Re: [PATCH] pch: Fix streaming of strings with embedded null bytes

[Jakub Jelinek <jakub@redhat.com>]

On Wed, Oct 19, 2022 at 12:54:11PM +0100, Richard Sandiford via Gcc-patches wrote:
> Lewis Hyatt via Gcc-patches <gcc-patches@gcc.gnu.org> writes:
> > When a GTY'ed struct is streamed to PCH, any plain char* pointers it contains
> > (whether they live in GC-controlled memory or not) will be marked for PCH
> > output by the routine gt_pch_note_object in ggc-common.cc. This routine
> > special-cases plain char* strings, and in particular it uses strlen() to get
> > their length. Thus it does not handle strings with embedded null bytes, but it
> > is possible for something PCH cares about (such as a string literal token in a
> > macro definition) to contain such embedded nulls. To fix that up, add a new
> > GTY option "string_length" so that gt_pch_note_object can be informed the
> > actual length it ought to use, and use it in the relevant libcpp structs
> > (cpp_string and ht_identifier) accordingly.
> 
> This isn't really my area, as I'm about to demonstrate with this
> question, but: regarding
> 
>   if (note_ptr_fn == gt_pch_p_S)
>     (*slot)->size = strlen ((const char *)obj) + 1;
>   else
>     (*slot)->size = ggc_get_size (obj);
> 
> do you know why the PCH code goes out of its way to handle the sizes of
> strings specially?  Are there enough garbage strings in the string pool
> that it's worth optimising the size of the saved memory for strings but
> not for other types of object?  Or is the gt_pch_p_S test needed for
> correctness, rather than just being an optimisation?

Just guessing, not all GC strings live in the stringpool.
Isn't e.g. ggc_strdup just a GC allocation where the string length
isn't stored anywhere?  And sometimes it isn't even GC allocated,
e.g. ggc_strdup ("") just returns "";
I guess const char * pointers in GC memory can also point to string literals
in .rodata and for PCH we move them.

	Jakub