[llvm-dev] RFC: LTO should use -disable-llvm-verifier

[Sean Silva via llvm-dev]

On Mon, Aug 31, 2015 at 11:40 AM, Duncan P. N. Exon Smith via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

>
> > On 2015-Aug-31, at 10:42, Reid Kleckner <rnk at google.com> wrote:
> >
> > On Mon, Aug 31, 2015 at 10:12 AM, Rafael Espíndola <
> llvm-dev at lists.llvm.org> wrote:
> >
> > > Not sure I follow? Generally LTO inputs are going to be "user
> provided" (in the sense that they're not produced immediately prior by the
> same process - or you'd have just produced a single Module in the first
> place, I would imagine) so changing the default still seems problematic in
> the sense of allowing 'unbounded' input without verification...
> >
> > The common case is for the bitcode to be generated by a paired clang.
> Even when it is an old bitcode compiled module, the Module itself is
> created by the bitcode reader.
> >
> > Sure, but it is not uncommon to LTO with old bitcode. We all know it's
> pretty easy to crash LLVM with bad bitcode or bad IR. These interfaces are
> not thoroughly tested.
> >
> > I think verifying the result of the bitcode reader by default during LTO
> is probably the right thing for the foreseeable future. It's the only thing
> that has any hope of telling the user something useful when things go wrong.
> >
> > I'd like it if we spent a little effort understanding why it's slow
> before flipping it off. Maybe the verifier is running multiple times,
> instead of after deserialization. We shouldn't need that in release builds.
>
> LTO runs the verifier three times:
>
>  1. On each input module after it's parsed from bitcode.
>  2. At the beginning of the optimization pipeline (post lib/Linker).
>  3. At the end of the optimization pipeline.
>
> If we're worried about user input, then I agree we should still run it
> at (1).  But I don't think we need to run it at (2) and (3).  Maybe we
> agree on this?
>

That sounds fine.


>
> Someone asked elsewhere in the thread about numbers: I had a look at a
> CPU profile (for linking verify-uselistorder with debug info).  For
> this (not-necessarily-representative) sample, (1) takes 1.6% of ld64
> runtime, and (2) and (3) combined take 3.2% of ld64 runtime.  Total of
> 4.8%.
>

Okay, this makes more sense where the 5% is coming from. Thanks for digging
in!

-- Sean Silva


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