malloc cannot alias preexisting pointers

[Richard Sandiford]

Marc Glisse <marc.glisse@inria.fr> writes:
> Hello,
>
> this patch lets gcc know that if a pointer existed before the call to 
> malloc, the result of malloc cannot alias it. This is a bit ad hoc and 
> fragile. A small improvement would be, when the 2 statements are in the 
> same bb but in the wrong order, to check if there is any statement in 
> between that might prevent from reordering them. But that's more 
> complicated, and the patch as it is already does help.
>
> I expect people may not like the call to a function from 
> tree-ssa-loop-niter too much, but it is convenient. And if someone 
> improves it, they will likely have to rewrite something not quite 
> equivalent to stmt_dominates_stmt_p.

It has linear complexity for statements in the same block though.
(reassoc_stmt_dominates_stmt_p avoids that, but relies on uids
being up-to-date.)

> The testcase uses libstdc++ quite a bit. I thought about putting it in the 
> libstdc++ testsuite, but it does not know scan-tree-dump, and in the 
> assembly it may be too late, memcpy may get expanded to something 
> target-specific. So it is in g++.dg. I could write a more artificial 
> testcase, but the behavior of std::vector is really what I want to test...
>
> Bootstrap+regtest on x86_64-pc-linux-gnu.
>
> 2019-05-13  Marc Glisse  <marc.glisse@inria.fr>
>
> gcc/
>  	* tree-ssa-loop-niter.c (stmt_dominates_stmt_p): Handle NULL
>  	basic block.
>  	* tree-ssa-alias.c: Include tree-ssa-loop-niter.h.
>  	(ptr_derefs_may_alias_p): Detect malloc and an older pointer.
>
> gcc/testsuite/
>  	* g++.dg/tree-ssa/ldist-2.C: New file.