compiler: optimize runtime.stringToBytes calls

This optimization makes sure the following pattern doesn't do a heap
allocation (assuming Write doesn't modify the slice):

    var w *machine.UART = ...
    w.Write([]byte("foo"))

As long as Write doesn't modify the slice and LLVM can detect this, a
call to runtime.stringToBytes with the necessary allocation + copy is
avoided.

compiler/optimizer.go

@@ -37,6 +37,7 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) {
 		goPasses.Run(c.mod)
 
 		// Run Go-specific optimization passes.
+		c.OptimizeStringToBytes()
 		c.OptimizeAllocs()
 		c.Verify()
 	}
@@ -48,6 +49,56 @@ func (c *Compiler) Optimize(optLevel, sizeLevel int, inlinerThreshold uint) {
 	modPasses.Run(c.mod)
 }
 
+// Transform runtime.stringToBytes(...) calls into const []byte slices whenever
+// possible. This optimizes the following pattern:
+//     w.Write([]byte("foo"))
+// where Write does not store to the slice.
+func (c *Compiler) OptimizeStringToBytes() {
+	stringToBytes := c.mod.NamedFunction("runtime.stringToBytes")
+	if stringToBytes.IsNil() {
+		// nothing to optimize
+		return
+	}
+
+	for _, call := range getUses(stringToBytes) {
+		strptr := call.Operand(0)
+		strlen := call.Operand(1)
+
+		// strptr is always constant because strings are always constant.
+
+		convertedAllUses := true
+		for _, use := range getUses(call) {
+			nilValue := llvm.Value{}
+			if use.IsAExtractValueInst() == nilValue {
+				convertedAllUses = false
+				continue
+			}
+			switch use.Type().TypeKind() {
+			case llvm.IntegerTypeKind:
+				// A length (len or cap). Propagate the length value.
+				use.ReplaceAllUsesWith(strlen)
+				use.EraseFromParentAsInstruction()
+			case llvm.PointerTypeKind:
+				// The string pointer itself.
+				if !c.isReadOnly(use) {
+					convertedAllUses = false
+					continue
+				}
+				use.ReplaceAllUsesWith(strptr)
+				use.EraseFromParentAsInstruction()
+			default:
+				// should not happen
+				panic("unknown return type of runtime.stringToBytes: " + use.Type().String())
+			}
+		}
+		if convertedAllUses {
+			// Call to runtime.stringToBytes can be eliminated: both the input
+			// and the output is constant.
+			call.EraseFromParentAsInstruction()
+		}
+	}
+}
+
 // Basic escape analysis: translate runtime.alloc calls into alloca
 // instructions.
 func (c *Compiler) OptimizeAllocs() {
@@ -122,25 +173,8 @@ func (c *Compiler) doesEscape(value llvm.Value) bool {
 			// Call only escapes when the (pointer) parameter is not marked
 			// "nocapture". This flag means that the parameter does not escape
 			// the give function.
-			fn := use.CalledValue()
-			if fn.IsAFunction() == nilValue {
-				// This is not a function but something else, like a function
-				// pointer.
-				return false
-			}
-			nocaptureKind := llvm.AttributeKindID("nocapture")
-			for i := 0; i < fn.ParamsCount(); i++ {
-				if use.Operand(i) != value {
-					// This is not the parameter we're checking.
-					continue
-				}
-				index := i + 1 // param attributes start at 1
-				nocapture := fn.GetEnumAttributeAtIndex(index, nocaptureKind)
-				nilAttribute := llvm.Attribute{}
-				if nocapture == nilAttribute {
-					// Parameter doesn't have the nocapture flag, so may escape.
-					return true
-				}
+			if !c.hasFlag(use, value, "nocapture") {
+				return true
 			}
 		} else {
 			// Unknown instruction, might escape.
@@ -152,6 +186,56 @@ func (c *Compiler) doesEscape(value llvm.Value) bool {
 	return false
 }
 
+// Check whether the given value (which is of pointer type) is never stored to.
+func (c *Compiler) isReadOnly(value llvm.Value) bool {
+	uses := getUses(value)
+	for _, use := range uses {
+		nilValue := llvm.Value{}
+		if use.IsAGetElementPtrInst() != nilValue {
+			if !c.isReadOnly(use) {
+				return false
+			}
+		} else if use.IsACallInst() != nilValue {
+			if !c.hasFlag(use, value, "readonly") {
+				return false
+			}
+		} else {
+			// Unknown instruction, might not be readonly.
+			return false
+		}
+	}
+	return true
+}
+
+// Check whether all uses of this param as parameter to the call have the given
+// flag. In most cases, there will only be one use but a function could take the
+// same parameter twice, in which case both must have the flag.
+// A flag can be any enum flag, like "readonly".
+func (c *Compiler) hasFlag(call, param llvm.Value, kind string) bool {
+	fn := call.CalledValue()
+	nilValue := llvm.Value{}
+	if fn.IsAFunction() == nilValue {
+		// This is not a function but something else, like a function pointer.
+		return false
+	}
+	kindID := llvm.AttributeKindID(kind)
+	for i := 0; i < fn.ParamsCount(); i++ {
+		if call.Operand(i) != param {
+			// This is not the parameter we're checking.
+			continue
+		}
+		index := i + 1 // param attributes start at 1
+		attr := fn.GetEnumAttributeAtIndex(index, kindID)
+		nilAttribute := llvm.Attribute{}
+		if attr == nilAttribute {
+			// At least one parameter doesn't have the flag (there may be
+			// multiple).
+			return false
+		}
+	}
+	return true
+}
+
 // Return a list of values (actually, instructions) where this value is used as
 // an operand.
 func getUses(value llvm.Value) []llvm.Value {