compiler: add function and global section pragmas

This patch adds a new pragma for functions and globals to set the
section name. This can be useful to place a function or global in a
special device specific section, for example:

  * Functions may be placed in RAM to make them run faster, or in flash
    (if RAM is the default) to not let them take up RAM.
  * DMA memory may only be placed in a special memory area.
  * Some RAM may be faster than other RAM, and some globals may be
    performance critical thus placing them in this special RAM area can
    help.
  * Some (large) global variables may need to be placed in external RAM,
    which can be done by placing them in a special section.

To use it, you have to place a function or global in a special section,
for example:

    //go:section .externalram
    var externalRAMBuffer [1024]byte

This can then be placed in a special section of the linker script, for
example something like this:

    .bss.extram (NOLOAD) : {
        *(.externalram)
    } > ERAM

compiler/compiler.go

@@ -762,6 +762,9 @@ func (c *compilerContext) createPackage(irbuilder llvm.Builder, pkg *ssa.Package
 			if !info.extern {
 				global.SetInitializer(llvm.ConstNull(global.Type().ElementType()))
 				global.SetVisibility(llvm.HiddenVisibility)
+				if info.section != "" {
+					global.SetSection(info.section)
+				}
 			}
 		}
 	}
@@ -787,6 +790,9 @@ func (b *builder) createFunction() {
 		b.llvmFn.SetVisibility(llvm.HiddenVisibility)
 		b.llvmFn.SetUnnamedAddr(true)
 	}
+	if b.info.section != "" {
+		b.llvmFn.SetSection(b.info.section)
+	}
 	if b.info.exported && strings.HasPrefix(b.Triple, "wasm") {
 		// Set the exported name. This is necessary for WebAssembly because
 		// otherwise the function is not exported.

compiler/symbol.go

@@ -24,6 +24,7 @@ type functionInfo struct {
 	module     string     // go:wasm-module
 	importName string     // go:linkname, go:export - The name the developer assigns
 	linkName   string     // go:linkname, go:export - The name that we map for the particular module -> importName
+	section    string     // go:section - object file section name
 	exported   bool       // go:export, CGo
 	nobounds   bool       // go:nobounds
 	variadic   bool       // go:variadic (CGo only)
@@ -270,6 +271,10 @@ func (info *functionInfo) parsePragmas(f *ssa.Function) {
 				if hasUnsafeImport(f.Pkg.Pkg) {
 					info.linkName = parts[2]
 				}
+			case "//go:section":
+				if len(parts) == 2 && hasUnsafeImport(f.Pkg.Pkg) {
+					info.section = parts[1]
+				}
 			case "//go:nobounds":
 				// Skip bounds checking in this function. Useful for some
 				// runtime functions.
@@ -325,6 +330,7 @@ type globalInfo struct {
 	linkName string // go:extern
 	extern   bool   // go:extern
 	align    int    // go:align
+	section  string // go:section
 }
 
 // loadASTComments loads comments on globals from the AST, for use later in the
@@ -438,6 +444,10 @@ func (info *globalInfo) parsePragmas(doc *ast.CommentGroup) {
 			if err == nil {
 				info.align = align
 			}
+		case "//go:section":
+			if len(parts) == 2 {
+				info.section = parts[1]
+			}
 		}
 	}
 }

compiler/testdata/pragma.go

@@ -39,3 +39,28 @@ func inlineFunc() {
 //go:noinline
 func noinlineFunc() {
 }
+
+// This function should have the specified section.
+//go:section .special_function_section
+func functionInSection() {
+}
+
+//export exportedFunctionInSection
+//go:section .special_function_section
+func exportedFunctionInSection() {
+}
+
+// This function should not: it's only a declaration and not a definition.
+//go:section .special_function_section
+func undefinedFunctionNotInSection()
+
+//go:section .special_global_section
+var globalInSection uint32
+
+//go:section .special_global_section
+//go:extern undefinedGlobalNotInSection
+var undefinedGlobalNotInSection uint32
+
+//go:align 1024
+//go:section .global_section
+var multipleGlobalPragmas uint32

compiler/testdata/pragma.ll

@@ -6,6 +6,9 @@ target triple = "wasm32--wasi"
 @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
+@main.globalInSection = hidden global i32 0, section ".special_global_section", align 4
+@undefinedGlobalNotInSection = external global i32, align 4
+@main.multipleGlobalPragmas = hidden global i32 0, section ".global_section", align 1024
 
 declare noalias nonnull i8* @runtime.alloc(i32, i8*, i8*)
 
@@ -38,6 +41,19 @@ entry:
   ret void
 }
 
+define hidden void @main.functionInSection(i8* %context, i8* %parentHandle) unnamed_addr section ".special_function_section" {
+entry:
+  ret void
+}
+
+define void @exportedFunctionInSection() #3 section ".special_function_section" {
+entry:
+  ret void
+}
+
+declare void @main.undefinedFunctionNotInSection(i8*, i8*)
+
 attributes #0 = { "wasm-export-name"="extern_func" }
 attributes #1 = { inlinehint }
 attributes #2 = { noinline }
+attributes #3 = { "wasm-export-name"="exportedFunctionInSection" }

transform/globals.go

@@ -11,7 +11,7 @@ import "tinygo.org/x/go-llvm"
 func ApplyFunctionSections(mod llvm.Module) {
 	llvmFn := mod.FirstFunction()
 	for !llvmFn.IsNil() {
-		if !llvmFn.IsDeclaration() {
+		if !llvmFn.IsDeclaration() && llvmFn.Section() == "" {
 			name := llvmFn.Name()
 			llvmFn.SetSection(".text." + name)
 		}