[PATCH] Checked arithmetic builtins

[Michael Gottesman]

On Jun 20, 2013, at 2:16 PM, Eli Friedman <eli.friedman at gmail.com> wrote:

> On Tue, Jun 18, 2013 at 3:11 PM, Michael Gottesman <mgottesman at apple.com> wrote:
> Hello cfe commits!
> 
> The attached patch adds checked-arithmetic builtins to ameliorate such code in security critical applications (for instance webkit). It simply exposes {u,s}{add,sub,mul}.
> 
> *NOTE* The u{add,sub} overlaps with the multi precision built-ins. I decided to add in the additional builtin since users are going to see s{add,sub} and look for u{add,sub}. We could add in a Builtins.h header where I could implement the checked arithmetic with the multi precision arithmetic builtins but I felt that was a bigger change than this.
> 
> 
> It looks like you copy-pasted some stuff you didn't mean to in the changes to LanguageExtensions.rst.

= /. Fixed.

> 
> +    return RValue::get(Builder.CreateZExt(Carry, X->getType()));
> 
> These builtins all return bool, right?  Why are you zero-extending here?  (You might be able to get away with this in C because of integer promotions, but I'm pretty sure it'll explode in C++.)

= /. Fixed. The reason I was zero extending was that I was basing this off of the multiprecision builtin code and forgot to remove it. 

> 
> IIRC, we don't actually support CodeGen of llvm.smul.with.overflow with 64-bit operands on x86-32; it would be nice to print a proper error message instead of crashing.  (At least, we didn't at one point; I don't recall if it ever got fixed.)

I just codegened my test file on OS X with -arch i386 and it codegened fine. So it looks like it was fixed.

Hows this look:



Thanks again,
Michael

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