Andy H schreef:
RFC
A problem has come up trying to fix function pointer arthmetic bugs.
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=35013
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27192
I created patch to solve this, but Anatoly has found a problem.
Without patch we had &func returning word address and (&func + 2)
returning byte address. This occurred because AVR backend did not
recognise (func+2) as program memory and did not use linker directive
gs(func+2).
With patch we get &func and &func+2 both returning word address. Which
solved the bugs reported.
Now if such a word pointer undergoes further arithmetic in c, then it
will, of course, be adjusting it by words.
The problem that Anatoly discovered is that optimization can break
this. His example involved volatile pointers but it will happen on more
realistic cases. For example if we create pointers to Foo, Foo+2,
Foo+4, optimization will try and use one pointer to create values of
other pointers. So we will get word address of Foo offset by words -
or word address of Foo offset by bytes! This just depends if the offset
calculation is done in gcc or the linker. Aghhhh!
Just for my understanding. The programpointer calculation is thus
translated to datapointer calculation?
There probably is no way of telling GCC we are dealing with different
pointer types here? Since GCC can not handle different memory types at all.
To fix is not simple. The following are ideas only - please add some more
1) One way to to get linker to perform maths on gs() using word offsets.
gs(foo+2) would be word address of foo+ 2 words. Then it would not
matter if gcc or linker performed maths. I do not know if this is
possible or what problems this might create.
2) We could avoid using gs() and get function address as byte address +
byte offset. This would require extra run-time code for function
pointer calls (to divide by two). It is useful in that same pointers
could be used to read bytes from program memory.
3) Give up and dont permit any arithmetic. (Perhaps error or warning
would still be needed)
4) Like (1) but use new directive gsw() to permit this method?
5) Like (2) but use attribute to permit this method?
6) Get gcc to recognize constant pointer maths and exclude it from
linker gs()
7) Get gcc to recognize constant pointer maths and pass to linker as
gs(Foo) + n instead of gs(Foo+n) - if this is possible.
Please add to discussion.
So they main point is to keep the knowledge that the add is in words not
bytes. Right?
Is it not possible to do all programspace addition on byte level, until
the linker? Which converts them to words? e.g. foo+2 becomes
foo+linker_know_this_is_2_words.
Wouter
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