transform: optimize string comparisons against ""

This optimizes a common pattern like:

    if s != "" {
        ...
    }

to:

    if len(s) != 0 {
        ...
    }

This avoids a runtime call and thus produces slightly better code.

transform/rtcalls.go

@@ -0,0 +1,87 @@
+package transform
+
+// This file implements several small optimizations of runtime calls.
+
+import (
+	"tinygo.org/x/go-llvm"
+)
+
+// OptimizeStringToBytes transforms 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 OptimizeStringToBytes(mod llvm.Module) {
+	stringToBytes := 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) {
+			if use.IsAExtractValueInst().IsNil() {
+				// Expected an extractvalue, but this is something else.
+				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 !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()
+		}
+	}
+}
+
+// OptimizeStringEqual transforms runtime.stringEqual(...) calls into simple
+// integer comparisons if at least one of the sides of the comparison is zero.
+// Ths converts str == "" into len(str) == 0 and "" == "" into false.
+func OptimizeStringEqual(mod llvm.Module) {
+	stringEqual := mod.NamedFunction("runtime.stringEqual")
+	if stringEqual.IsNil() {
+		// nothing to optimize
+		return
+	}
+
+	builder := mod.Context().NewBuilder()
+	defer builder.Dispose()
+
+	for _, call := range getUses(stringEqual) {
+		str1len := call.Operand(1)
+		str2len := call.Operand(3)
+
+		zero := llvm.ConstInt(str1len.Type(), 0, false)
+		if str1len == zero || str2len == zero {
+			builder.SetInsertPointBefore(call)
+			icmp := builder.CreateICmp(llvm.IntEQ, str1len, str2len, "")
+			call.ReplaceAllUsesWith(icmp)
+			call.EraseFromParentAsInstruction()
+			continue
+		}
+	}
+}