Efficient Integer Overflow Checking in LLVM (2016)

https://blog.regehr.org/archives/1384

by ot • 6 years ago

84 22 6 years ago

blog.regehr.org

pizlonator2• 6 years ago

Maybe some day llvm will know how to optimize overflow checks as well as JavaScript engines do.

nly• 6 years ago

Is this tongue in cheek? JavaScript numbers are floating point, are they not?

titzer• 6 years ago

No, JS engines optimize heavily for small integers (31 or 32 bits) and dynamically overflow into double range as necessary.

chrisseaton2• 6 years ago

They aren’t literally implemented as floating point in most cases. That’d be crazy! If the number fits in an integer then a sensible implementation uses an integer.

saagarjha2• 6 years ago

Maybe, but LLVM has the disadvantage that speculating “this won’t overflow” and creating a fast path with that information is not something that can be fixed if more profiling data comes in at runtime saying it was a poor choice.

im3w1l• 6 years ago

The C/C++ standards allow compilers to do whatever they want as long as it does not cause observable output changes.

So LLVM could do runtime profiling if it wanted. But I suspect that in the majority of the cases the cost of the profiling would be worse than the potential gains.

pizlonator2• 6 years ago

Those aren’t the optimizations I’m talking about. That’s not even how JSC optimizes overflow checks.

I’m talking about:

- abstract interpretation using a streamlined version of the octagon domain.

- instruction selection and register allocation hacks to emit the best possible code for checked math. I don’t think llvm has parity with B3 here, particularly in cases where inputs to the math are live on the failing case.

- probably other stuff. IRs that start out with checked math tend to include checked math reasoning in more optimizations.

MaxBarraclough3• 6 years ago

Interesting post, as Regehr's posts tend to be.

Presumably .Net goes to great lengths to optimise overflow-checking, as the checks are enabled for production use. I wonder if they manage to get the overhead to below the 8.7% mentioned in the article.

edit: I had put OpenJDK and .Net but tom_mellior is right to point out that Java has wrapping arithmetic

tom_mellior3• 6 years ago

Why would OpenJDK do integer overflow checking? Java integers have well-defined two's complement wrapping semantics. When you add two positive numbers and get a negative result, this might not be what the programmer wanted, but it is not undefined behavior like it is in C.

Are you maybe mixing this up with array bounds checking?

MaxBarraclough3• 6 years ago

Oops, you're right! I forgot Java arithmetic wraps, offering (rarely used) methods for overflow-checked arithmetic. [0]

I was right about .Net though. Assuming a checked context, .Net (and C#) throw on overflow, rather than wrapping. [1]

[0] https://docs.oracle.com/en/java/javase/11/docs/api/java.base...

[1] https://docs.microsoft.com/en-us/dotnet/csharp/language-refe...

chrisseaton2• 6 years ago

OpenJDK only does overflow checking in a tiny number of cases.

MaxBarraclough3• 6 years ago

Cunningham's Law strikes again!

You're right of course. Please see my response to tom_mellior.

Gaelan• 6 years ago

(2016)

stephc_int13• 6 years ago

Wow.

I don't want this in release mode.

Please tell me that this overflow checking is turned off by default.

commandlinefan• 6 years ago

Hm - I was wondering exactly what "ud2" does when it's triggered (looks like an "undefined opcode"... causes a seg fault?), but what's worse? An app that crashes with no apparent cause, or a bank account that resets to $0 if it gets too big?

saagarjha2• 6 years ago

You’ll get a SIGILL.

cbarrick• 6 years ago

You have to explicitly enable UBSan checks:

    -fsanitize=signed-integer-overflow

See the UBSan docs: https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html

rurban• 6 years ago

@dang Can we have a (2016) in the title please.

That's when I added them to perl's add operator. Being about 10% faster than manual checking the high bits. gcc added them about one year later.

KindOne• 6 years ago

Post from 2016:

https://news.ycombinator.com/item?id=11462308 (24 comments)

Efficient Integer Overflow Checking in LLVM (2016)

Related Stories