d/dmd/sideeffect.d

/**
 * Find side-effects of expressions.
 *
 * Copyright:   Copyright (C) 1999-2021 by The D Language Foundation, All Rights Reserved
 * Authors:     $(LINK2 http://www.digitalmars.com, Walter Bright)
 * License:     $(LINK2 http://www.boost.org/LICENSE_1_0.txt, Boost License 1.0)
 * Source:      $(LINK2 https://github.com/dlang/dmd/blob/master/src/dmd/sideeffect.d, _sideeffect.d)
 * Documentation:  https://dlang.org/phobos/dmd_sideeffect.html
 * Coverage:    https://codecov.io/gh/dlang/dmd/src/master/src/dmd/sideeffect.d
 */

module dmd.sideeffect;

import dmd.apply;
import dmd.astenums;
import dmd.declaration;
import dmd.dscope;
import dmd.expression;
import dmd.expressionsem;
import dmd.func;
import dmd.globals;
import dmd.identifier;
import dmd.init;
import dmd.mtype;
import dmd.tokens;
import dmd.visitor;

/**************************************************
 * Front-end expression rewriting should create temporary variables for
 * non trivial sub-expressions in order to:
 *  1. save evaluation order
 *  2. prevent sharing of sub-expression in AST
 */
extern (C++) bool isTrivialExp(Expression e)
{
    extern (C++) final class IsTrivialExp : StoppableVisitor
    {
        alias visit = typeof(super).visit;
    public:
        extern (D) this()
        {
        }

        override void visit(Expression e)
        {
            /* https://issues.dlang.org/show_bug.cgi?id=11201
             * CallExp is always non trivial expression,
             * especially for inlining.
             */
            if (e.op == TOK.call)
            {
                stop = true;
                return;
            }
            // stop walking if we determine this expression has side effects
            stop = lambdaHasSideEffect(e);
        }
    }

    scope IsTrivialExp v = new IsTrivialExp();
    return walkPostorder(e, v) == false;
}

/********************************************
 * Determine if Expression has any side effects.
 *
 * Params:
 *   e = the expression
 *   assumeImpureCalls = whether function calls should always be assumed to
 *                       be impure (e.g. debug is allowed to violate purity)
 */
extern (C++) bool hasSideEffect(Expression e, bool assumeImpureCalls = false)
{
    extern (C++) final class LambdaHasSideEffect : StoppableVisitor
    {
        alias visit = typeof(super).visit;
    public:
        extern (D) this()
        {
        }

        override void visit(Expression e)
        {
            // stop walking if we determine this expression has side effects
            stop = lambdaHasSideEffect(e, assumeImpureCalls);
        }
    }

    scope LambdaHasSideEffect v = new LambdaHasSideEffect();
    return walkPostorder(e, v);
}

/********************************************
 * Determine if the call of f, or function type or delegate type t1, has any side effects.
 * Returns:
 *      0   has any side effects
 *      1   nothrow + constant purity
 *      2   nothrow + strong purity
 */
int callSideEffectLevel(FuncDeclaration f)
{
    /* https://issues.dlang.org/show_bug.cgi?id=12760
     * ctor call always has side effects.
     */
    if (f.isCtorDeclaration())
        return 0;
    assert(f.type.ty == Tfunction);
    TypeFunction tf = cast(TypeFunction)f.type;
    if (tf.isnothrow)
    {
        PURE purity = f.isPure();
        if (purity == PURE.strong)
            return 2;
        if (purity == PURE.const_)
            return 1;
    }
    return 0;
}

int callSideEffectLevel(Type t)
{
    t = t.toBasetype();
    TypeFunction tf;
    if (t.ty == Tdelegate)
        tf = cast(TypeFunction)(cast(TypeDelegate)t).next;
    else
    {
        assert(t.ty == Tfunction);
        tf = cast(TypeFunction)t;
    }
    if (!tf.isnothrow)  // function can throw
        return 0;

    tf.purityLevel();
    PURE purity = tf.purity;
    if (t.ty == Tdelegate && purity > PURE.weak)
    {
        if (tf.isMutable())
            purity = PURE.weak;
        else if (!tf.isImmutable())
            purity = PURE.const_;
    }

    if (purity == PURE.strong)
        return 2;
    if (purity == PURE.const_)
        return 1;
    return 0;
}

private bool lambdaHasSideEffect(Expression e, bool assumeImpureCalls = false)
{
    switch (e.op)
    {
    // Sort the cases by most frequently used first
    case TOK.assign:
    case TOK.plusPlus:
    case TOK.minusMinus:
    case TOK.declaration:
    case TOK.construct:
    case TOK.blit:
    case TOK.addAssign:
    case TOK.minAssign:
    case TOK.concatenateAssign:
    case TOK.concatenateElemAssign:
    case TOK.concatenateDcharAssign:
    case TOK.mulAssign:
    case TOK.divAssign:
    case TOK.modAssign:
    case TOK.leftShiftAssign:
    case TOK.rightShiftAssign:
    case TOK.unsignedRightShiftAssign:
    case TOK.andAssign:
    case TOK.orAssign:
    case TOK.xorAssign:
    case TOK.powAssign:
    case TOK.in_:
    case TOK.remove:
    case TOK.assert_:
    case TOK.halt:
    case TOK.delete_:
    case TOK.new_:
    case TOK.newAnonymousClass:
        return true;
    case TOK.call:
        {
            if (assumeImpureCalls)
                return true;

            if (e.type && e.type.ty == Tnoreturn)
                return true;

            CallExp ce = cast(CallExp)e;
            /* Calling a function or delegate that is pure nothrow
             * has no side effects.
             */
            if (ce.e1.type)
            {
                Type t = ce.e1.type.toBasetype();
                if (t.ty == Tdelegate)
                    t = (cast(TypeDelegate)t).next;
                if (t.ty == Tfunction && (ce.f ? callSideEffectLevel(ce.f) : callSideEffectLevel(ce.e1.type)) > 0)
                {
                }
                else
                    return true;
            }
            break;
        }
    case TOK.cast_:
        {
            CastExp ce = cast(CastExp)e;
            /* if:
             *  cast(classtype)func()  // because it may throw
             */
            if (ce.to.ty == Tclass && ce.e1.op == TOK.call && ce.e1.type.ty == Tclass)
                return true;
            break;
        }
    default:
        break;
    }
    return false;
}

/***********************************
 * The result of this expression will be discarded.
 * Print error messages if the operation has no side effects (and hence is meaningless).
 * Returns:
 *      true if expression has no side effects
 */
bool discardValue(Expression e)
{
    if (lambdaHasSideEffect(e)) // check side-effect shallowly
        return false;
    switch (e.op)
    {
    case TOK.cast_:
        {
            CastExp ce = cast(CastExp)e;
            if (ce.to.equals(Type.tvoid))
            {
                /*
                 * Don't complain about an expression with no effect if it was cast to void
                 */
                return false;
            }
            break; // complain
        }
    case TOK.error:
        return false;
    case TOK.variable:
        {
            VarDeclaration v = (cast(VarExp)e).var.isVarDeclaration();
            if (v && (v.storage_class & STC.temp))
            {
                // https://issues.dlang.org/show_bug.cgi?id=5810
                // Don't complain about an internal generated variable.
                return false;
            }
            break;
        }
    case TOK.call:
        /* Issue 3882: */
        if (global.params.warnings != DiagnosticReporting.off && !global.gag)
        {
            CallExp ce = cast(CallExp)e;
            if (e.type.ty == Tvoid)
            {
                /* Don't complain about calling void-returning functions with no side-effect,
                 * because purity and nothrow are inferred, and because some of the
                 * runtime library depends on it. Needs more investigation.
                 *
                 * One possible solution is to restrict this message to only be called in hierarchies that
                 * never call assert (and or not called from inside unittest blocks)
                 */
            }
            else if (ce.e1.type)
            {
                Type t = ce.e1.type.toBasetype();
                if (t.ty == Tdelegate)
                    t = (cast(TypeDelegate)t).next;
                if (t.ty == Tfunction && (ce.f ? callSideEffectLevel(ce.f) : callSideEffectLevel(ce.e1.type)) > 0)
                {
                    const(char)* s;
                    if (ce.f)
                        s = ce.f.toPrettyChars();
                    else if (ce.e1.op == TOK.star)
                    {
                        // print 'fp' if ce.e1 is (*fp)
                        s = (cast(PtrExp)ce.e1).e1.toChars();
                    }
                    else
                        s = ce.e1.toChars();
                    e.warning("calling `%s` without side effects discards return value of type `%s`; prepend a `cast(void)` if intentional", s, e.type.toChars());
                }
            }
        }
        return false;
    case TOK.andAnd:
    case TOK.orOr:
        {
            LogicalExp aae = cast(LogicalExp)e;
            return discardValue(aae.e2);
        }
    case TOK.question:
        {
            CondExp ce = cast(CondExp)e;
            /* https://issues.dlang.org/show_bug.cgi?id=6178
             * https://issues.dlang.org/show_bug.cgi?id=14089
             * Either CondExp::e1 or e2 may have
             * redundant expression to make those types common. For example:
             *
             *  struct S { this(int n); int v; alias v this; }
             *  S[int] aa;
             *  aa[1] = 0;
             *
             * The last assignment statement will be rewitten to:
             *
             *  1 in aa ? aa[1].value = 0 : (aa[1] = 0, aa[1].this(0)).value;
             *
             * The last DotVarExp is necessary to take assigned value.
             *
             *  int value = (aa[1] = 0);    // value = aa[1].value
             *
             * To avoid false error, discardValue() should be called only when
             * the both tops of e1 and e2 have actually no side effects.
             */
            if (!lambdaHasSideEffect(ce.e1) && !lambdaHasSideEffect(ce.e2))
            {
                return discardValue(ce.e1) |
                       discardValue(ce.e2);
            }
            return false;
        }
    case TOK.comma:
        {
            CommaExp ce = cast(CommaExp)e;
            // Don't complain about compiler-generated comma expressions
            if (ce.isGenerated)
                return false;

            // Don't check e1 until we cast(void) the a,b code generation.
            // This is concretely done in expressionSemantic, if a CommaExp has Tvoid as type
            return discardValue(ce.e2);
        }
    case TOK.tuple:
        /* Pass without complaint if any of the tuple elements have side effects.
         * Ideally any tuple elements with no side effects should raise an error,
         * this needs more investigation as to what is the right thing to do.
         */
        if (!hasSideEffect(e))
            break;
        return false;
    default:
        break;
    }
    e.error("`%s` has no effect", e.toChars());
    return true;
}

/**************************************************
 * Build a temporary variable to copy the value of e into.
 * Params:
 *  stc = storage classes will be added to the made temporary variable
 *  name = name for temporary variable
 *  e = original expression
 * Returns:
 *  Newly created temporary variable.
 */
VarDeclaration copyToTemp(StorageClass stc, const char[] name, Expression e)
{
    assert(name[0] == '_' && name[1] == '_');
    auto vd = new VarDeclaration(e.loc, e.type,
        Identifier.generateId(name),
        new ExpInitializer(e.loc, e));
    vd.storage_class = stc | STC.temp | STC.ctfe; // temporary is always CTFEable
    return vd;
}

/**************************************************
 * Build a temporary variable to extract e's evaluation, if e is not trivial.
 * Params:
 *  sc = scope
 *  name = name for temporary variable
 *  e0 = a new side effect part will be appended to it.
 *  e = original expression
 *  alwaysCopy = if true, build new temporary variable even if e is trivial.
 * Returns:
 *  When e is trivial and alwaysCopy == false, e itself is returned.
 *  Otherwise, a new VarExp is returned.
 * Note:
 *  e's lvalue-ness will be handled well by STC.ref_ or STC.rvalue.
 */
Expression extractSideEffect(Scope* sc, const char[] name,
    ref Expression e0, Expression e, bool alwaysCopy = false)
{
    //printf("extractSideEffect(e: %s)\n", e.toChars());

    /* The trouble here is that if CTFE is running, extracting the side effect
     * results in an assignment, and then the interpreter says it cannot evaluate the
     * side effect assignment variable. But we don't have to worry about side
     * effects in function calls anyway, because then they won't CTFE.
     * https://issues.dlang.org/show_bug.cgi?id=17145
     */
    if (!alwaysCopy &&
        ((sc.flags & SCOPE.ctfe) ? !hasSideEffect(e) : isTrivialExp(e)))
        return e;

    auto vd = copyToTemp(0, name, e);
    vd.storage_class |= e.isLvalue() ? STC.ref_ : STC.rvalue;

    e0 = Expression.combine(e0, new DeclarationExp(vd.loc, vd)
                                .expressionSemantic(sc));

    return new VarExp(vd.loc, vd)
           .expressionSemantic(sc);
}