irutil: add instruction and terminator operand use-tracking and value update API

example_test.go

@@ -0,0 +1,154 @@
+package irutil_test
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/llir/irutil"
+	"github.com/llir/llvm/asm"
+	"github.com/llir/llvm/ir"
+	"github.com/llir/llvm/ir/value"
+	"github.com/pkg/errors"
+)
+
+func Example() {
+	// Parse LLVM IR module.
+	const module = `
+define void @g() {
+	; potential side effects
+	ret void
+}
+
+define i32 @f(i32 %x, i32 %y) {
+; <label>:0
+	%1 = add i32 %x, %y
+	%2 = xor i32 %1, 10
+	%3 = sub i32 %x, %y
+	call void @g()
+	%4 = mul i32 3, 5
+	ret i32 %3
+}`
+	m, err := asm.ParseString("foo.ll", module)
+	if err != nil {
+		log.Fatalf("%+v", err)
+	}
+	// Perform dead code elimination (dce).
+	for _, f := range m.Funcs {
+		if err := dce(f); err != nil {
+			log.Fatalf("%+v", err)
+		}
+	}
+	// Print LLVM IR module after dce.
+	fmt.Println(m)
+
+	// Output:
+	//
+	// define void @g() {
+	// ; <label>:0
+	// 	ret void
+	// }
+	//
+	// define i32 @f(i32 %x, i32 %y) {
+	// ; <label>:0
+	// 	%1 = sub i32 %x, %y
+	// 	call void @g()
+	// 	ret i32 %1
+	// }
+}
+
+// dce performs dead code elimination on f, pruning value instructions with
+// unused results in f. Call instructions are never pruned as they may have side
+// effects.
+func dce(f *ir.Func) error {
+	// Prune until we reach fixed-point. If we prune an unused value instruction,
+	// this may result in another local variable becoming unused.
+	for {
+		// Get names of local variables used in f.
+		usedIdents, err := getUsedIdents(f)
+		if err != nil {
+			return errors.WithStack(err)
+		}
+		// Prune value instructions with unused results in f.
+		if !pruneUnusedInsts(f, usedIdents) {
+			// Fix-point reached, no more pruning possible.
+			break
+		}
+	}
+	return nil
+}
+
+// pruneUnusedInsts prunes value instructions with unused results in f. Call
+// instructions are never pruned as they may have side effects. The boolean
+// return value indicates whether an instruction was pruned.
+func pruneUnusedInsts(f *ir.Func, usedIdents map[string]bool) bool {
+	pruned := false
+	// Prune value instructions with unused results in f.
+	for _, block := range f.Blocks {
+		insts := block.Insts[:0] // filter instructions in-place.
+		for _, inst := range block.Insts {
+			if isUnused(inst, usedIdents) {
+				// Prune unused value instruction, skip append to insts.
+				pruned = true
+				continue
+			}
+			insts = append(insts, inst)
+		}
+		block.Insts = insts
+	}
+	if pruned {
+		// Reset IDs of unnamed local variables (as one or more instructions have
+		// been pruned).
+		irutil.ResetNames(f)
+	}
+	return pruned
+}
+
+// ValueInstruction is an instruction producing a local variable result.
+//
+// Note: call instructions are value instructions, but only produce a result if
+// the return type of their callee is non-void.
+type ValueInstruction interface {
+	ir.Instruction
+	value.Named
+}
+
+// isUnused reports whether the given instruction is a value instruction with
+// unused results. Call instructions are never considered unused, as they may
+// have side effects.
+func isUnused(inst ir.Instruction, usedIdents map[string]bool) bool {
+	valueInst, ok := inst.(ValueInstruction)
+	if !ok {
+		// Non-value instructions are never considered unused.
+		return false
+	}
+	if _, ok := inst.(*ir.InstCall); ok {
+		// Call instructions are never considered unused as they may have side
+		// effects.
+		return false
+	}
+	return !usedIdents[valueInst.Name()]
+}
+
+// getUsedIdents returns the names of local variables used in f.
+func getUsedIdents(f *ir.Func) (map[string]bool, error) {
+	// Assign IDs to unnamed local variables (as we need to record the names of
+	// local variables used in f).
+	if err := f.AssignIDs(); err != nil {
+		return nil, errors.WithStack(err)
+	}
+	// Pre-allocate backing array on stack for uses slice.
+	var backing [100]irutil.Use
+	// Record names of local variables used used in f.
+	uses := irutil.FuncUses(backing[:0], f) // the backing array is optional, pass nil if not needed.
+	usedIdents := make(map[string]bool)
+	for _, use := range uses {
+		valueInst, ok := (*use.Val).(ValueInstruction)
+		if !ok {
+			// Only record the names of local variables produced by value
+			// instructions.
+			continue
+		}
+		usedIdents[valueInst.Name()] = true
+	}
+	return usedIdents, nil
+}

go.mod

@@ -2,4 +2,7 @@ module github.com/llir/irutil
 
 go 1.12
 
-require github.com/llir/llvm v0.3.0-pre7.0.20191220191840-fc6d0e8d9bc6
+require (
+	github.com/llir/llvm v0.3.0
+	github.com/pkg/errors v0.8.1
+)

go.sum

@@ -1,27 +1,27 @@
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