Hi,
For now, SCEV may compute iv base in the form of "(signed T)((unsigned
T)base + step))".  This complicates other optimizations/analysis depending
on SCEV because it's hard to dive into type conversions.  For many cases,
such type conversions can be simplified with additional range information
implied by loop initial conditions.  This patch does such simplification.
With simplified iv base, loop niter analysis can compute more accurate bound
information since sensible value range can be derived for "base+step".  For
example, accurate loop bound&may_be_zero information is computed for cases
added by this patch.
The code is actually borrowed from loop_exits_before_overflow.  Moreover,
with simplified iv base, the second case handled in that function now
becomes the first case.  I didn't remove that part of code because it may(?)
still be visited in scev analysis itself and simple_iv isn't an interface
for that.

Is it OK?

Thanks,
bin

2015-07-28  Bin Cheng  <bin.cheng@arm.com>

	* tree-ssa-loop-niter.c (tree_simplify_using_condition): Export
	the interface.
	* tree-ssa-loop-niter.h (tree_simplify_using_condition): Declare.
	* tree-scalar-evolution.c (simple_iv): Simplify type conversions
	in iv base using loop initial conditions.

gcc/testsuite/ChangeLog
2015-07-28  Bin Cheng  <bin.cheng@arm.com>

	* gcc.dg/tree-ssa/loop-bound-2.c: New test.
	* gcc.dg/tree-ssa/loop-bound-4.c: New test.
	* gcc.dg/tree-ssa/loop-bound-6.c: New test.