Re: [PATCH] Check the STRING_CSTs in varasm.c

[Bernd Edlinger <bernd.edlinger@hotmail.de>]

On 08/17/18 14:19, Richard Biener wrote:
> On Fri, 17 Aug 2018, Bernd Edlinger wrote:
> 
>> Richard Biener wrote:
>>> +embedded @code{NUL} characters.  However, the
>>> +@code{TREE_STRING_LENGTH} always includes a trailing @code{NUL} that
>>> +is not part of the language string literal but appended by the front end.
>>> +If the string shall not be @code{NUL}-terminated the @code{TREE_TYPE}
>>> +is one character shorter than @code{TREE_STRING_LENGTH}.
>>> +Excess caracters other than one trailing @code{NUL} character are not
> 
> characters btw.
> 

thanks, updated.

> I read the above that the string literal for
> 
> char x[2] = "1";
> 
> is actually "1\0\0" - there's one NUL that is not part of the language
> string literal.  The second sentence then suggests that both \0
> are removed because 2 is less than 3?
> 

maybe 2 is a bad example, lets consider:
char x[2000000000] = "1";

That is a string_cst with STRING_LENGTH = 2, content = "2\0\0"
the array_type is used on both x, and the string_cst,
I was assuming that both tree objects refer to the same type object.
which is char[0..2000000000-1]

varasm assembles the bytes that are given by STRING_LENGTH
and appends zeros as appropriate.

> As said, having this extra semantics of a STRING_CST tied to
> another tree node (its TREE_TYPE) looks ugly.
> 
>>> +permitted.
>>>
>>> I find this very confusing and oppose to that change.  Can we get
>>> back to the drawing board please?  If we want an easy way to
>>> see whether a string is "properly" terminated then maybe we can
>>> simply use a flag that gets set by build_string?
>>>
>>
>> What I mean with that is the case like
>> char x[2] = "123456";
>>
>> which is build_string(7, "123456"), but with a type char[2],
>> so varasm throws away "3456\0".
> 
> I think varasm throws away chars not because of the type of
> the STRING_CST but because of the available storage in x.
> 

But at other places we look at the type of the string_cst, don't we?
Shouldn't those be the same?

>> I want to say that this is not okay, the excess precision
>> should only be used to strip the nul termination, in cases
>> where it is intended to be a assembled as a not zero terminated
>> string.  But maybe the wording could be improved?
> 
> ISTR we always assemble a NUL in .strings to get string merging
> working.
> 

String merging is not working when the string is not explicitly
NUL terminated, my followup patch here tries to fix that:

[PATCH] Handle not explicitly zero terminated strings in merge sections
https://gcc.gnu.org/ml/gcc-patches/2018-08/msg00481.html


Bernd.