Re: [PATCH] Avoid reuse of string buffer when concatening adjacent string litterals

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Sat, Jan 31 2015, Luc Van Oostenryck <luc.vanoostenryck@gmail.com> wrote:

> In get_string_constant(), the code tried to reuse the storage for the string
> but only if the expansion of the string was not bigger than its unexpanded form.
> But this fail when the string constant is a sequence of adjacent string litterals
> (each being possibly shared, used elsewhere, isolated or in another order).
> The minimal exemple would be something like this:
>
> #define P "\001"
> const char a[] = P "a";
> const char b[] = P "b";
>
> The expansion for 'a' will produce a string which is smaller than
> the unexpanded "\001" (2 instead of 4).
> By trying to reuse the storage, all further occurrence of "\001"
> (probably only from the same 'origin', here the macro P) will then be replaced by "\001a".
>
> The fix is thus to not try to reuse the storage for the string if it consit of
> several adjacent litterals.
>

Thanks, but there's still something wrong. Using your show-data feature
on this:

===
#define BACKSLASH "\\"
#define LETTER_t "t"

static const char s1[] = BACKSLASH;
/* static const char s2[] = BACKSLASH; */
static const char s3[] = BACKSLASH LETTER_t;
static const char s4[] = "a" BACKSLASH LETTER_t "b";
===

I get

symbol s1:
        char static const [toplevel] s1[0]
        bit_size = 16
        val = "\\"
symbol s3:
        char static const [toplevel] s3[0]
        bit_size = 24
        val = "\0t"
symbol s4:
        char static const [toplevel] s4[0]
        bit_size = 40
        val = "a\0tb"

Now if I do the same with s2 not commented out, I get


symbol s1:
        char static const [toplevel] s1[0]
        bit_size = 16
        val = "\0"
symbol s2:
        char static const [toplevel] s2[0]
        bit_size = 16
        val = "\0"
symbol s3:
        char static const [toplevel] s3[0]
        bit_size = 24
        val = "\0t"
symbol s4:
        char static const [toplevel] s4[0]
        bit_size = 40
        val = "a\0tb"

So the expansion of BACKSLASH changes depending on how often it is
expanded...

The LETTER_t thing above is because I thought I had somehow provoked a
double expansion, making BACKSLASH LETTER_t (or some variant) expand to
a single-character string containing just a tab. But I can't seem to
reproduce that particular behaviour, so maybe I'm imagining
stuff. Anyway, the above is certainly real.

Thanks,
Rasmus