Funnel shift in 64-bit doesn't get matched to an intrinsic

[AZero13]

Yes I know this has two cases of subpar optimization: One where the funnel shift is 64-bit based, and the other is left funnel shifts that can be converted to a right funnel shift. This one is about the latter. However, this was lifted from one of gcc's test suites.

https://godbolt.org/z/Yd63ed3sv

[llvmbot]

Hi!

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[llvmbot]

@llvm/issue-subscribers-good-first-issue

Author: AZero13 (AZero13)

Yes I know this has two cases of subpar optimization: One where the funnel shift is 64-bit based, and the other is left funnel shifts that can be converted to a right funnel shift. This one is about the latter. However, this was lifted from one of gcc's test suites.

https://godbolt.org/z/Yd63ed3sv

[RKSimon]

For funnel shifts representing rotations, we can remove negations and flip the funnel shift direction
Alive2: https://alive2.llvm.org/ce/z/tX9iyE

----------------------------------------
define i4 @src_fshl(i4 %x, i4 %y) {
#0:
  %n = sub i4 0, %y
  %r = fshl i4 %x, i4 %x, i4 %n
  ret i4 %r
}
=>
define i4 @tgt_fshl(i4 %x, i4 %y) {
#0:
  %r = fshr i4 %x, i4 %x, i4 %y
  ret i4 %r
}
Transformation seems to be correct!

----------------------------------------
define i4 @src_fshr(i4 %x, i4 %y) {
#0:
  %n = sub i4 0, %y
  %r = fshr i4 %x, i4 %x, i4 %n
  ret i4 %r
}
=>
define i4 @tgt_fshr(i4 %x, i4 %y) {
#0:
  %r = fshl i4 %x, i4 %x, i4 %y
  ret i4 %r
}
Transformation seems to be correct!

[sallto]

Hi, I would like to work on this. Since this is composed of 2 subtasks, I would start with combining the negation + shifts and after that try to get the 64bit rotates to be converted to funnel shifts.