[PowerPC] LI folding opportunity

[chenzheng1030]

See https://github.com/llvm/llvm-project/pull/86857/files#r1548822138

If we know the lowest bit of gpr5 is 0( slwi 5, 6, 1), PPC should be able to fold

li 3, 1
rlwimi 3, 5, 0, 0, 30

to

addi 3, 5, 1

Or

ori 3, 5, 1 

[ecnelises]

Optimized legalized selection DAG: %bb.0 'testDoubleword:entry'
SelectionDAG has 19 nodes:
  t0: ch,glue = EntryToken
    t10: i32,ch = CopyFromReg t0, Register:i32 %4
  t21: i32 = shl t10, Constant:i32<1>
            t2: v2i64,ch = CopyFromReg t0, Register:v2i64 %0
          t19: v4i32 = bitcast t2
          t4: i32,ch = CopyFromReg t0, Register:i32 %1
        t22: v4i32 = insert_vector_elt t19, t4, t21
        t6: i32,ch = CopyFromReg t0, Register:i32 %2
        t26: i32 = or disjoint t21, Constant:i32<1>
      t24: v4i32 = insert_vector_elt t22, t6, t26
    t25: v2i64 = bitcast t24
  t16: ch,glue = CopyToReg t0, Register:v2i64 $v2, t25
  t17: ch = PPCISD::RET_GLUE t16, Register:v2i64 $v2, t16:1

ISEL: Starting selection on root node: t26: i32 = or disjoint t21, Constant:i32<1>
Creating constant: t33: i32 = TargetConstant<31>
Creating new machine node: t34: i32 = RLWIMI t21, Constant:i32<1>, TargetConstant:i32<0>, TargetConstant:i32<31>, TargetConstant:i32<31>

It's strange that or X, 1 was selected into RLWIMI X, 1, 0, 31, 31.

[llvmbot]

@llvm/issue-subscribers-backend-powerpc

Author: Chen Zheng (chenzheng1030)

See https://github.com/llvm/llvm-project/pull/86857/files#r1548822138 If we know the lowest bit of gpr5 is 0( slwi 5, 6, 1), PPC should be able to fold ``` li 3, 1 rlwimi 3, 5, 0, 0, 30 ``` to ``` addi 3, 5, 1 ``` Or ``` ori 3, 5, 1 ```

[diggerlin]

we do it in more generically.

slwi 5, 6, n
li 3, Val
rlwimi 3, 5, 0, 0, K    (K=31-n...31)

if 2^(31-n) >val

we can convert to

addi 3, 5, Val

or

ori 3, 5, Val

[chenzheng1030]

I agree. And a more generic pattern is we don't need to check the n or slwi instruction, we just need to check the trailing zero number of gpr5 in the rlwimi instruction. If the trailing zero number is able to contain the Val in li, we can use addi/ori to combine li + rlwimi.