[PATCH] Checked arithmetic builtins

[Eli Friedman]

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.

+    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++.)

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.)

-Eli
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