On Fri, Nov 3, 2017 at 1:35 PM, Bin Cheng <bin.ch...@arm.com> wrote:
> Hi,
> This is a simple patch exploiting more undefined pointer overflow behavior in
> loop niter analysis. Originally, it only supports POINTER_PLUS_EXPR if the
> offset part is IV. This patch also handles the case if pointer is IV. With
> this patch, the while(true) loop in test can be removed by cddce pass now.
>
> Bootstrap and test on x86_64 and AArch64. This patch introduces two failures:
> FAIL: g++.dg/pr79095-1.C -std=gnu++98 (test for excess errors)
> FAIL: g++.dg/pr79095-2.C -std=gnu++11 (test for excess errors)
> I believe this exposes inaccurate value range information issue. For below
> code:
> /* { dg-do compile } */
> /* { dg-options "-Wall -O3" } */
>
> typedef long unsigned int size_t;
>
> inline void
> fill (int *p, size_t n, int)
> {
> while (n--)
> *p++ = 0;
> }
>
> struct B
> {
> int* p0, *p1, *p2;
>
> size_t size () const {
> return size_t (p1 - p0);
> }
>
> void resize (size_t n) {
> if (n > size())
> append (n - size());
> }
>
> void append (size_t n)
> {
> if (size_t (p2 - p1) >= n) {
> fill (p1, n, 0);
> }
> }
> };
>
> void foo (B &b)
> {
> if (b.size () != 0)
> b.resize (b.size () - 1);
> }
>
> GCC gives below warning with this patch:
> pr79095-1.C: In function ‘void foo(B&)’:
> pr79095-1.C:10:7: warning: iteration 4611686018427387903 invokes undefined
> behavior [-Waggressive-loop-optimizations]
> *p++ = 0;
> ~^~
> pr79095-1.C:9:11: note: within this loop
> while (n--)
> ^~
>
> Problem is VRP should understand that it's never the case with condition:
> (size_t (p2 - p1) >= n)
> in function B::append.
>
> So, any comment?
I'm looking hard but I can't see you changed anything in
infer_loop_bounds_from_pointer_arith
besides adding a expr_invariant_in_loop_p (loop, rhs2) check.
What am I missing?
Richard.
> Thanks,
> bin
> 2017-11-02 Bin Cheng <bin.ch...@arm.com>
>
> PR tree-optimization/82776
> * tree-ssa-loop-niter.c (infer_loop_bounds_from_pointer_arith): Handle
> POINTER_PLUS_EXPR in which the pointer is an IV.
> (infer_loop_bounds_from_signedness): Refine comment.
>
> gcc/testsuite
> 2017-11-02 Bin Cheng <bin.ch...@arm.com>
>
> PR tree-optimization/82776
> * g++.dg/pr82776.C: New test.
> * gcc.dg/tree-ssa/split-path-6.c: Refine test.
