compiler: avoid load/store on zero-length data

These loads/stores probably would get optimized away anyway, but not
emitting them helps the init interpreter.

compiler/compiler.go

@@ -499,6 +499,8 @@ func (c *Compiler) Compile(mainPath string) error {
 	}
 	c.builder.CreateRetVoid()
 
+	// Add a wrapper for the main.main function, either calling it directly or
+	// setting up the scheduler with it.
 	mainWrapper := c.ir.GetFunction(c.ir.Program.ImportedPackage("runtime").Members["mainWrapper"].(*ssa.Function))
 	mainWrapper.LLVMFn.SetLinkage(llvm.InternalLinkage)
 	mainWrapper.LLVMFn.SetUnnamedAddr(true)
@@ -1650,6 +1652,10 @@ func (c *Compiler) parseInstr(frame *Frame, instr ssa.Instruction) error {
 		if err != nil {
 			return err
 		}
+		if c.targetData.TypeAllocSize(llvmVal.Type()) == 0 {
+			// nothing to store
+			return nil
+		}
 		store := c.builder.CreateStore(llvmVal, llvmAddr)
 		valType := instr.Addr.Type().(*types.Pointer).Elem()
 		if c.ir.IsVolatile(valType) {
@@ -2141,12 +2147,14 @@ func (c *Compiler) parseExpr(frame *Frame, expr ssa.Value) (llvm.Value, error) {
 			buf = c.builder.CreateBitCast(buf, llvm.PointerType(typ, 0), "")
 		} else {
 			buf = c.builder.CreateAlloca(typ, expr.Comment)
+			if c.targetData.TypeAllocSize(typ) != 0 {
 				zero, err := c.getZeroValue(typ)
 				if err != nil {
 					return llvm.Value{}, err
 				}
 				c.builder.CreateStore(zero, buf) // zero-initialize var
 			}
+		}
 		return buf, nil
 	case *ssa.BinOp:
 		x, err := c.parseExpr(frame, expr.X)
@@ -3215,12 +3223,17 @@ func (c *Compiler) parseUnOp(frame *Frame, unop *ssa.UnOp) (llvm.Value, error) {
 		}
 	case token.MUL: // *x, dereference pointer
 		valType := unop.X.Type().(*types.Pointer).Elem()
+		if c.targetData.TypeAllocSize(x.Type().ElementType()) == 0 {
+			// zero-length data
+			return c.getZeroValue(x.Type().ElementType())
+		} else {
 			load := c.builder.CreateLoad(x, "")
 			if c.ir.IsVolatile(valType) {
 				// Volatile load, for memory-mapped registers.
 				load.SetVolatile(true)
 			}
 			return load, nil
+		}
 	case token.XOR: // ^x, toggle all bits in integer
 		return c.builder.CreateXor(x, llvm.ConstInt(x.Type(), ^uint64(0), false), ""), nil
 	default:

compiler/interface.go

@@ -37,6 +37,8 @@ func (c *Compiler) parseMakeInterface(val llvm.Value, typ types.Type, global str
 			c.builder.CreateStore(val, itfValueCast)
 			itfValue = c.builder.CreateBitCast(itfValueCast, c.i8ptrType, "")
 		}
+	} else if size == 0 {
+		itfValue = llvm.ConstPointerNull(c.i8ptrType)
 	} else {
 		// Directly place the value in the interface.
 		switch val.Type().TypeKind() {
@@ -152,10 +154,16 @@ func (c *Compiler) parseTypeAssert(frame *Frame, expr *ssa.TypeAssert) (llvm.Val
 		// Type assert on concrete type. Extract the underlying type from
 		// the interface (but only after checking it matches).
 		valuePtr := c.builder.CreateExtractValue(itf, 1, "typeassert.value.ptr")
-		if c.targetData.TypeAllocSize(assertedType) > c.targetData.TypeAllocSize(c.i8ptrType) {
+		size := c.targetData.TypeAllocSize(assertedType)
+		if size > c.targetData.TypeAllocSize(c.i8ptrType) {
 			// Value was stored in an allocated buffer, load it from there.
 			valuePtrCast := c.builder.CreateBitCast(valuePtr, llvm.PointerType(assertedType, 0), "")
 			valueOk = c.builder.CreateLoad(valuePtrCast, "typeassert.value.ok")
+		} else if size == 0 {
+			valueOk, err = c.getZeroValue(assertedType)
+			if err == nil {
+				return llvm.Value{}, err
+			}
 		} else {
 			// Value was stored directly in the interface.
 			switch assertedType.TypeKind() {