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From: Toon Knapen (toon.knapen_at_[hidden])
Date: 2004-08-10 14:12:38

Hi all,

I got segmentation faults when trying to build my fortran project (yep,
I need to build fortran sources too so I am extending bjam v2) and got
segmentation faults. So I decided to valgrind bjam and I got quite a few
warnings. And thus I dived into the code.

Now I spotted soth and I would like to ask if my suspicion is correct.
Inside function extend_full in strings.c the new_size is calculated from
adding the length of the attachment (finish - start) to the currenc
_capacity_. I suppose it should be added to current _size_? Or am I
missing soth (for your convenience I have copied the body of exten_full

static void extend_full( string* self, char* start, char *finish )
size_t new_size = self->capacity + ( finish - start );
size_t new_capacity = self->capacity;
size_t old_size = self->capacity;
while ( new_capacity < new_size + 1)
new_capacity <<= 1;
string_reserve_internal( self, new_capacity );
memcpy( self->value + old_size, start, new_size - old_size );
self->value[new_size] = 0;
self->size = new_size;

Additionally I have a question about string_reserve_internal that is
called inside extend_full but before that I need to understand the opt
field inside the string.

IIUC the opt char-array is intended to store short (shorter than 31
chars) on the stack instead of on the heap. Is this an optimisation to
prevent allocation of short strings? But if the string is longer than 31
chars, the 32chars (of the opt-array) on the stack are wasted, right?
Can't we rely on small-object-allocation optimisation of malloc as many
compilers do now for the new operator?

So looking at string_reserve_internal, I also saw that whenever the
function is called and the opt char-array is still in use, an allocation
will be done (on the heap) regardless whether the requested capacity is
still <=32 chars. Should'nt we check first if the requested capacity is
bigger than the 32chars before mallocing?

With the answers to my question I hope to be able to hunt down my
segmentation fault and remove the valgrind warnings. If I can improve
performance in the mean time, that would be great too of course (because
currently the time between invoking bjam and the first compilation takes
at least 3 minutes on my project).



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