From: Jeff Law <law@redhat.com>
To: Joseph Myers <joseph@codesourcery.com>
Cc: gcc-patches <gcc-patches@gcc.gnu.org>
Subject: Re: [PR middle-end/82123] 00/06 Use EVRP range data in sprintf warnings
Date: Wed, 21 Feb 2018 02:38:00 -0000 [thread overview]
Message-ID: <f218eb8d-d9a6-d958-63b2-099738e3dff3@redhat.com> (raw)
In-Reply-To: <alpine.DEB.2.20.1802202359090.7674@digraph.polyomino.org.uk>
On 02/20/2018 05:00 PM, Joseph Myers wrote:
> Does this help with any of the cases in bug 80776 that weren't already
> fixed, or are those distinct despite looking similar?
>
I don't think so.
THe __builtin_unreachable markers are removed by vrp1 -- well before the
sprintf warning code gets run.
So the sprintf warning code never gets to exploit the properties implied
by the __builtin_unreachable calls.
It doesn't look like VRP records the narrowed ranges implied by the
__builtin_unreachable calls.
After ASSERT_EXPR insertion we have:
;; basic block 6, loop depth 0, count 1072883002 (estimated locally),
maybe hot
;; prev block 5, next block 1, flags: (NEW, REACHABLE, VISITED)
;; pred: 4 [100.0% (guessed)] count:1072883003 (estimated
locally) (FALSE_VALUE,EXECUTABLE)
i_7 = ASSERT_EXPR <i_6, (unsigned int) i_6 <= 999999>;
__builtin___sprintf_chk (&number, 1, 7, "%d", i_7);
return;
ANd the ranges computed by VRP:
i.0_1: [0, 999999]
i_4: [0, +INF]
i_6: [0, +INF] EQUIVALENCES: { i_4 } (1 elements)
i_7: [0, 999999] EQUIVALENCES: { i_4 i_6 } (2 elements)
So VRP does identify the narrow range for i_7.
But then we remove the ASSERT_EXPRs and we're left with:
i_4 = somerandom ();
i.0_1 = (unsigned int) i_4;
__builtin___sprintf_chk (&number, 1, 7, "%d", i_4);
return;
Subsequent EVRP analysis will start with the range of i_4 as a seed.
BUt there's nothing to further narrow that range.
If ASSERT_EXPR removal could be taught to use i_7 I suspect the right
things would "just happen". I haven't thought at all about what might
be required to have VRP do-the-right-thing. Given the overall desire to
drop ASSERT_EXPRs and the range propagation step in VRP in favor of EVRP
style analysis I doubt anyone is likely to spend much time on fixing
this in the old style VRP analysis.
jeff
prev parent reply other threads:[~2018-02-21 2:38 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-02-20 18:48 Jeff Law
2018-02-21 0:00 ` Joseph Myers
2018-02-21 2:38 ` Jeff Law [this message]
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