fields(comment.Text)
 		switch parts[0] {
+		case "//go:linkname":
+			if len(parts) == 3 && g.Name() == parts[1] {
+				info.linkName = parts[2]
+				info.extern = true
+			}
 		case "//go:extern":
 			info.extern = true
 			if len(parts) == 2 {

compiler/testdata/pragma.go

@@ -2,6 +2,12 @@ package main
 
 import _ "unsafe"
 
+// Use the go:linkname mechanism to link this global to a different package.
+// This is used in math/bits.
+//
+//go:linkname linknamedGlobal runtime.testLinknamedGlobal
+var linknamedGlobal int
+
 // Creates an external global with name extern_global.
 //
 //go:extern extern_global

compiler/testdata/pragma.ll

@@ -3,6 +3,7 @@ source_filename = "pragma.go"
 target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20"
 target triple = "wasm32-unknown-wasi"
 
+@runtime.testLinknamedGlobal = external global i32, align 4
 @extern_global = external global [0 x i8], align 1
 @main.alignedGlobal = hidden global [4 x i32] zeroinitializer, align 32
 @main.alignedGlobal16 = hidden global [4 x i32] zeroinitializer, align 16