[cfe-dev] [RFC] automatic variable initialization

Richard Smith via cfe-dev cfe-dev at lists.llvm.org
Wed Jan 16 19:21:34 PST 2019


On Wed, 16 Jan 2019, 18:13 Kostya Serebryany via cfe-dev <
cfe-dev at lists.llvm.org wrote:

> On Wed, Jan 16, 2019 at 5:06 PM Richard Smith <richard at metafoo.co.uk>
> wrote:
>
>> On Wed, 16 Jan 2019 at 16:48, Kostya Serebryany via cfe-dev <
>> cfe-dev at lists.llvm.org> wrote:
>>
>>> we've collected the performance and code size data on one of our major
>>> server-side benchmarks (huge x86_64 binary)
>>>
>>> Stack Init Method -O1 -O2 -O3 -O2, ThinLTO -O2, ThinLTO, FDO
>>> zero-init Size Regression 0.5% 0.6% 0.6% 0.4% 0.3%
>>> Perf Regression 0.7% -0.9% 1.1% 1.0% 1.7%
>>> pattern-init Size Regression 1.6% 1.6% 1.6% 1.4% 1.6%
>>> Perf Regression 1.1% 1.2% 1.2% 0.8% 1.1%
>>>
>>> so, already not bad, but we pay too much code size for the pattern-init.
>>>
>>> The following seems to be the dominating code pattern where we have to
>>> insert initialization:
>>>
>>> struct Bar {  // think std::vector
>>>   Bar() : a(nullptr), b(nullptr), c(nullptr) {}
>>>
>>
>> Wait, should this default constructor be here?
>>
>
> Hmm?
> A user-defined Foo::Foo is called here.
>

Sure, but it calls the Bar default constructor to initialize its three Bar
members. Nothing is uninitialised in this example, so I think there should
be no cost from automatic initialization.

If so, why are we doing any pattern init in this example? I'd think only
>> otherwise-uninitialized members should receive pattern init.
>>
>
> When we are looking at main(), how do we know that e.g. foo.v1.a is
> initialized?
> (especially if some of the CTORs here is in another CU)
>

Should main be involved, though? It's normally the job of the constructor
to initialize the class members, and that's where the information about
whether the members are uninitialised lives.

  long *a, *b, *c;
>>> };
>>> struct Foo {
>>>   Foo() *__attribute__((noinline))* {}
>>>   Bar v1, v2, v3;
>>> };
>>> int main() { Foo foo; }
>>>
>>> The Foo CTOR does not get inlined and so we insert initialization for
>>> all Foo's fields.
>>> What's worse, is that -ftrivial-auto-var-init*=pattern* produces very
>>> sub-optimal code here:
>>>
>>> x86_64:
>>> movq .L__const.main.foo+64(%rip), %rax
>>> movq %rax, 64(%rsp)
>>> movups .L__const.main.foo+48(%rip), %xmm0
>>> movaps %xmm0, 48(%rsp)
>>> movups .L__const.main.foo+32(%rip), %xmm0
>>> movaps %xmm0, 32(%rsp)
>>> movups .L__const.main.foo+16(%rip), %xmm0
>>> movaps %xmm0, 16(%rsp)
>>> movups .L__const.main.foo(%rip), %xmm0
>>> movaps %xmm0, (%rsp)
>>>
>>> aarch64:
>>> adrp x8, .L__const.main.foo
>>> add x8, x8, :lo12:.L__const.main.foo
>>> ldp q0, q3, [x8]
>>> ldr x9, [x8, #64]
>>> ldp q1, q2, [x8, #32]
>>> mov x0, sp
>>> stp q0, q3, [sp]
>>> str x9, [sp, #64]
>>> stp q1, q2, [sp, #32]
>>>
>>> I.e. instead of materializing the 16-byte pattern once, it does it many
>>> times.
>>>
>>> Compare to -ftrivial-auto-var-init*=zero*:
>>>
>>> xorps %xmm0, %xmm0
>>> movaps %xmm0, 48(%rsp)
>>> movaps %xmm0, 32(%rsp)
>>> movaps %xmm0, 16(%rsp)
>>> movaps %xmm0, (%rsp)
>>> movq $0, 64(%rsp)
>>>
>>> movi v0.2d, #0000000000000000
>>> mov x0, sp
>>> str xzr, [sp, #64]
>>> stp q0, q0, [sp, #32]
>>> stp q0, q0, [sp]
>>>
>>>
>>> OTOH, with a relatively simple LTO-based analysis we can get rid of
>>> these extra auto-inits completely.
>>>
>>> CC @Vitaly Buka <vitalybuka at google.com> @Peter Collingbourne
>>> <pcc at google.com> @Evgeniy Stepanov <eugenis at google.com>
>>>
>>>
>>> --kcc
>>>
>>> On Tue, Dec 18, 2018 at 11:38 AM Kostya Serebryany <kcc at google.com>
>>> wrote:
>>>
>>>> 2019 is going to have one bug class fewer. :)
>>>> (unless Jann&Co find new bug classes again)
>>>>
>>>> Looking forward to the followup patches (e.g. padding gaps)
>>>> and to doing the measurements.
>>>>
>>>> --kcc
>>>>
>>>> On Tue, Dec 18, 2018 at 10:53 AM JF Bastien <jfbastien at apple.com>
>>>> wrote:
>>>>
>>>>> Hello fans of the pre-C++11 `auto` keyword!
>>>>>
>>>>> Thanks to reviews from Peter and Richard, automatic variable
>>>>> initialization landed last night:
>>>>>
>>>>> https://reviews.llvm.org/rL349442
>>>>>
>>>>>
>>>>> Chandler pointed out that I should have circled back to the RFC in
>>>>> case any of the finer points needed tuning. Let me know if there’s any
>>>>> follow-up you’d like! I’ve made zero-init harder / uglier to use as was
>>>>> requested, while allowing folks like Kostya to test it out.
>>>>>
>>>>> Let me know if you find bugs (in your code, or in the implementation),
>>>>> and please file bugs on any missed optimization opportunities. I know of a
>>>>> handful of missing optimizations which I intend to tackle in the near
>>>>> future. I’d also love to hear what kind of performance / size impact you
>>>>> see from various codebases.
>>>>>
>>>>> As pointed out in the patch there’s a few other hardenings that I’ll
>>>>> want to tackle next. I’ll do so in the new year.
>>>>>
>>>>> Thanks,
>>>>>
>>>>> JF
>>>>>
>>>>>
>>>>> On Dec 11, 2018, at 3:18 PM, Joe Bialek via cfe-dev <
>>>>> cfe-dev at lists.llvm.org> wrote:
>>>>>
>>>>> We went with zero initialization for production builds in Visual
>>>>> Studio because we anticipate zero being the fastest, smallest code gen, and
>>>>> safest value for the following cases:
>>>>>
>>>>>    - Pointer: A non-zero pointer could potentially point at valid
>>>>>    code. On Windows x64, the first 64kb of the virtual address space is
>>>>>    guaranteed to not be mappable and the last 512GB of the virtual address
>>>>>    space (today) has the space guarantee. So null pointer or near-null pointer
>>>>>    dereferences are denial of service at worst.
>>>>>    - Size: If you forget to set a size as an out parameter we’d
>>>>>    rather it’s set to 0 so you don’t index out of bounds.
>>>>>    - Index: Same as size.
>>>>>
>>>>>
>>>>> We are using this mitigation to downgrade vulnerabilities from remote
>>>>> code execution, elevation of privilege, or information disclosure, down to
>>>>> denial of service at worst.
>>>>>
>>>>> Assuming the denial of service isn’t for a scenario where it matters
>>>>> (there are many types of DOS we don’t really worry about for security), we
>>>>> will not bother servicing the vulnerabilities down-level and instead only
>>>>> fix them for new code.
>>>>>
>>>>> It is helpful for us to have deterministic behavior to help accomplish
>>>>> this goal. If an engineer knows a value is always set to 0, they can
>>>>> quickly determine if the bug report is actually exploitable or not. If it
>>>>> is initialized to a random or compile time constant value, we’ll need to do
>>>>> more work to determine if the bug is exploitable.
>>>>>
>>>>> We are not zero initializing CHK builds to prevent folks from taking a
>>>>> dependency on this feature.
>>>>>
>>>>>
>>>>> Joe
>>>>>
>>>>> *From:* Kostya Serebryany <kcc at google.com>
>>>>> *Sent:* Tuesday, December 11, 2018 3:04 PM
>>>>> *To:* Mehdi AMINI <joker.eph at gmail.com>; Jann Horn <jannh at google.com>
>>>>> *Cc:* David Blaikie <dblaikie at gmail.com>; Richard Smith <
>>>>> richard at metafoo.co.uk>; Clang Dev <cfe-dev at lists.llvm.org>; Joe
>>>>> Bialek <jobialek at microsoft.com>
>>>>> *Subject:* Re: [cfe-dev] [RFC] automatic variable initialization
>>>>>
>>>>> One more dimension in this discussion (zero-init vs pattern-init) is
>>>>> what will security get from those options.
>>>>> My second-hand knowledge here suggests that zero-init may have *on
>>>>> average* better security guarantees than non-zero init.
>>>>> For example if the uninitialized data is interpreted as a length of
>>>>> something, it better be zero than any large number or, worse, a negative
>>>>> signed number.
>>>>> +Jann Horn <jannh at google.com> and Joe Bialek who have first-hand
>>>>> knowledge here.
>>>>>
>>>>> --kcc
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> On Thu, Dec 6, 2018 at 3:58 PM Kostya Serebryany <kcc at google.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>> On Thu, Dec 6, 2018 at 3:55 PM Mehdi AMINI <joker.eph at gmail.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>> On Thu, Dec 6, 2018 at 3:43 PM Kostya Serebryany <kcc at google.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>> On Thu, Dec 6, 2018 at 3:03 PM Mehdi AMINI <joker.eph at gmail.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>> On Thu, Dec 6, 2018 at 1:01 PM Kostya Serebryany <kcc at google.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>> On Wed, Dec 5, 2018 at 6:07 PM Mehdi AMINI via cfe-dev <
>>>>> cfe-dev at lists.llvm.org> wrote:
>>>>>
>>>>> I have the same question as David: I don't understand why this isn't
>>>>> just an experimental build option disabled by default? (Google, Apple, etc.
>>>>> can just enable it in their build script, no need to patch the source).
>>>>>
>>>>>
>>>>> I will need to rebuild half a dozen compiler binaries to do the
>>>>> measurements I need.
>>>>> This is going to double the cost of the effort for me because it adds
>>>>> too many extra moving pieces.
>>>>>
>>>>>
>>>>> Not sure I follow, you have to build the compiler anyway to get it
>>>>> after the code is patched?
>>>>> The only thing you would have to do is one CL to enable the build flag
>>>>> (that expose the command line flag) inside your codebase and then you get
>>>>> your toolchain as usual?
>>>>>
>>>>>
>>>>> Once the JF's patch is in, I'll have it in most production compilers I
>>>>> care about in 2-6 weeks, w/o any effort on my side.
>>>>> None of those builders (easily) support adding custom patches, and
>>>>> even a build flag is going to be very non-trivial.
>>>>> I can deal with one or two of those builds relatively easily, but not
>>>>> with the rest.
>>>>>
>>>>>
>>>>> What builders are you referring to? So far I was assuming we were
>>>>> talking about your internal infrastructure that produces your production
>>>>> compilers.
>>>>>
>>>>>
>>>>> I am talking about our internal infra.
>>>>> But we have multiple independent builders from multiple independent
>>>>> compiler users (e.g. Chrome and Android are using different compiler
>>>>> builds).
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Mehdi
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> The only argument I read in the thread was about allowing people to
>>>>> make performance measurement without rebuilding the compiler, but I have a
>>>>> hard time reconciliation this with the fact that we're talking about not
>>>>> shipping this before performing the actual measurements?
>>>>>
>>>>> I expect that anyone that cares enough about the perf impact of this
>>>>> to influence the development of the feature in clang should already be
>>>>> rebuilding their compiler today.
>>>>>
>>>>> --
>>>>> Mehdi
>>>>>
>>>>>
>>>>> On Mon, Dec 3, 2018 at 5:58 PM David Blaikie via cfe-dev <
>>>>> cfe-dev at lists.llvm.org> wrote:
>>>>>
>>>>> Just out of curiosity - what's the major difference between a
>>>>> build-time off-by-default feature and a build-time
>>>>> on-by-default-but-off-in-release branches feature? If we're only targeting
>>>>> groups that build/release the compiler themselves, then they're likely able
>>>>> to opt-in to a build-time feature easily enough, I'd think? & then there's
>>>>> no need to make our releases different from day-to-day builds?
>>>>>
>>>>> But sounds like folks are in general agreement of a way forward, so I
>>>>> don't want to disrupt/delay that.
>>>>> On Wed, Nov 28, 2018 at 11:14 PM Chandler Carruth <chandlerc at gmail.com>
>>>>> wrote:
>>>>>
>>>>> Suggested compromise technique to at least get an initial set of
>>>>> numbers:
>>>>>
>>>>> 1) Require a special, long, ugly flag name.
>>>>> 2) Make it a CC1 flag, requiring -Xclang ... to use.
>>>>> 3) Emit a warning by default (that cannot be suppressed with a
>>>>> -Wno-... flag) when this flag is enabled.
>>>>> 4) Commit to never including this flag in any upstream release. Either
>>>>> we remove it before the next release branches or we revert it on the branch.
>>>>>
>>>>> Most of the folks we're hoping to get performance data with are
>>>>> willing to use a not-yet-released build of Clang. They won't have to
>>>>> actually patch it in any way. They will have strong reminders to not deploy
>>>>> this in any way due to the warning.
>>>>>
>>>>> Thoughts?
>>>>>
>>>>> On Wed, Nov 28, 2018 at 4:34 PM Kostya Serebryany via cfe-dev <
>>>>> cfe-dev at lists.llvm.org> wrote:
>>>>>
>>>>> On Wed, Nov 28, 2018 at 3:28 PM David Blaikie <dblaikie at gmail.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>> On Wed, Nov 28, 2018 at 3:17 PM Kostya Serebryany <kcc at google.com>
>>>>> wrote:
>>>>>
>>>>>
>>>>> Seems easier to me to separate the two pieces - move ahead with the
>>>>> non-zero options, and separate the discussion on the zero option. You can
>>>>> present performance numbers from what you can measure without shipping a
>>>>> compiler with the feature - and if those numbers are sufficiently
>>>>> compelling compared to the risks of slicing the language, then perhaps we
>>>>> go that way.
>>>>>
>>>>>
>>>>> This approach will significantly impair my ability to do the
>>>>> measurements I need.
>>>>>
>>>>>
>>>>> I'm aware waht I'm proposing would make it more difficult for some
>>>>> people to take measurements - that's a tradeoff to be sure - one where I
>>>>> err in this direction.
>>>>>
>>>>> Specifically for Google though - would it be that difficult for Google
>>>>> to opt-in to a certain build configuration of LLVM?
>>>>>
>>>>>
>>>>> Absolutely yes.
>>>>> Google is not just a single monolithic code base
>>>>> <https://nam06.safelinks.protection.outlook.com/?url=http%3A%2F%2Fdelivery.acm.org%2F10.1145%2F2860000%2F2854146%2Fp78-potvin.pdf%3Fip%3D104.133.8.94%26id%3D2854146%26acc%3DOA%26key%3D4D4702B0C3E38B35%252E4D4702B0C3E38B35%252E4D4702B0C3E38B35%252E5945DC2EABF3343C%26__acm__%3D1543446999_3aadcd36f657e2297430c38bee93f16c&data=02%7C01%7Cjobialek%40microsoft.com%7C533e0fe1444d4b5a3a6b08d65fbcf479%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636801662510848537&sdata=dd4I1xQ5qfawzLwsnBM3iqsypqZ%2BlFswj5IgKa%2FLHbw%3D&reserved=0>
>>>>> .
>>>>>
>>>>>
>>>>> Couldn't access this link ("An error occurred while processing your
>>>>> request" - but yeah, I understand there's a bunch of different pieces of
>>>>> Google beyond the "stuff that runs in data centers" piece we mostly support.
>>>>>
>>>>>
>>>>> Besides that monolithic thing, we have Android, Chrome, ChromeOS,
>>>>> Fuchsia, and a bazillion of smaller efforts that use their own toolchains.
>>>>>
>>>>>
>>>>> Still, most/all of these build their own compilers, I think? But yeah,
>>>>> that adds an opt-in overhead to each project, for sure.
>>>>>
>>>>>
>>>>> In some cases the most reliable and complete way of measuring
>>>>> performance changes is to submit the changes to revision control,
>>>>> and let the performance bots shew it for a couple of days. That's how
>>>>> we iterated with the LLVM's CFI in Chrome.
>>>>> We will also need to work with the upstream Linux kernel -- it's hard
>>>>> enough for them to use clang and a modified clang will cost us much more
>>>>> effort.
>>>>>
>>>>>
>>>>> Yeah, I can imagine that one's a bit trickier - how's performance
>>>>> evaluation of the kernel done?
>>>>>
>>>>>
>>>>> I don't think anyone knows that. :-|
>>>>> And requiring a compiler patch will shift the problem from "hard" to
>>>>> "I'd better do something else".
>>>>>
>>>>>
>>>>> (though, again, I imagine a fair amount of progress could be made
>>>>> without the zero-init feature - perhaps enough to say "hey, here are all
>>>>> the places we have run tests & seen the performance tradeoff is worthwhile
>>>>> for us (& possibly that it's close to the zero-init case, but that's sort
>>>>> of orthogonal, imho - that it's worthwhile is the main thing) - perhaps
>>>>> other folks would be willing to test it (non-zero init) & see if it's
>>>>> worthwhile to them - and if it isn't/they're interested in more
>>>>> performance, maybe all that evidence we can gain from the places where it's
>>>>> easy for us to rebuild compilers, etc, would make it interesting enough to
>>>>> motivate someone to do build the kernel with a custom compiler & do some
>>>>> performance measurements, etc...
>>>>>
>>>>> Sorry that was a bit rambly, anyway.
>>>>>
>>>>> - Dave
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> We/Google do build the compiler from scratch, I assume we pick the
>>>>> configuration options we build with & some of them probably aren't the
>>>>> defaults for a release build of LLVM. So if it was important that Google's
>>>>> production compiler had these features enabled (rather than building a test
>>>>> compiler for running some experiments), that doesn't seem (at least to me,
>>>>> at this moment) especially prohibitive, is it?
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>>
>>>>>
>>>>> cfe-dev mailing list
>>>>> cfe-dev at lists.llvm.org
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>>>>>
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