[cfe-dev] [RFC] automatic variable initialization

Kostya Serebryany via cfe-dev cfe-dev at lists.llvm.org
Wed Jan 16 18:12:49 PST 2019


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.



> 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)


>
>
>>   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?
>>>>
>>>>
>>>> _______________________________________________
>>>>
>>>>
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