assigned to @rotateright

Don't need the trunc unless we are seeing multi-use-of-the-zext examples.

https://reviews.llvm.org/rG0f8b6686ac28

Maybe we want something similar for cttz too?

define i32 @​src(i16 %x) {

%z = zext i16 %x to i32
%p = call i32 @​llvm.cttz.i32(i32 %z, i1 true)
ret i32 %p
}

define i32 @​tgt(i16 %x) {

%p = call i16 @​llvm.cttz.i16(i16 %x, i1 true)
%z = zext i16 %p to i32
ret i32 %z
}

declare i32 @​llvm.cttz.i32(i32, i1)
declare i16 @​llvm.cttz.i16(i16, i1)

define i32 @​src(i16 %x) {
%0:
%z = zext i16 %x to i32
%p = cttz i32 %z, 1
ret i32 %p
}
=>
define i32 @​tgt(i16 %x) {
%0:
%p = cttz i16 %x, 0
%z = zext i16 %p to i32
ret i32 %z
}
Transformation seems to be correct!

src: # @​src
movzx eax, di
bsf eax, eax
ret
tgt: # @​tgt
bsf ax, di
movzx eax, ax
ret

also

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!

Sure - cttz/ctlz can be improved too. Can you open another bug, so we can track that?

Ok.

llvm/llvm-bugzilla-archive#50172 , llvm/llvm-bugzilla-archive#50173