[Bug c/66552] New: Missed optimization when shift amount is result of signed modulus

["rv at rasmusvillemoes dot dk" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552

            Bug ID: 66552
           Summary: Missed optimization when shift amount is result of
                    signed modulus
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rv at rasmusvillemoes dot dk
  Target Milestone: ---
            Target: x86/generic

In code such as

unsigned f(unsigned x, int n)
{
  return x >> (n % 32);
}

I think gcc should be allowed to assume that n is either positive or divisible
by 32 (i.e., that the result of the % is non-negative). So it should actually
compile this the same as it does x >> (n & 31). However, it seems to generate
code to compute the actual value of n%32 and then uses that.

Even if one doesn't want to optimize based on UB, this is silly on a platform
like x86: There, gcc already knows that the shift instruction only depends on
the lower 5 bits, and those 5 bits do not depend on the full result of the
modulus (whether following C99 or not).