Failure to optimize out redundant compare

[GabrielRavier]

Bugzilla Link	46160
Version	trunk
OS	Linux
CC	@RKSimon,@rotateright

Extended Description

bool f(int a, int b, int c)
{
return (a == b) && (b == c) && (a == c);
}

This can be optimized to return (a == b) && (a == c);. GCC does this transformation, but LLVM does not.

[rotateright]

In the minimal form of 3 comparisons, this is the kind of reassociation that instcombine dabbles in for binops, but it doesn't generalize for comparisons.

The more general optimization should probably go in the -reassociate pass. That pass also handles binops, but does nothing for icmp eq.

Any solution should hopefully also solve the DeMorgan inverse pattern with 'ne' and 'or'.