compiler,runtime: fix multiple definitions of a single function

strings.IndexByte was implemented in the runtime up to Go 1.11. It is
implemented using a direct call to internal/bytealg.IndexByte since Go
1.12.

Make sure we remain compatible with both.

compiler/compiler.go

@@ -820,6 +820,9 @@ func (c *Compiler) parseFunc(frame *Frame) {
 	if c.DumpSSA {
 		fmt.Printf("\nfunc %s:\n", frame.fn.Function)
 	}
+	if !frame.fn.LLVMFn.IsDeclaration() {
+		panic("function is already defined: " + frame.fn.LLVMFn.Name())
+	}
 	if !frame.fn.IsExported() {
 		frame.fn.LLVMFn.SetLinkage(llvm.InternalLinkage)
 		frame.fn.LLVMFn.SetUnnamedAddr(true)

src/runtime/string.go

@@ -166,9 +166,10 @@ func decodeUTF8(s string, index uintptr) (rune, uintptr) {
 	}
 }
 
-// indexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
-//go:linkname indexByte strings.IndexByte
-func indexByte(s string, c byte) int {
+// indexByteString returns the index of the first instance of c in s, or -1 if c
+// is not present in s.
+//go:linkname indexByteString internal/bytealg.IndexByteString
+func indexByteString(s string, c byte) int {
 	for i := 0; i < len(s); i++ {
 		if s[i] == c {
 			return i

src/runtime/strings_go111.go

@@ -0,0 +1,12 @@
+// +build !go1.12
+
+package runtime
+
+// indexByte provides compatibility with Go 1.11.
+// See the following:
+// https://github.com/tinygo-org/tinygo/issues/351
+// https://github.com/golang/go/commit/ad4a58e31501bce5de2aad90a620eaecdc1eecb8
+//go:linkname indexByte strings.IndexByte
+func indexByte(s string, c byte) int {
+	return indexByteString(s, c)
+}

testdata/stdlib.go

@@ -4,12 +4,18 @@ import (
 	"fmt"
 	"math/rand"
 	"os"
+	"strings"
 )
 
 func main() {
+	// package os, fmt
 	fmt.Println("stdin: ", os.Stdin.Fd())
 	fmt.Println("stdout:", os.Stdout.Fd())
 	fmt.Println("stderr:", os.Stderr.Fd())
 
+	// package math/rand
 	fmt.Println("pseudorandom number:", rand.Int31())
+
+	// package strings
+	fmt.Println("strings.IndexByte:", strings.IndexByte("asdf", 'd'))
 }

testdata/stdlib.txt

@@ -2,3 +2,4 @@ stdin:  0
 stdout: 1
 stderr: 2
 pseudorandom number: 1298498081
+strings.IndexByte: 2