This transform is broken for undef or poison inputs without a freeze.
This is also broken in lots of other places where shifts are split
into 32-bit pieces.
Amt < 32 case:
; Broken: https://alive2.llvm.org/ce/z/7bb4vc
; Freezing the low half of the bits makes it correct
; Fixed: https://alive2.llvm.org/ce/z/zJAZFr
define i64 @src(i64 %val) {
%shl = shl i64 %val, 55
%shr = ashr i64 %shl, 55
ret i64 %shr
}
define i64 @tgt(i64 %val) {
%lo32 = trunc i64 %val to i32
%shr.half = lshr i64 %val, 32
%hi32 = trunc i64 %shr.half to i32
%inreg.0 = shl i32 %lo32, 23
%new.lo = ashr i32 %inreg.0, 23
%new.hi = ashr i32 %new.lo, 31
%zext.lo = zext i32 %new.lo to i64
%zext.hi = zext i32 %new.hi to i64
%hi.ins = shl i64 %zext.hi, 32
%or = or i64 %hi.ins, %zext.lo
ret i64 %or
}
Amt == 32 case:
Broken: https://alive2.llvm.org/ce/z/5f4qwQ
Fixed: https://alive2.llvm.org/ce/z/A2hWWF
This one times out alive; works if argument is made undef or scaled down
to a smaller bitwidth.
define i64 @src(i64 %val) {
%shl = shl i64 %val, 32
%shr = ashr i64 %shl, 32
ret i64 %shr
}
define i64 @tgt(i64 %val) {
%lo32 = trunc i64 %val to i32
%shr.half = lshr i64 %val, 32
%hi32 = trunc i64 %shr.half to i32
%new.hi = ashr i32 %lo32, 31
%zext.lo = zext i32 %lo32 to i64
%zext.hi = zext i32 %new.hi to i64
%hi.ins = shl i64 %zext.hi, 32
%or = or i64 %hi.ins, %zext.lo
ret i64 %or
}
Amt > 32 case:
; Correct: https://alive2.llvm.org/ce/z/tvrhPf
define i64 @src(i64 %val) {
%shl = shl i64 %val, 9
%shr = ashr i64 %shl, 9
ret i64 %shr
}
define i64 @tgt(i64 %val) {
%lo32 = trunc i64 %val to i32
%lshr = lshr i64 %val, 32
%hi32 = trunc i64 %lshr to i32
%inreg.0 = shl i32 %hi32, 9
%new.hi = ashr i32 %inreg.0, 9
%zext.lo = zext i32 %lo32 to i64
%zext.hi = zext i32 %new.hi to i64
%hi.ins = shl i64 %zext.hi, 32
%or = or i64 %hi.ins, %zext.lo
ret i64 %or
}
I would add some context to describe why freeze is needed here, since it's not an instruction that's often seen (at least for me)