aleks-tmb wrote:
@VigneshwarJ
Hi, before your MR was reverted, our downstream testing detected an issue.
Below is a minimal reproducer:
```
define i32 @main(i1 %cond) {
entry:
%a = load i32, ptr addrspace(1) null, align 8
br label %loop0
loop0:
%iv0 = phi i32 [ 0, %entry ], [ %iv0.next, %loop0 ]
%iv0.next = add i32 %iv0, 1
br i1 %cond, label %mid, label %loop0
mid:
%add = add i32 %a, %iv0
ret i32 %add
}
```
Command to reproduce:
```
bin/opt -passes='print<scalar-evolution>,loop-mssa(licm)' -verify-scev test.ll
```
Output:
```
Printing analysis 'Scalar Evolution Analysis' for function 'main':
Classifying expressions for: @main
%a = load i32, ptr addrspace(1) null, align 8
--> %a U: full-set S: full-set
%iv0 = phi i32 [ 0, %entry ], [ %iv0.next, %loop0 ]
--> {0,+,1}<%loop0> U: full-set S: full-set Exits: <<Unknown>>
LoopDispositions: { %loop0: Computable }
%iv0.next = add i32 %iv0, 1
--> {1,+,1}<%loop0> U: full-set S: full-set Exits: <<Unknown>>
LoopDispositions: { %loop0: Computable }
%add = add i32 %a, %iv0
--> {%a,+,1}<%loop0> U: full-set S: full-set
Determining loop execution counts for: @main
Loop %loop0: Unpredictable backedge-taken count.
Loop %loop0: Unpredictable constant max backedge-taken count.
Loop %loop0: Unpredictable symbolic max backedge-taken count.
opt:
/remote-home/apopov/old_work/llvm-project/llvm/lib/Analysis/ScalarEvolution.cpp:3706:
const llvm::SCEV*
llvm::ScalarEvolution::getAddRecExpr(llvm::SmallVectorImpl<const llvm::SCEV*>&,
const llvm::Loop*, llvm::SCEV::NoWrapFlags): Assertion
`isAvailableAtLoopEntry(Op, L) && "SCEVAddRecExpr operand is not available at
loop entry!"' failed.
```
The root cause of the issue is that after sinking load `%a` to the loop exit,
the SCEV `{%a,+,1}<%loop0>` became invalid, since `%a` is no longer available
at the `%loop0` entry.
A possible fix is to forget the SCEV after load sinking
```cpp
...
moveInstructionBefore(I, ExitBlock->getFirstInsertionPt(), *SafetyInfo,
MSSAU, SE, MemorySSA::Beginning);
if (SE)
SE->forgetValue(&I);
```
https://github.com/llvm/llvm-project/pull/157559
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