[llvm-commits] [PATCH] Simplify negated bit test

[Nuno Lopes]

Please add a comment stating that this transformation is only correct 
because shifting by a number larger than the number of bits is undefined. 
Otherwise this transformation wouldn't be sound.

As an example, take a look to the output of the Z3 SMT solver (comment line 
5 to get a counterexample):
http://rise4fun.com/Z3/RuuZ

Nuno


----- Original Message -----
> The attached patched takes a sequence like:
> %neg = xor i32 %A, -1
> %shl = shl i32 1, %B
> %and = and i32 %shl, %neg
> %cmp = icmp ne i32 %and, 0
>
> and turns it into:
> %shl = shl i32 1, %B
> %and = and i32 %shl, %A
> %cmp = icmp eq i32 %and, 0
>
> The patch includes a test case that exercises this transform.
>
> Examples of frontend code that could generate sequence 1 is:
>
> bool fun1(unsigned A, unsigned B) { return ~A & (1 << B); }
>
> An example of sequence 2 is:
> bool fun2(unsigned A, unsigned B) { return !(A & (1 << B)); }
>
> With this patch, they would have the same IR.