[llvm-bugs] [Bug 33898] New: shr <reg>, 1 sets flags in useful ways

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33898

            Bug ID: 33898
           Summary: shr <reg>, 1 sets flags in useful ways
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: rjmccall at apple.com
                CC: llvm-bugs at lists.llvm.org

"shr <reg>, 1" sets:
  ZF if (<reg> >> 1) == 0, i.e. if (<reg> & ~1) == 0
  CF if (<reg> & 1) == 1

A branch predicated on (x == 0 || x == 1), ((x & ~1) == 0), or (x ULE 1) can be
emitted as shr x, 1; jz.

A branch predicated on ((x & 1) == 1) can be emitted as shr x, 1; jb.  This is
not useful on its own because test x, 1 exists, but it can be combined:

A branch predicated on (x == 0 || (x & 1) == 1) can be emitted as shr x, 1;
jbe.

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