[cfe-dev] [RFC] Introduce overflow builtins

[Xi Wang]

On Apr 2, 2012, at 1:30 AM, Dave Zarzycki wrote:
> Also, this error could be more clear:
> 
> +def err_overflow_builtin_pointer_size : Error<
> +  "first argument to overflow builtin must be a pointer to 2, 4, or 8 "
> +  "type (%0 invalid)">;
> 
> To:
> 
> +def err_overflow_builtin_pointer_size : Error<
> +  "first argument to overflow builtin must be a pointer to a 16-bit, 32-bit, or 64-bit integer "
> +  "type (%0 invalid)">;

Nice catch!  Thanks.  The word "byte" was missing when copied from err_atomic_builtin_pointer_size, my bad..

Again, the updated patch (including sign inference) is available at

https://github.com/xiw/clang/compare/master...builtin-overflow

> 1) Make __builtin_mul_with_overflow() return the "high" half of the result instead of a bool. You should be able to use the normal LLVM multiplication intrinsics for this instead of llvm.*mul.with.overflow.*.

I guess you mean something like MSVC's intrinsics:

__int64 _mul128( 
   __int64 Multiplier, 
   __int64 Multiplicand, 
   __int64 *HighProduct 
);

__int64 __mulh( 
   __int64 a, 
   __int64 b 
);

http://msdn.microsoft.com/en-us/library/82cxdw50.aspx

http://msdn.microsoft.com/en-us/library/17wa28cw.aspx

I don't think you need a new builtin here.  For example, converting a and b to __int128_t should do the trick.

> 2) Make __builtin_*_with_overflow work with 8-bit integers.

Actually I doubt if types smaller than `int' would require overflow checking in practice..

> 3) Add __builtin_add_with_carry() and __builtin_sub_with_borrow().

What could benefit from them?

- xi