[SCEV] Assertion `isAvailableAtLoopEntry(Op, L) && "SCEVAddRecExpr operand is not available at loop entry!"' failed.

[Nirhar]

Faulty IR:

; ModuleID = 'test.ll'
source_filename = "/nfs/build-cache/zvm-dev-4598/jdk11/x86_64/fastdebug/lib/server/boilerplate.ll"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128-ni:1-p2:32:8:8:32-ni:2"
target triple = "x86_64-unknown-linux-gnu"

define void @test(ptr addrspace(1) %p) {
entry:
  %check = icmp eq ptr addrspace(1) %p, null
  br label %bb0

bb0:                                              ; preds = %loop0, %entry
  br i1 %check, label %loop0, label %latch

latch:                                            ; preds = %bb0
  br i1 %check, label %exit0, label %loop0

exit0:                                            ; preds = %latch
  ret void

loop0:                                            ; preds = %latch, %bb0
  %0 = load atomic i32, ptr addrspace(1) %p unordered, align 8
  %add = add i32 %0, 3
  br i1 true, label %preheader, label %bb0

preheader:                                        ; preds = %loop0
  br label %loop1

loop1:                                            ; preds = %backedge, %preheader
  %iv1 = phi i32 [ %add, %preheader ], [ %1, %backedge ]
  %1 = add i32 %iv1, -33
  br label %loop2

backedge:                                         ; preds = %loop2
  br i1 true, label %exit1, label %loop1

loop2:                                            ; preds = %loop2, %loop1
  %iv0 = phi i32 [ %iv1, %loop1 ], [ %2, %loop2 ]
  %2 = add nsw i32 %iv0, 1
  %3 = icmp sgt i32 %2, 0
  br i1 %3, label %backedge, label %loop2

exit1:                                            ; preds = %backedge
  ret void
}

fails when run with opt -passes='print<scalar-evolution>,simple-loop-unswitch<nontrivial>,print<scalar-evolution>'
Demo of failure: https://godbolt.org/z/qhboK5r3j

[Nirhar]

Here is the IR after simple-loop-unswitch pass:

At the point of crash, the SCEV for Exit Value of %iv0(which I assume, as we crashed while printing it) is (-4 + (-1 * %1) + %.us-phi), which is incorrect. The crash happens because %1 is in a block that does not properly dominate %iv0. Looks like, simple-loop-unswitch is either incorrectly updating the SCEV in the pass, or is incorrectly reporting SCEV to be preserved. As of now, I'm not sure which is the case.

[Nirhar]

@nikic @dtcxzyw @EugeneZelenko any thoughts? I'm inclined to having a patch that removes SCEV from simple-loop-unswitch 's list of preserved analyses.

[dtcxzyw]

I'm inclined to having a patch that removes SCEV from simple-loop-unswitch 's list of preserved analyses.

Some SCEVs should be invalidated in unswitchNontrivialInvariants. Removing SCEV from the list of preserved analyses doesn't fix the issue.