[Bug rtl-optimization/66706] New: Redundant shift instruction on x >> (n & 32)

[msebor at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66706

            Bug ID: 66706
           Summary: Redundant shift instruction on x >> (n & 32)
           Product: gcc
           Version: 5.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: msebor at gcc dot gnu.org
                CC: mpolacek at gcc dot gnu.org, msebor at gcc dot gnu.org,
                    rv at rasmusvillemoes dot dk, segher at gcc dot gnu.org,
                    unassigned at gcc dot gnu.org
        Depends on: 66552
  Target Milestone: ---
            Target: x86, powerpc64

+++ This bug was initially created as a clone of Bug #66552 +++

Compiling the following function with -O2:

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

results in an unnecessary instruction to zero out the already clear high order
bits of register 3 after the shift instruction:

        rldicl 4,4,0,59
        srw 3,3,4
        rldicl 3,3,0,32
        blr

Clang emits an optimal sequence:

        rlwinm 4, 4, 0, 27, 31
        srw 3, 3, 4
        blr


Referenced Bugs:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66552
[Bug 66552] Missed optimization when shift amount is result of signed modulus