[all-commits] [llvm/llvm-project] d9a9c9: [LoopReroll] Fix rerolling loop with extra instruc...

[KAWASHIMA Takahiro via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: d9a9c992d190dd6645ea911b66cf0cadba0dadc3
      https://github.com/llvm/llvm-project/commit/d9a9c992d190dd6645ea911b66cf0cadba0dadc3
  Author: KAWASHIMA Takahiro <t-kawashima at fujitsu.com>
  Date:   2021-04-23 (Fri, 23 Apr 2021)

  Changed paths:
    M llvm/lib/Transforms/Scalar/LoopRerollPass.cpp
    A llvm/test/Transforms/LoopReroll/extra_instr.ll

  Log Message:
  -----------
  [LoopReroll] Fix rerolling loop with extra instructions

Fixes PR47627

This fix suppresses rerolling a loop which has an unrerollable
instruction.

Sample IR for the explanation below:

```
define void @foo([2 x i32]* nocapture %a) {
entry:
  br label %loop

loop:
  ; base instruction
  %indvar = phi i64 [ 0, %entry ], [ %indvar.next, %loop ]

  ; unrerollable instructions
  %stptrx = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 %indvar, i64 0
  store i32 999, i32* %stptrx, align 4

  ; extra simple arithmetic operations, used by root instructions
  %plus20 = add nuw nsw i64 %indvar, 20
  %plus10 = add nuw nsw i64 %indvar, 10

  ; root instruction 0
  %ldptr0 = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 %plus20, i64 0
  %value0 = load i32, i32* %ldptr0, align 4
  %stptr0 = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 %plus10, i64 0
  store i32 %value0, i32* %stptr0, align 4

  ; root instruction 1
  %ldptr1 = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 %plus20, i64 1
  %value1 = load i32, i32* %ldptr1, align 4
  %stptr1 = getelementptr inbounds [2 x i32], [2 x i32]* %a, i64 %plus10, i64 1
  store i32 %value1, i32* %stptr1, align 4

  ; loop-increment and latch
  %indvar.next = add nuw nsw i64 %indvar, 1
  %exitcond = icmp eq i64 %indvar.next, 5
  br i1 %exitcond, label %exit, label %loop

exit:
  ret void
}
```

In the loop rerolling pass, `%indvar` and `%indvar.next` are appended
to the `LoopIncs` vector in the `LoopReroll::DAGRootTracker::findRoots`
function.

Before this fix, two instructions with `unrerollable instructions`
comment above are marked as `IL_All` at the end of the
`LoopReroll::DAGRootTracker::collectUsedInstructions` function,
as well as instructions with `extra simple arithmetic operations`
comment and `loop-increment and latch` comment. It is incorrect
because `IL_All` means that the instruction should be executed in all
iterations of the rerolled loop but the `store` instruction should
not.

This fix rejects instructions which may have side effects and don't
belong to def-use chains of any root instructions and reductions.

See https://bugs.llvm.org/show_bug.cgi?id=47627 for more information.