[cfe-dev] [RFC] automatic variable initialization

Wed Jan 16 16:48:21 PST 2019

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) {}
  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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