Patch for misleading comment in src/share/vm/opto/node.cpp

[Peter B. Kessler]

Peter B. Kessler wrote:
> This comment is so wrong as almost to qualify as malicious.  Here's a 
> patch that might help.
> 
>      src/share/vm/opto/node.cpp
> 
>      971 // If you modify the 'this' pointer's inputs, you must use 'set_req' with
>      972 // def-use info.  If you are making a new Node (either as the new root or
>      973 // some new internal piece) you must NOT use set_req with def-use info.
>      974 // You can make a new Node with either 'new' or 'clone'.  In either case,
>      975 // def-use info is (correctly) not generated.
>      976 // Example: reshape "(X+3)+4" into "X+7":
>      977 //    set_req(1,in(1)->in(1) /* grab X */, du /* must use DU on 'this' */);
>      978 //    set_req(2,phase->intcon(7),du);
>      979 //    return this;
> -    980 // Example: reshape "X*4" into "X<<1"
> -    981 //    return new (C,3) LShiftINode( in(1), phase->intcon(1) );
> +    980 // Example: reshape "X*4" into "X<<2"
> +    981 //    return new (C,3) LShiftINode( in(1), phase->intcon(2) );
>      982 //
>      983 // You must call 'phase->transform(X)' on any new Nodes X you make, except
> -    984 // for the returned root node.  Example: reshape "X*31" with "(X<<5)-1".
> -    985 //    Node *shift=phase->transform(new(C,3)LShiftINode(in(1),phase->intcon(5)));
> -    986 //    return new (C,3) AddINode(shift, phase->intcon(-1));
> +    984 // for the returned root node.  Example: reshape "X*31" with "(X<<5)-X".
> +    985 //    Node *shift=phase->transform(new(C,3)LShiftINode(in(1),phase->intcon(5)));
> +    986 //    return new (C,3) SubINode(shift, phase->intcon(in(1)));

This last line should probably be 

 +    986 //    return new (C,3) SubINode(shift, in(1));

but I admit I'm a noob in this part of the code.  (Which is why I was reading the comments. :-)

			... peter

> The code *does* implement the transforms shown, but the transforms 
> themselves are so wrong that it doesn't give me warm fuzzy feelings 
> about trusting the author of this comment to transform my code.  (And I 
> have some worries about overflows and sign bits on the last transform, 
> but I haven't checked them out.)
> 
>             ... peter