[SCEV] Consider non-volatile memory intrinsics as not having side-effect for forward progress

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -7435,7 +7435,15 @@ ScalarEvolution::getLoopProperties(const Loop *L) {
       if (auto *SI = dyn_cast<StoreInst>(I))
         return !SI->isSimple();
 
-      return I->mayThrow() || I->mayWriteToMemory();
+      if (I->mayThrow())
+        return true;
+
+      // Non-volatile memset / memcpy do not count as side-effect for forward
+      // progress.
+      if (isa<MemIntrinsic>(I) && !I->isVolatile())
+        return false;
+
+      return I->mayWriteToMemory();
     };
 
     LoopProperties LP = {/* HasNoAbnormalExits */ true,

llvm/test/Transforms/LoopIdiom/introduce-memset-in-outerloop.ll

@@ -0,0 +1,47 @@
+; RUN: opt <%s -p "print<scalar-evolution>" -disable-output 2>&1 | FileCheck %s
+; RUN: opt <%s -p "loop(loop-idiom),print<scalar-evolution>" -disable-output 2>&1 | FileCheck %s
+
+; CHECK: backedge-taken count is i64 1
+
+; 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;
+; }
+
+@a = external global [0 x i8]
+
+define void @foo()  {
+entry:
+  br label %outerL
+
+outerL:                              ; preds = %entry, %outerLatch
+  %e = phi i64 [ poison, %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
+
+exit:                                         ; preds = %outerLatch
+  ret void
+}
+
+!0 = distinct !{!0, !2}
+!1 = distinct !{!1, !2}
+!2 = !{!"llvm.loop.mustprogress"}