From: Richard Biener <richard.guenther@gmail.com>
To: "bin.cheng" <bin.cheng@linux.alibaba.com>
Cc: GCC Patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PATCH PR100740]Fix overflow check in simplifying exit cond comparing two IVs.
Date: Wed, 2 Jun 2021 09:27:08 +0200 [thread overview]
Message-ID: <CAFiYyc0S_3xZsR9q7Lc3ECaWQ-_=6j=i54JYAnM_oRJxEPb+CQ@mail.gmail.com> (raw)
In-Reply-To: <b348cd70-f131-4efc-b980-95ae68bd4a8e.bin.cheng@linux.alibaba.com>
On Tue, Jun 1, 2021 at 4:00 PM bin.cheng via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
>
> Hi,
> As described in patch summary, this fixes the wrong code issue by adding overflow-ness
> check for iv1.step - iv2.step.
>
> Bootstrap and test on x86_64. Any comments?
+ bool wrap_p = TYPE_OVERFLOW_WRAPS (step_type);
+ if (wrap_p)
+ {
+ tree t = fold_binary_to_constant (GE_EXPR, step_type,
+ iv0->step, iv1->step);
+ wrap_p = integer_zerop (t);
+ }
I think we can't use TYPE_OVERFLOW_WRAPS/TYPE_OVERFLOW_UNDEFINED since
that's only relevant for expressions written by the user - we're
computing iv0.step - iv1.step
which can even overflow when TYPE_OVERFLOW_UNDEFINED (in fact we may not
even generate this expression then!). So I think we have to do sth like
/* If the iv0->step - iv1->step wraps, fail. */
if (!operand_equal_p (iv0->step, iv1->step)
&& (TREE_CODE (iv0->step) != INTEGER_CST || TREE_CODE
(iv1->step) != INTEGER_CST)
&& !wi::gt (wi::to_widest (iv0->step), wi::to_widest (iv1->step))
return false;
which only handles equality and all integer constant steps. You could
also use ranges
like
wide_int min0, max0, min1, max1;
if (!operand_equal_p (iv->step, iv1->step)
&& (determine_value_range (iv0->step, &min0, &max0) != VR_RANGE
|| determine_value_range (iv1->step, &min1, &max1) != VR_RANGE
|| !wi::ge (min0, max1)))
return false;
Note I'm not sure why
iv0->step = step;
if (!POINTER_TYPE_P (type))
iv0->no_overflow = false;
here the no_overflow reset does not happen for pointer types? Or
rather why does
it happen at all? Don't we strictly make the step less in absolute value?
> Thanks,
> bin
next prev parent reply other threads:[~2021-06-02 7:27 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-06-01 11:15 bin.cheng
2021-06-02 7:27 ` Richard Biener [this message]
2021-06-06 10:01 ` Bin.Cheng
2021-06-07 14:35 ` Richard Biener
2021-07-01 12:19 ` Richard Biener
2021-07-02 1:37 ` Bin.Cheng
2021-07-02 7:57 ` Richard Biener
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