[llvm-bugs] [Bug 50173] New: Missing fold trunc(ctlz(zext(x))) -> add(ctlz(x), C)

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50173

            Bug ID: 50173
           Summary: Missing fold trunc(ctlz(zext(x))) -> add(ctlz(x), C)
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: david.bolvansky at gmail.com
                CC: llvm-bugs at lists.llvm.org

define i16 @src(i16 %x) {

  %z = zext i16 %x to i32
  %p = call i32 @llvm.ctlz.i32(i32 %z, i1 false)
  %zz = trunc i32 %p to i16
  ret i16 %zz
}

define i16 @tgt(i16 %x) {

  %p = call i16 @llvm.ctlz.i16(i16 %x, i1 false)
  %z = add i16 %p, 16
  ret i16 %z
}


declare i32 @llvm.ctlz.i32(i32, i1)
declare i16 @llvm.ctlz.i16(i16, i1)



----------------------------------------
define i16 @src(i16 %x) {
%0:
  %z = zext i16 %x to i32
  %p = ctlz i32 %z, 0
  %zz = trunc i32 %p to i16
  ret i16 %zz
}
=>
define i16 @tgt(i16 %x) {
%0:
  %p = ctlz i16 %x, 0
  %z = add i16 %p, 16
  ret i16 %z
}
Transformation seems to be correct!

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