Translate bootstrapping main from C to LLVM IR

This avoids needing a C compiler for every platform.

Makefile

@@ -9,6 +9,7 @@ tgo: build/tgo
 LLVM := $(shell go env GOPATH)/src/llvm.org/llvm/bindings/go/llvm/workdir/llvm_build/bin/
 LINK = $(LLVM)llvm-link
 LLC = $(LLVM)llc
+LLAS = $(LLVM)llvm-as
 
 CFLAGS = -Wall -Werror -Os -g -fno-exceptions -flto -ffunction-sections -fdata-sections $(LLFLAGS)
 
@@ -16,7 +17,14 @@ RUNTIME_PARTS = build/runtime.bc
 
 TARGET ?= unix
 
-ifeq ($(TARGET),pca10040)
+ifeq ($(TARGET),unix)
+# Regular *nix system.
+GCC = gcc
+LD = clang
+SIZE = size
+
+else ifeq ($(TARGET),pca10040)
+# PCA10040: nRF52832 development board
 GCC = arm-none-eabi-gcc
 LD = arm-none-eabi-ld -T arm.ld --gc-sections
 SIZE = arm-none-eabi-size
@@ -30,14 +38,12 @@ CFLAGS += -I$(CURDIR)/lib/CMSIS/CMSIS/Include
 CFLAGS += -DNRF52832_XXAA
 CFLAGS += -Wno-uninitialized
 RUNTIME_PARTS += build/runtime_nrf.bc
-RUNTIME_PARTS += build/system_nrf52.bc
-OBJ += build/startup_nrf51.o # TODO nrf52, see https://bugs.llvm.org/show_bug.cgi?id=31601
+RUNTIME_PARTS += build/nrfx_system_nrf52.bc
+OBJ += build/nrfx_startup_nrf51.o # TODO nrf52, see https://bugs.llvm.org/show_bug.cgi?id=31601
+
+else
+$(error Unknown target)
 
-else ifeq ($(TARGET),unix)
-# Regular *nix system.
-GCC = gcc
-LD = clang
-SIZE = size
 endif
 
 
@@ -75,13 +81,17 @@ build/%.bc: src/runtime/%.c src/runtime/*.h
 	@mkdir -p build
 	clang $(CFLAGS) -c -o $@ $<
 
+# Compile LLVM bitcode from LLVM source.
+build/%.bc: src/runtime/%.ll
+	$(LLAS) -o $@ $<
+
 # Compile system_* file for the nRF.
-build/%.bc: lib/nrfx/mdk/%.c
+build/nrfx_%.bc: lib/nrfx/mdk/%.c
 	@mkdir -p build
 	clang $(CFLAGS) -c -o $@ $^
 
 # Compile startup_* file for the nRF.
-build/%.o: lib/nrfx/mdk/gcc_%.S
+build/nrfx_%.o: lib/nrfx/mdk/gcc_%.S
 	@mkdir -p build
 	clang $(CFLAGS) -c -o $@ $^
 

src/runtime/runtime.c

@@ -1,21 +0,0 @@
-
-#include "runtime.h"
-#include <string.h>
-
-void go_init() __asm__("runtime.initAll");
-void go_main() __asm__("main.main");
-
-int main() {
-	go_init();
-	go_main();
-
-	return 0;
-}
-
-__attribute__((weak))
-void * memset(void *s, int c, size_t n) {
-	for (size_t i = 0; i < n; i++) {
-		((uint8_t*)s)[i] = c;
-	}
-	return s;
-}

src/runtime/runtime.ll

@@ -0,0 +1,10 @@
+source_filename = "runtime/runtime.ll"
+
+declare void @runtime.initAll()
+declare void @main.main()
+
+define i32 @main() {
+	call void @runtime.initAll()
+	call void @main.main()
+	ret i32 0
+}

tgo.go

@@ -199,8 +199,12 @@ func (c *Compiler) Parse(mainPath string, buildTags []string) error {
 
 	// After all packages are imported, add a synthetic initializer function
 	// that calls the initializer of each package.
+	initFn := c.mod.NamedFunction("runtime.initAll")
+	if initFn.IsNil() {
 		initType := llvm.FunctionType(llvm.VoidType(), nil, false)
-	initFn := llvm.AddFunction(c.mod, "runtime.initAll", initType)
+		initFn = llvm.AddFunction(c.mod, "runtime.initAll", initType)
+	}
+	initFn.SetLinkage(llvm.PrivateLinkage)
 	block := c.ctx.AddBasicBlock(initFn, "entry")
 	c.builder.SetInsertPointAtEnd(block)
 	for _, fn := range c.initFuncs {
@@ -208,6 +212,11 @@ func (c *Compiler) Parse(mainPath string, buildTags []string) error {
 	}
 	c.builder.CreateRetVoid()
 
+	// Set functions referenced in runtime.ll to internal linkage, to improve
+	// optimization (hopefully).
+	main := c.mod.NamedFunction("main.main")
+	main.SetLinkage(llvm.PrivateLinkage)
+
 	return nil
 }
 
@@ -518,6 +527,7 @@ func (c *Compiler) parseFuncDecl(f *ssa.Function) (*Frame, error) {
 // Special function parser for generated package initializers (which also
 // initializes global variables).
 func (c *Compiler) parseInitFunc(frame *Frame, f *ssa.Function) error {
+	frame.llvmFn.SetLinkage(llvm.PrivateLinkage)
 	llvmBlock := c.ctx.AddBasicBlock(frame.llvmFn, "entry")
 	c.builder.SetInsertPointAtEnd(llvmBlock)
 
@@ -605,10 +615,7 @@ func (c *Compiler) parseInitFunc(frame *Frame, f *ssa.Function) error {
 }
 
 func (c *Compiler) parseFunc(frame *Frame, f *ssa.Function) error {
-	if frame.llvmFn.Name() != "main.main" {
-		// This function is only used from within Go.
 	frame.llvmFn.SetLinkage(llvm.PrivateLinkage)
-	}
 
 	// Pre-create all basic blocks in the function.
 	for _, block := range f.DomPreorder() {
@@ -1341,15 +1348,7 @@ func Compile(pkgName, runtimePath, outpath, target string, printIR bool) error {
 		return err
 	}
 
-	parseErr := c.Parse(pkgName, buildTags)
-	if printIR {
-		fmt.Println(c.IR())
-	}
-	if parseErr != nil {
-		return parseErr
-	}
-
-	// Add C runtime.
+	// Add C/LLVM runtime.
 	runtime, err := llvm.ParseBitcodeFile(runtimePath)
 	if err != nil {
 		return err
@@ -1359,6 +1358,15 @@ func Compile(pkgName, runtimePath, outpath, target string, printIR bool) error {
 		return err
 	}
 
+	// Compile Go code to IR.
+	parseErr := c.Parse(pkgName, buildTags)
+	if printIR {
+		fmt.Println(c.IR())
+	}
+	if parseErr != nil {
+		return parseErr
+	}
+
 	c.ApplyFunctionSections() // -ffunction-sections
 
 	if err := c.Verify(); err != nil {