[PATCH] InstCombine: Combine two icmps into one if the RHSs agree

[Meador Inge]

On 07/31/2014 01:27 PM, David Majnemer wrote:

> consider:
> %cmp1 = icmp CC %V1, C
> %cmp2 = icmp CC %V2, C
> %and  = and %cmp1, %cmp2
> 
> we can transform this into:
> %or  = or %V1, %V2
> %cmp = icmp CC %or, C

Maybe I am missing something, but this transformation seems invalid.
Consider CC = eq, V1 = 1, V2 = 3, C = 3:

  %cmp1 = icmp eq %V1, 3
  %cmp2 = icmp eq %V2, 3
  %and  = and %cmp1, %cmp2

Therefore and = 0.

Now consider the optimized sequence:

  %or  = or %V1, %V2
  %cmp = icmp eq %or, 3

In this case cmp = 1, which is incorrect.

-- 
Meador Inge
CodeSourcery / Mentor Embedded