[llvm] [LoopInterchange] Fix assertion failure when LCSSA PHI is used by non-PHI (PR #201059)

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<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Ryotaro Kasuga (kasuga-fj)

<details>
<summary>Changes</summary>

This patch fixes one of the assertion failures reported in #<!-- -->200819. The root cause is that the assertion assumes all uses of LCSSA PHIs are PHI nodes, which is not always true, so `cast<PHINode>(U)` can fail. In fact, the user does not have to be a PHI, and the assertion should apply the special check only when the user is a PHI node.

---
Full diff: https://github.com/llvm/llvm-project/pull/201059.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Scalar/LoopInterchange.cpp (+4-2) 
- (added) llvm/test/Transforms/LoopInterchange/non-phi-uses-lcssa-phi.ll (+72) 


``````````diff
diff --git a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
index 5143e2f497bc5..b0a9fc339bb35 100644
--- a/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInterchange.cpp
@@ -2213,12 +2213,14 @@ static void moveLCSSAPhis(BasicBlock *InnerExit, BasicBlock *InnerHeader,
 
     assert(all_of(P.users(),
                   [OuterHeader, OuterExit, IncI, InnerHeader](User *U) {
+                    if (!isa<PHINode>(U))
+                      return true;
                     return (cast<PHINode>(U)->getParent() == OuterHeader &&
                             IncI->getParent() == InnerHeader) ||
                            cast<PHINode>(U)->getParent() == OuterExit;
                   }) &&
-           "Can only replace phis iff the uses are in the loop nest exit or "
-           "the incoming value is defined in the inner header (it will "
+           "Can only replace phis iff the phi-uses are in the loop nest exit "
+           "or the incoming value is defined in the inner header (it will "
            "dominate all loop blocks after interchanging)");
     P.replaceAllUsesWith(IncI);
     P.eraseFromParent();
diff --git a/llvm/test/Transforms/LoopInterchange/non-phi-uses-lcssa-phi.ll b/llvm/test/Transforms/LoopInterchange/non-phi-uses-lcssa-phi.ll
new file mode 100644
index 0000000000000..32ad3e46d7923
--- /dev/null
+++ b/llvm/test/Transforms/LoopInterchange/non-phi-uses-lcssa-phi.ll
@@ -0,0 +1,72 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -passes=loop-interchange -loop-interchange-profitabilities=ignore -S | FileCheck %s
+
+; Ensure that non-PHI uses of an LCSSA PHI does not cause havoc.
+
+define i64 @non_phi_lcssa_phi_user(ptr %A) {
+; CHECK-LABEL: define i64 @non_phi_lcssa_phi_user(
+; CHECK-SAME: ptr [[A:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[INNER_PREHEADER:.*]]
+; CHECK:       [[OUTER_HEADER_PREHEADER:.*]]:
+; CHECK-NEXT:    br label %[[OUTER_HEADER:.*]]
+; CHECK:       [[OUTER_HEADER]]:
+; CHECK-NEXT:    [[I:%.*]] = phi i64 [ [[I_NEXT:%.*]], %[[OUTER_LATCH:.*]] ], [ 0, %[[OUTER_HEADER_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[INNER_SPLIT1:.*]]
+; CHECK:       [[INNER_PREHEADER]]:
+; CHECK-NEXT:    br label %[[INNER:.*]]
+; CHECK:       [[INNER]]:
+; CHECK-NEXT:    [[J:%.*]] = phi i64 [ [[TMP1:%.*]], %[[INNER_SPLIT:.*]] ], [ 0, %[[INNER_PREHEADER]] ]
+; CHECK-NEXT:    br label %[[OUTER_HEADER_PREHEADER]]
+; CHECK:       [[INNER_SPLIT1]]:
+; CHECK-NEXT:    [[PTR:%.*]] = getelementptr inbounds [100 x i32], ptr [[A]], i64 [[J]], i64 [[I]]
+; CHECK-NEXT:    store i32 0, ptr [[PTR]], align 4
+; CHECK-NEXT:    [[J_NEXT:%.*]] = add i64 [[J]], 1
+; CHECK-NEXT:    [[EXITCOND_INNER:%.*]] = icmp eq i64 [[J]], 99
+; CHECK-NEXT:    br label %[[INNER_EXIT:.*]]
+; CHECK:       [[INNER_SPLIT]]:
+; CHECK-NEXT:    [[TMP0:%.*]] = phi i64 [ [[USE_J:%.*]], %[[OUTER_LATCH]] ]
+; CHECK-NEXT:    [[TMP1]] = add i64 [[J]], 1
+; CHECK-NEXT:    [[TMP2:%.*]] = icmp eq i64 [[J]], 99
+; CHECK-NEXT:    br i1 [[TMP2]], label %[[EXIT:.*]], label %[[INNER]]
+; CHECK:       [[INNER_EXIT]]:
+; CHECK-NEXT:    br label %[[OUTER_LATCH]]
+; CHECK:       [[OUTER_LATCH]]:
+; CHECK-NEXT:    [[USE_J]] = add i64 [[J]], 1
+; CHECK-NEXT:    [[I_NEXT]] = add i64 [[I]], 1
+; CHECK-NEXT:    [[EXITCOND_OUTER:%.*]] = icmp eq i64 [[I]], 99
+; CHECK-NEXT:    br i1 [[EXITCOND_OUTER]], label %[[INNER_SPLIT]], label %[[OUTER_HEADER]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    [[RESULT:%.*]] = phi i64 [ [[TMP0]], %[[INNER_SPLIT]] ]
+; CHECK-NEXT:    ret i64 [[RESULT]]
+;
+entry:
+  br label %outer.header
+
+outer.header:
+  %i = phi i64 [ 0, %entry ], [ %i.next, %outer.latch ]
+  br label %inner
+
+inner:
+  %j = phi i64 [ 0, %outer.header ], [ %j.next, %inner ]
+  %ptr = getelementptr inbounds [100 x i32], ptr %A, i64 %j, i64 %i
+  store i32 0, ptr %ptr
+  %j.next = add i64 %j, 1
+  %exitcond.inner = icmp eq i64 %j, 99
+  br i1 %exitcond.inner, label %inner.exit, label %inner
+
+inner.exit:
+  %j.lcssa = phi i64 [ %j, %inner ]
+  br label %outer.latch
+
+outer.latch:
+  %j.latch = phi i64 [ %j.lcssa, %inner.exit ]
+  %use.j = add i64 %j.latch, 1
+  %i.next = add i64 %i, 1
+  %exitcond.outer = icmp eq i64 %i, 99
+  br i1 %exitcond.outer, label %exit, label %outer.header
+
+exit:
+  %result = phi i64 [ %use.j, %outer.latch ]
+  ret i64 %result
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/201059