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/optimizer.go

@@ -97,6 +97,7 @@ func Optimize(mod llvm.Module, config *compileopts.Config, optLevel, sizeLevel i
 		// Run TinyGo-specific interprocedural optimizations.
 		OptimizeAllocs(mod)
 		OptimizeStringToBytes(mod)
+		OptimizeStringEqual(mod)
 
 	} else {
 		// Must be run at any optimization level.

transform/rtcalls.go

@@ -1,5 +1,7 @@
 package transform
 
+// This file implements several small optimizations of runtime calls.
+
 import (
 	"tinygo.org/x/go-llvm"
 )
@@ -55,3 +57,31 @@ func OptimizeStringToBytes(mod llvm.Module) {
 		}
 	}
 }
+
+// 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
+		}
+	}
+}

transform/rtcalls_test.go

@@ -13,3 +13,11 @@ func TestOptimizeStringToBytes(t *testing.T) {
 		OptimizeStringToBytes(mod)
 	})
 }
+
+func TestOptimizeStringEqual(t *testing.T) {
+	t.Parallel()
+	testTransform(t, "testdata/stringequal", func(mod llvm.Module) {
+		// Run optimization pass.
+		OptimizeStringEqual(mod)
+	})
+}

transform/testdata/stringequal.ll

@@ -0,0 +1,19 @@
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7m-none-eabi"
+
+@zeroString = constant [0 x i8] zeroinitializer
+
+declare i1 @runtime.stringEqual(i8*, i32, i8*, i32, i8*, i8*)
+
+define i1 @main.stringCompareEqualConstantZero(i8* %s1.data, i32 %s1.len, i8* %context, i8* %parentHandle) {
+entry:
+  %0 = call i1 @runtime.stringEqual(i8* %s1.data, i32 %s1.len, i8* getelementptr inbounds ([0 x i8], [0 x i8]* @zeroString, i32 0, i32 0), i32 0, i8* undef, i8* null)
+  ret i1 %0
+}
+
+define i1 @main.stringCompareUnequalConstantZero(i8* %s1.data, i32 %s1.len, i8* %context, i8* %parentHandle) {
+entry:
+  %0 = call i1 @runtime.stringEqual(i8* %s1.data, i32 %s1.len, i8* getelementptr inbounds ([0 x i8], [0 x i8]* @zeroString, i32 0, i32 0), i32 0, i8* undef, i8* null)
+  %1 = xor i1 %0, true
+  ret i1 %1
+}

transform/testdata/stringequal.out.ll

@@ -0,0 +1,19 @@
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+target triple = "armv7m-none-eabi"
+
+@zeroString = constant [0 x i8] zeroinitializer
+
+declare i1 @runtime.stringEqual(i8*, i32, i8*, i32, i8*, i8*)
+
+define i1 @main.stringCompareEqualConstantZero(i8* %s1.data, i32 %s1.len, i8* %context, i8* %parentHandle) {
+entry:
+  %0 = icmp eq i32 %s1.len, 0
+  ret i1 %0
+}
+
+define i1 @main.stringCompareUnequalConstantZero(i8* %s1.data, i32 %s1.len, i8* %context, i8* %parentHandle) {
+entry:
+  %0 = icmp eq i32 %s1.len, 0
+  %1 = xor i1 %0, true
+  ret i1 %1
+}