compiler: do not check for impossible type asserts

Previously there was code to avoid impossible type asserts but it wasn't
great and in fact was too aggressive when combined with reflection.

This commit improves this by checking all types that exist in the
program that may appear in an interface (even struct fields and the
like) but without creating runtime.typecodeID objects with the type
assert. This has two advantages:

  * As mentioned, it optimizes impossible type asserts away.
  * It allows methods on types that were only asserted on (in
    runtime.typeAssert) but never used in an interface to be optimized
    away using GlobalDCE. This may have a cascading effect so that other
    parts of the code can be further optimized.

This sometimes massively improves code size and mostly negates the code
size regression of the previous commit.

compiler/compiler.go

@@ -23,7 +23,7 @@ import (
 // Version of the compiler pacakge. Must be incremented each time the compiler
 // package changes in a way that affects the generated LLVM module.
 // This version is independent of the TinyGo version number.
-const Version = 3 // last change: remove runtime.typeInInterface
+const Version = 4 // last change: runtime.typeAssert signature
 
 func init() {
 	llvm.InitializeAllTargets()

compiler/interface.go

@@ -338,10 +338,16 @@ func (b *builder) createTypeAssert(expr *ssa.TypeAssert) llvm.Value {
 		commaOk = b.createRuntimeCall("interfaceImplements", []llvm.Value{actualTypeNum, methodSet}, "")
 
 	} else {
+		globalName := "reflect/types.type:" + getTypeCodeName(expr.AssertedType) + "$id"
+		assertedTypeCodeGlobal := b.mod.NamedGlobal(globalName)
+		if assertedTypeCodeGlobal.IsNil() {
+			// Create a new typecode global.
+			assertedTypeCodeGlobal = llvm.AddGlobal(b.mod, b.ctx.Int8Type(), globalName)
+			assertedTypeCodeGlobal.SetGlobalConstant(true)
+		}
 		// Type assert on concrete type.
 		// Call runtime.typeAssert, which will be lowered to a simple icmp or
 		// const false in the interface lowering pass.
-		assertedTypeCodeGlobal := b.getTypeCode(expr.AssertedType)
 		commaOk = b.createRuntimeCall("typeAssert", []llvm.Value{actualTypeNum, assertedTypeCodeGlobal}, "typecode")
 	}
 

interp/interpreter.go

@@ -286,16 +286,10 @@ func (r *runner) run(fn *function, params []value, parentMem *memoryView, indent
 				if r.debug {
 					fmt.Fprintln(os.Stderr, indent+"typeassert:", operands[1:])
 				}
-				actualType, err := operands[1].asPointer(r)
-				if err != nil {
-					return nil, mem, r.errorAt(inst, err)
-				}
-				assertedType, err := operands[2].asPointer(r)
-				if err != nil {
-					return nil, mem, r.errorAt(inst, err)
-				}
-				result := assertedType.asRawValue(r).equal(actualType.asRawValue(r))
-				if result {
+				assertedType := operands[2].toLLVMValue(inst.llvmInst.Operand(1).Type(), &mem)
+				actualTypePtrToInt := operands[1].toLLVMValue(inst.llvmInst.Operand(0).Type(), &mem)
+				actualType := actualTypePtrToInt.Operand(0)
+				if actualType.Name()+"$id" == assertedType.Name() {
 					locals[inst.localIndex] = literalValue{uint8(1)}
 				} else {
 					locals[inst.localIndex] = literalValue{uint8(0)}

interp/testdata/interface.ll

@@ -6,10 +6,11 @@ target triple = "x86_64--linux"
 
 @main.v1 = global i1 0
 @"reflect/types.type:named:main.foo" = private constant %runtime.typecodeID { %runtime.typecodeID* @"reflect/types.type:basic:int", i64 0, %runtime.interfaceMethodInfo* null }
+@"reflect/types.type:named:main.foo$id" = external constant i8
 @"reflect/types.type:basic:int" = external constant %runtime.typecodeID
 
 
-declare i1 @runtime.typeAssert(i64, %runtime.typecodeID*, i8*, i8*)
+declare i1 @runtime.typeAssert(i64, i8*, i8*, i8*)
 
 define void @runtime.initAll() unnamed_addr {
 entry:
@@ -20,7 +21,7 @@ entry:
 define internal void @main.init() unnamed_addr {
 entry:
   ; Test type asserts.
-  %typecode = call i1 @runtime.typeAssert(i64 ptrtoint (%runtime.typecodeID* @"reflect/types.type:named:main.foo" to i64), %runtime.typecodeID* @"reflect/types.type:named:main.foo", i8* undef, i8* null)
+  %typecode = call i1 @runtime.typeAssert(i64 ptrtoint (%runtime.typecodeID* @"reflect/types.type:named:main.foo" to i64), i8* @"reflect/types.type:named:main.foo$id", i8* undef, i8* null)
   store i1 %typecode, i1* @main.v1
   ret void
 }

src/runtime/interface.go

@@ -124,7 +124,7 @@ type structField struct {
 // lowering, to assign the lowest type numbers to the types with the most type
 // asserts. Also, it is replaced with const false if this type assert can never
 // happen.
-func typeAssert(actualType uintptr, assertedType *typecodeID) bool
+func typeAssert(actualType uintptr, assertedType *uint8) bool
 
 // Pseudo function call that returns whether a given type implements all methods
 // of the given interface.

transform/interface-lowering.go

@@ -194,8 +194,11 @@ func (p *lowerInterfacesPass) run() error {
 	typeAssertUses := getUses(typeAssert)
 	for _, use := range typeAssertUses {
 		typecode := use.Operand(1)
-		name := typecode.Name()
-		p.types[name].countTypeAsserts++
+		name := typecode.Name()            // name with $id suffix
+		name = name[:len(name)-len("$id")] // remove $id suffix
+		if t, ok := p.types[name]; ok {
+			t.countTypeAsserts++
+		}
 	}
 
 	// Find all interface method calls.
@@ -371,13 +374,27 @@ func (p *lowerInterfacesPass) run() error {
 		}
 	}
 
-	// Replace each type assert with an actual type comparison.
+	// Replace each type assert with an actual type comparison or (if the type
+	// assert is impossible) the constant false.
+	llvmFalse := llvm.ConstInt(p.ctx.Int1Type(), 0, false)
 	for _, use := range typeAssertUses {
 		actualType := use.Operand(0)
-		assertedTypeGlobal := use.Operand(1)
-		p.builder.SetInsertPointBefore(use)
-		commaOk := p.builder.CreateICmp(llvm.IntEQ, llvm.ConstPtrToInt(assertedTypeGlobal, p.uintptrType), actualType, "typeassert.ok")
-		use.ReplaceAllUsesWith(commaOk)
+		name := use.Operand(1).Name()      // name with $id suffix
+		name = name[:len(name)-len("$id")] // remove $id suffix
+		if t, ok := p.types[name]; ok {
+			// The type exists in the program, so lower to a regular integer
+			// comparison.
+			p.builder.SetInsertPointBefore(use)
+			commaOk := p.builder.CreateICmp(llvm.IntEQ, llvm.ConstPtrToInt(t.typecode, p.uintptrType), actualType, "typeassert.ok")
+			use.ReplaceAllUsesWith(commaOk)
+		} else {
+			// The type does not exist in the program, so lower to a constant
+			// false. This is trivially further optimized.
+			// TODO: eventually it'll be necessary to handle reflect.PtrTo and
+			// reflect.New calls which create new types not present in the
+			// original program.
+			use.ReplaceAllUsesWith(llvmFalse)
+		}
 		use.EraseFromParentAsInstruction()
 	}
 

transform/testdata/interface.ll

@@ -5,6 +5,8 @@ target triple = "armv7m-none-eabi"
 %runtime.interfaceMethodInfo = type { i8*, i32 }
 
 @"reflect/types.type:basic:uint8" = external constant %runtime.typecodeID
+@"reflect/types.type:basic:uint8$id" = external constant i8
+@"reflect/types.type:basic:int16$id" = external constant i8
 @"reflect/types.type:basic:int" = external constant %runtime.typecodeID
 @"func NeverImplementedMethod()" = external constant i8
 @"Unmatched$interface" = private constant [1 x i8*] [i8* @"func NeverImplementedMethod()"]
@@ -14,9 +16,10 @@ target triple = "armv7m-none-eabi"
 @"reflect/types.type:named:Number" = private constant %runtime.typecodeID { %runtime.typecodeID* @"reflect/types.type:basic:int", i32 0, %runtime.interfaceMethodInfo* getelementptr inbounds ([1 x %runtime.interfaceMethodInfo], [1 x %runtime.interfaceMethodInfo]* @"Number$methodset", i32 0, i32 0) }
 
 declare i1 @runtime.interfaceImplements(i32, i8**)
-declare i1 @runtime.typeAssert(i32, %runtime.typecodeID*)
+declare i1 @runtime.typeAssert(i32, i8*)
 declare i32 @runtime.interfaceMethod(i32, i8**, i8*)
 declare void @runtime.printuint8(i8)
+declare void @runtime.printint16(i16)
 declare void @runtime.printint32(i32)
 declare void @runtime.printptr(i32)
 declare void @runtime.printnl()
@@ -52,7 +55,7 @@ typeswitch.Doubler:
   ret void
 
 typeswitch.notDoubler:
-  %isByte = call i1 @runtime.typeAssert(i32 %typecode, %runtime.typecodeID* nonnull @"reflect/types.type:basic:uint8")
+  %isByte = call i1 @runtime.typeAssert(i32 %typecode, i8* nonnull @"reflect/types.type:basic:uint8$id")
   br i1 %isByte, label %typeswitch.byte, label %typeswitch.notByte
 
 typeswitch.byte:
@@ -62,6 +65,17 @@ typeswitch.byte:
   ret void
 
 typeswitch.notByte:
+  ; this is a type assert that always fails
+  %isInt16 = call i1 @runtime.typeAssert(i32 %typecode, i8* nonnull @"reflect/types.type:basic:int16$id")
+  br i1 %isInt16, label %typeswitch.int16, label %typeswitch.notInt16
+
+typeswitch.int16:
+  %int16 = ptrtoint i8* %value to i16
+  call void @runtime.printint16(i16 %int16)
+  call void @runtime.printnl()
+  ret void
+
+typeswitch.notInt16:
   ret void
 }
 

transform/testdata/interface.out.ll

@@ -5,6 +5,8 @@ target triple = "armv7m-none-eabi"
 %runtime.interfaceMethodInfo = type { i8*, i32 }
 
 @"reflect/types.type:basic:uint8" = external constant %runtime.typecodeID
+@"reflect/types.type:basic:uint8$id" = external constant i8
+@"reflect/types.type:basic:int16$id" = external constant i8
 @"reflect/types.type:basic:int" = external constant %runtime.typecodeID
 @"func NeverImplementedMethod()" = external constant i8
 @"func Double() int" = external constant i8
@@ -12,12 +14,14 @@ target triple = "armv7m-none-eabi"
 
 declare i1 @runtime.interfaceImplements(i32, i8**)
 
-declare i1 @runtime.typeAssert(i32, %runtime.typecodeID*)
+declare i1 @runtime.typeAssert(i32, i8*)
 
 declare i32 @runtime.interfaceMethod(i32, i8**, i8*)
 
 declare void @runtime.printuint8(i8)
 
+declare void @runtime.printint16(i16)
+
 declare void @runtime.printint32(i32)
 
 declare void @runtime.printptr(i32)
@@ -63,6 +67,15 @@ typeswitch.byte:                                  ; preds = %typeswitch.notDoubl
   ret void
 
 typeswitch.notByte:                               ; preds = %typeswitch.notDoubler
+  br i1 false, label %typeswitch.int16, label %typeswitch.notInt16
+
+typeswitch.int16:                                 ; preds = %typeswitch.notByte
+  %int16 = ptrtoint i8* %value to i16
+  call void @runtime.printint16(i16 %int16)
+  call void @runtime.printnl()
+  ret void
+
+typeswitch.notInt16:                              ; preds = %typeswitch.notByte
   ret void
 }