BPF Archive on lore.kernel.org
 help / color / Atom feed
From: Yonghong Song <yhs@fb.com>
To: Jiong Wang <jiong.wang@netronome.com>
Cc: "alexei.starovoitov@gmail.com" <alexei.starovoitov@gmail.com>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>,
	"oss-drivers@netronome.com" <oss-drivers@netronome.com>
Subject: Re: [LLVM PATCH] bpf: fix wrong truncation elimination when there is back-edge/loop
Date: Wed, 16 Oct 2019 19:03:59 +0000
Message-ID: <e12b61aa-9348-60a5-8948-517bf8781d6c@fb.com> (raw)
In-Reply-To: <4l08k8n8.fsf@cbtest28.netronome.com>



On 10/16/19 8:33 AM, Jiong Wang wrote:
> 
> Jiong Wang writes:
> 
>> Yonghong Song writes:
>>
>>> On 10/12/19 12:18 AM, Jiong Wang wrote:
>>>> Currently, BPF backend is doing truncation elimination. If one truncation
>>>> is performed on a value defined by narrow loads, then it could be redundant
>>>> given BPF loads zero extend the destination register implicitly.
>>>>
>>>> When the definition of the truncated value is a merging value (PHI node)
>>>> that could come from different code paths, then checks need to be done on
>>>> all possible code paths.
>>>>
>>>> Above described optimization was introduced as r306685, however it doesn't
>>>> work when there is back-edge, for example when loop is used inside BPF
>>>> code.
>>>>
>>>> For example for the following code, a zero-extended value should be stored
>>>> into b[i], but the "and reg, 0xffff" is wrongly eliminated which then
>>>> generates corrupted data.
>>>>
>>>> void cal1(unsigned short *a, unsigned long *b, unsigned int k)
>>>> {
>>>>     unsigned short e;
>>>>
>>>>     e = *a;
>>>>     for (unsigned int i = 0; i < k; i++) {
>>>>       b[i] = e;
>>>>       e = ~e;
>>>>     }
>>>> }
>>>>
>>>> The reason is r306685 was trying to do the PHI node checks inside isel
>>>> DAG2DAG phase, and the checks are done on MachineInstr. This is actually
>>>> wrong, because MachineInstr is being built during isel phase and the
>>>> associated information is not completed yet. A quick search shows none
>>>> target other than BPF is access MachineInstr info during isel phase.
>>>>
>>>> For an PHI node, when you reached it during isel phase, it may have all
>>>> predecessors linked, but not successors. It seems successors are linked to
>>>> PHI node only when doing SelectionDAGISel::FinishBasicBlock and this
>>>> happens later than PreprocessISelDAG hook.
>>>>
>>>> Previously, BPF program doesn't allow loop, there is probably the reason
>>>> why this bug was not exposed.
>>>>
>>>> This patch therefore fixes the bug by the following approach:
>>>>    - The existing truncation elimination code and the associated
>>>>      "load_to_vreg_" records are removed.
>>>>    - Instead, implement truncation elimination using MachineSSA pass, this
>>>>      is where all information are built, and keep the pass together with other
>>>>      similar peephole optimizations inside BPFMIPeephole.cpp. Redundant move
>>>>      elimination logic is updated accordingly.
>>>>    - Unit testcase included + no compilation errors for kernel BPF selftest.
>>>
>>> Thanks for the fix. The code looks good. Just two minor comments.
>>
>> Thanks Yonghong. Your comments make sense to me, will fix them.
>>
>>> After the fix, could you directly push to the llvm repo?
>>
>> Sure will do.
>>
>> (And I will update my llvm account email first, should be quick, if it takes
>> too long will come back to you for committing help)
> 
> Fix pushed after two minor comments addressed and re-unit-tested:
> 
>    https://github.com/llvm/llvm-project/commit/ec51851026a55e1cfc7f006f0e75f0a19acb32d3

Thanks!

      reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-10-12  7:18 Jiong Wang
2019-10-12 15:07 ` Yonghong Song
2019-10-15  2:41 ` Yonghong Song
2019-10-15 10:03   ` Jiong Wang
2019-10-16 15:33     ` Jiong Wang
2019-10-16 19:03       ` Yonghong Song [this message]

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=e12b61aa-9348-60a5-8948-517bf8781d6c@fb.com \
    --to=yhs@fb.com \
    --cc=alexei.starovoitov@gmail.com \
    --cc=bpf@vger.kernel.org \
    --cc=jiong.wang@netronome.com \
    --cc=oss-drivers@netronome.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

BPF Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/bpf/0 bpf/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 bpf bpf/ https://lore.kernel.org/bpf \
		bpf@vger.kernel.org
	public-inbox-index bpf

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.bpf


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git