builder: add real ThinLTO mode

We use ThinLTO for linking, but we use it in a way that doesn't give
most of its benefits: we merge all the bitcode files into a single LLVM
module and run some optimizations on it before linking. Therefore, this
works more like a traditional "full" LTO link rather than a true thin
link.

This commit adds a new experimental -lto=thin option to do a true
ThinLTO link. The main benefit is that linking will be a lot faster,
especially for large programs consisting of many packages.

At the moment, it only works for programs that don't do interface type
asserts and don't call interface methods. It also probably won't work on
WebAssembly and baremetal systems. But it's part of a larger goal
towards a truly incremental build system:
https://github.com/tinygo-org/tinygo/issues/2870
Once interface type asserts and method calls are converted to a
vtable-like implementation, most programs should just work on
linux/darwin/windows.

compileopts/config.go

@@ -165,6 +165,14 @@ func (c *Config) OptLevels() (optLevel, sizeLevel int, inlinerThreshold uint) {
 	}
 }
 
+// LTO returns one of the possible LTO configurations: legacy or thin.
+func (c *Config) LTO() string {
+	if c.Options.LTO != "" {
+		return c.Options.LTO
+	}
+	return "legacy"
+}
+
 // PanicStrategy returns the panic strategy selected for this target. Valid
 // values are "print" (print the panic value, then exit) or "trap" (issue a trap
 // instruction).

compileopts/options.go

@@ -14,6 +14,7 @@ var (
 	validPrintSizeOptions     = []string{"none", "short", "full"}
 	validPanicStrategyOptions = []string{"print", "trap"}
 	validOptOptions           = []string{"none", "0", "1", "2", "s", "z"}
+	validLTOOptions           = []string{"legacy", "thin"}
 )
 
 // Options contains extra options to give to the compiler. These options are
@@ -25,6 +26,7 @@ type Options struct {
 	GOARM           string // environment variable (only used with GOARCH=arm)
 	Target          string
 	Opt             string
+	LTO             string
 	GC              string
 	PanicStrategy   string
 	Scheduler       string
@@ -107,6 +109,12 @@ func (o *Options) Verify() error {
 		}
 	}
 
+	if o.LTO != "" {
+		if !isInArray(validLTOOptions, o.LTO) {
+			return fmt.Errorf("invalid -lto=%s: valid values are %s", o.LTO, strings.Join(validLTOOptions, ", "))
+		}
+	}
+
 	return nil
 }