On Fri, Dec 16, 2016 at 3:41 PM, Aldy Hernandez <al...@redhat.com> wrote:
> Hi folks.
>
> This is a follow-up on Jeff and Richi's interaction on the aforementioned PR
> here:
>
> https://gcc.gnu.org/ml/gcc-patches/2016-08/msg01397.html
>
> I decided to explore the idea of analyzing may-undefness on-demand, which
> actually looks rather cheap.
>
> BTW, I don't understand why we don't have auto_bitmap's, as we already have
> auto_sbitmap's. I've implemented the former based on auto_sbitmap's code we
> already have.
>
> The attached patch fixes the bug without introducing any regressions.
>
> I also tested the patch by compiling 242 .ii files with -O3. These were
> gathered from a stage1 build with -save-temps. There is a slight time
> degradation of 4 seconds within 27 minutes of user time:
>
> tainted: 26:52
> orig: 26:48
>
> This was the average aggregate time of two runs compiling all 242 .ii files.
> IMO, this looks reasonable. It is after all, -O3. Is it acceptable?
+ while (!worklist.is_empty ())
+ {
+ name = worklist.pop ();
+ gcc_assert (TREE_CODE (name) == SSA_NAME);
+
+ if (ssa_undefined_value_p (name, true))
+ return true;
+
+ bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name));
it should be already set as we use visited_ssa as "was it ever on the worklist",
so maybe renaming it would be a good thing as well.
+ if (TREE_CODE (name) == SSA_NAME)
+ {
+ /* If an SSA has already been seen, this may be a loop.
+ Fail conservatively. */
+ if (bitmap_bit_p (visited_ssa, SSA_NAME_VERSION (name)))
+ return false;
so to me "conservative" is returning true, not false.
+ bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name));
+ worklist.safe_push (name);
but for loops we can just continue and ignore this use. And bitmap_set_bit
returns whether it set a bit, thus
if (bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name)))
worklist.safe_push (name);
should work?
+ /* Check that any SSA names used to define NAME is also fully
+ defined. */
+ use_operand_p use_p;
+ ssa_op_iter iter;
+ FOR_EACH_SSA_USE_OPERAND (use_p, def, iter, SSA_OP_USE)
+ {
+ name = USE_FROM_PTR (use_p);
+ if (TREE_CODE (name) == SSA_NAME)
always true.
+ {
+ /* If an SSA has already been seen, this may be a loop.
+ Fail conservatively. */
+ if (bitmap_bit_p (visited_ssa, SSA_NAME_VERSION (name)))
+ return false;
+ bitmap_set_bit (visited_ssa, SSA_NAME_VERSION (name));
+ worklist.safe_push (name);
See above.
In principle the thing is sound but I'd like to be able to pass in a bitmap of
known maybe-undefined/must-defined SSA names to have a cache for
multiple invocations from within a pass (like this unswitching case).
Also once you hit defs that are in a post-dominated region of the loop entry
you can treat them as not undefined (as their use invokes undefined
behavior). This is also how you treat function parameters (well,
ssa_undefined_value_p does), where the call site invokes undefined behavior
when passing in undefined values. So we need an extra parameter specifying
the post-dominance region.
You do not handle memory or calls conservatively which means the existing
testcase only needs some obfuscation to become a problem again. To fix
that before /* Check that any SSA names used to define NAME is also fully
defined. */ bail out conservatively, like
if (! is_gimple_assign (def)
|| gimple_assign_single_p (def))
return true;
Only unswitching on conditions that post-dominate the loop entry might be a
simpler solution for the PR in question. OTOH this may disable too much
unswitching in practice.
Richard.
> Aldy
