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


On Fri, 17 Aug 2018, Bernd Edlinger wrote:

> 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]

Oh, didn't realize we use char[200000000] for the STRING_CST.  Makes
my suggestion to use char[] instead not (very) much worse than the
existing practice then.

> 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 think most (all?) places look at TREE_TYPE (TREE_TYPE (string))
only.  I'm not aware of users of the array domain of the array type
of a string - but I'm far from knowing GCC inside-out ;)

> >> 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

I'd have expected sth as simple as

  if (merge_strings && str[thissize - 1] != '\0')
    thissize++;

being appended in output_constant.

Richard.


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