[llvm] [loop-idiom] Forget outer loop scev when loop-idiom introduces memset… (PR #150916)

[Sushant Gokhale via llvm-commits]

================
@@ -0,0 +1,48 @@
+; REQUIRES: asserts
+; RUN: opt <%s -p "loop(loop-idiom)" -verify-scev -o /dev/null
+
+
+; IR corresponds to the following C test:
+; extern char a[];
+; void foo() {
+;   for (long c = 0; c < 6; c += -2078836808675943215)
+;     for (long d; d < 6; d++)
+;       a[c + d] = 0;
+; }
+
+; Make sure we do not crash when verifying scev.
+
+ at a = external global [0 x i8]
+
+define void @foo()  {
+entry:
+  br label %outerL
+
+outerL:                              ; preds = %entry, %outerLatch
+  %e = phi i64 [ undef, %entry ], [ %lcssa, %outerLatch ]
+  %c = phi i64 [ 0, %entry ], [ %c.next, %outerLatch ]
+  %e.cmp = icmp slt i64 %e, 6
+  br i1 %e.cmp, label %innerL, label %outerLatch
+
+innerL:                                          ; preds = %outerL, %innerL
+  %d = phi i64 [ %d.next, %innerL ], [ %e, %outerL ]
+  %add = add nsw i64 %d, %c
+  %arrayidx = getelementptr inbounds [0 x i8], ptr @a, i64 0, i64 %add
+  store i8 0, ptr %arrayidx
+  %d.next = add nsw i64 %d, 1
+  %d.cmp = icmp slt i64 %d, 5
+  br i1 %d.cmp, label %innerL, label %outerLatch, !llvm.loop !0
+
+outerLatch:                                         ; preds = %innerL, %outerL
+  %lcssa = phi i64 [ %e, %outerL ], [ %d.next, %innerL ]
+  %c.next = add nsw i64 %c, -2078836808675943215
+  %c.cmp = icmp slt i64 %c, 2078836808675943221
+  br i1 %c.cmp, label %outerL, label %exit, !llvm.loop !1
----------------
sushgokh wrote:

Not for the patch to work, but for reproducing the crash

https://github.com/llvm/llvm-project/pull/150916