pass,printer: display required ISA features (#120)

Fixes #119

pass/isa.go

@@ -0,0 +1,31 @@
+package pass
+
+import (
+	"sort"
+
+	"github.com/mmcloughlin/avo/ir"
+)
+
+// RequiredISAExtensions determines ISA extensions required for the given
+// function. Populates the ISA field.
+func RequiredISAExtensions(fn *ir.Function) error {
+	// Collect ISA set.
+	set := map[string]bool{}
+	for _, i := range fn.Instructions() {
+		for _, isa := range i.ISA {
+			set[isa] = true
+		}
+	}
+
+	if len(set) == 0 {
+		return nil
+	}
+
+	// Populate the function's ISA field with the unique sorted list.
+	for isa := range set {
+		fn.ISA = append(fn.ISA, isa)
+	}
+	sort.Strings(fn.ISA)
+
+	return nil
+}