[cfe-dev] Disabling ubsan's object size check at -O0

[George Burgess IV via cfe-dev]

Hi,

> AFAICT, and please let me know if I've missed something here, @llvm.objectsize always conservatively returns "I don't know" (i.e -1) at -O0

Yup. The object-size sanitizer lowers directly to @llvm.objectsize
(bypassing clang's __builtin_object_size evaluator, which is the
right thing to do here), and the only places we try to lower
@llvm.objectsize to a non-default value, to my knowledge, are
in InstCombine and CodeGenPrepare. Looks like both of those
are disabled on -O0.

As you mentioned, if we did decide that we should try to
lower @llvm.objectsize calls more aggressively in -O0, I'd
imagine we'd still end up handing back -1 in ~99% of cases.
(With the remaining 1% looking something like:

struct Foo {int i;};
char c;
int bar() { return reinterpret_cast<Foo *>(&c)->i; } // error: c is
too small, ...
)

So, unless I'm missing something, I think this is a pure win for
ubsan in -O0. I'm in favor of it.

Thank you!
George

On Tue, Jun 20, 2017 at 4:57 PM, Vedant Kumar via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
> UBSan has an object size check (-fsanitize=object-size) which can determine
> when an object is not large enough to represent a value of its type. The
> check uses the @llvm.objectsize intrinsic to determine the size of objects.
>
> AFAICT, and please let me know if I've missed something here,
> @llvm.objectsize always conservatively returns "I don't know" (i.e -1) at
> -O0, which means that it can't catch any issues at -O0. This is a problem
> because there is a substantial compile-time cost to enabling the object size
> check in debug builds. It seems unlikely that we can make @llvm.objectsize
> more precise at -O0 without regressing compile time and the debugging
> experience in other ways.
>
> So, I'm proposing that we disable ubsan's object size check at -O0. This
> will speed up debug builds without compromising on diagnostic quality. E.g I
> measured a 26% decrease in the compile time for X86FastISel.cpp with this
> change, and a 32% decrease in the *.o size:
>
> No ubsan [1]
> -----------------
> Average compile time: 5.27 s
> X86FastISel.cpp.o size: 3.06 MB
>
> Ubsan [2]
> -------------
> Average compile time: 9.49 s
> X86FastISel.cpp.o size: 8.93 MB
>
> Ubsan without the object size check [3]
> ----------------------------------------------------
> Average compile time: 6.99 s
> X86FastISel.cpp.o size: 6.06 MB
>
>
> There's reason to expect similar compile-time / binary size savings with
> other *.cpp files. The object size check is in the same category of checks
> as the null check and the alignment check. This group of checks accounts for
> the vast majority of checks inserted by ubsan (over 90% in some macOS apps),
> so any savings here would be helpful.
>
> Any objections? Anyone else in favor?
>
> thanks,
> vedant
>
> [1] -O0 -g
> [2] -O0 -g -fsanitize=undefined -fno-sanitize-recover=all
> -fno-sanitize=vptr,function
> [3] -O0 -g -fsanitize=undefined -fno-sanitize-recover=all
> -fno-sanitize=vptr,function,object-size
>
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