Re: A CTFE Segfault (with explanation, but I'm not sure what the fix is).

[Gareth Charnock]

Done.

http://d.puremagic.com/issues/show_bug.cgi?id=5248

I'm sorry if I'm a bit over-cautious before submitting bugs.

On 21/11/10 22:18, Brad Roberts wrote:

On 11/21/2010 2:11 PM, Gareth Charnock wrote:

This code makes the DMD compiler segfault

struct LeafType {
     string Compile_not_ovloaded() {
         return "expression";
     }
};

struct MatrixASTNode {
     LeafType Right;

     string Compile() {
         return  Right.Compile_not_ovloaded();
     }
};

void main() {
     enum AST = MatrixASTNode();
     enum s=AST.Compile();
}

I'm not sure if it's valid D, but MatrixASTNode.Compile seems like it should be
evaluable at compile time. segfaults are bad anyway. A search with the keyword
"segfault" didn't seem to turn up any existing bugs with promising titles, so I
assume this is a new bug (I'll put it in bugzilla if nobody can think of an
existing bug).

I've tried compiling a debug version of svn dmd and I traced the bug down to
what appears to be a stack overflow. These two functions in interpret.c seem to
call each other repeatedly.

Expression *ThisExp::interpret(InterState *istate)
{
     if (istate&&  istate->localThis)
         return istate->localThis->interpret(istate);
     error("value of 'this' is not known at compile time");
     return EXP_CANT_INTERPRET;
}

Expression *DotVarExp::interpret(InterState *istate)
{   Expression *e = EXP_CANT_INTERPRET;

#if LOG
     printf("DotVarExp::interpret() %s\n", toChars());
#endif

     Expression *ex = e1->interpret(istate); //<- we never get past here
     if (ex != EXP_CANT_INTERPRET)
     {
         if (ex->op == TOKstructliteral)
         {   StructLiteralExp *se = (StructLiteralExp *)ex;
             VarDeclaration *v = var->isVarDeclaration();
             if (v)
             {   e = se->getField(type, v->offset);
                 if (!e)
                 {
                     error("couldn't find field %s in %s", v->toChars(),
type->toChars());
                     e = EXP_CANT_INTERPRET;
                 }
                 return e;
             }
         }
         else
             error("%s.%s is not yet implemented at compile time", 
e1->toChars(),
var->toChars());
     }


If you turn logging on for the file you get this:

CallExp::interpret() MatrixASTNode(LeafType()).Compile()

********
FuncDeclaration::interpret(istate = (nil)) Compile
cantInterpret = 0, semanticRun = 5
StructLiteralExp::interpret() MatrixASTNode(LeafType())
StructLiteralExp::interpret() LeafType()
CompoundStatement::interpret()
ExpStatement::interpret(assert(&this,"null this"))
AssertExp::interpret() assert(&this,"null this")
StructLiteralExp::interpret() MatrixASTNode(LeafType())
StructLiteralExp::interpret() LeafType()
ReturnStatement::interpret(this.Right.Compile_not_ovloaded())
CallExp::interpret() this.Right.Compile_not_ovloaded()

********
FuncDeclaration::interpret(istate = 0xbfe685a0) Compile_not_ovloaded
cantInterpret = 0, semanticRun = 5
DotVarExp::interpret() this.Right
StructLiteralExp::interpret() MatrixASTNode(LeafType())
StructLiteralExp::interpret() LeafType()
CompoundStatement::interpret()
ExpStatement::interpret(assert(&this,"null this"))
AssertExp::interpret() assert(&this,"null this")
DotVarExp::interpret() this.Right
DotVarExp::interpret() this.Right
DotVarExp::interpret() this.Right
DotVarExp::interpret() this.Right
...an so on until stack overflow


The reason for the recursion happens is that in the contect of
DotVarExp::interpret istate->localThis == this so in ThisExp::interpret the
statement

istate->localThis->interpret(istate);

goes right back to DotVarExp::interpret again.

Unfortunately I don't really know enough about the internals of dmd so say what
the fix is, but I hope this information is helpful.

Please file a bug report:  http://d.puremagic.com/issues/