[NFC] Add assert and test showing that revert of D109596 wasn't justified

All transforms of IndVars have prerequisite requirement of LCSSA and LoopSimplify
form and rely on it. Added test that shows that this actually stands.

Author: xortator

llvm/lib/Transforms/Scalar/IndVarSimplify.cpp

@@ -1311,6 +1311,7 @@ static void foldExit(const Loop *L, BasicBlock *ExitingBB, bool IsTaken,
 
 static void replaceLoopPHINodesWithPreheaderValues(
     Loop *L, SmallVectorImpl<WeakTrackingVH> &DeadInsts) {
+  assert(L->isLoopSimplifyForm() && "Should only do it in simplify form!");
   auto *LoopPreheader = L->getLoopPreheader();
   auto *LoopHeader = L->getHeader();
   for (auto &PN : LoopHeader->phis()) {

llvm/test/Transforms/IndVarSimplify/eliminate-backedge.ll

@@ -77,3 +77,88 @@ exit:
   [ %baz_ret, %latch ], [ 0, %entry ]
   ret i1 %val
 }
+
+define i1 @siblings(i8* align 1 %lhs, i8* align 1 %rhs, i32 %len) {
+; CHECK-LABEL: @siblings(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    %length_not_zero = icmp ne i32 %len, 0
+; CHECK-NEXT:    br i1 %length_not_zero, label %weird_loop.preheader, label %exit
+; CHECK:       weird_loop.preheader:
+; CHECK-NEXT:    br label %weird_loop
+; CHECK:       weird_loop:
+; CHECK-NEXT:    %weird.iv = phi i32 [ %weird.iv.next, %weird_loop ], [ 0, %weird_loop.preheader ]
+; CHECK-NEXT:    %weird.iv.next = add i32 %weird.iv, 1
+; CHECK-NEXT:    %weird.iv.wide = zext i32 %weird.iv to i64
+; CHECK-NEXT:    %weird.cond = call i1 @bar()
+; CHECK-NEXT:    br i1 %weird.cond, label %weird_loop, label %loop.preheader
+; CHECK:       loop.preheader:
+; CHECK-NEXT:    %weird.iv.lcssa = phi i32 [ %weird.iv, %weird_loop ]
+; CHECK-NEXT:    %weird.iv.wide.lcssa = phi i64 [ %weird.iv.wide, %weird_loop ]
+; CHECK-NEXT:    br label %loop
+; CHECK:       loop:
+; CHECK-NEXT:    %iv.next = add i32 %weird.iv.lcssa, 1
+; CHECK-NEXT:    %left_ptr = getelementptr inbounds i8, i8* %lhs, i32 %weird.iv.lcssa
+; CHECK-NEXT:    %right_ptr = getelementptr inbounds i8, i8* %rhs, i32 %weird.iv.lcssa
+; CHECK-NEXT:    %result = call i1 @foo(i8* %left_ptr, i8* %right_ptr)
+; CHECK-NEXT:    br i1 %result, label %exiting_1, label %exit.loopexit
+; CHECK:       exiting_1:
+; CHECK-NEXT:    %iv.wide.is_not_zero = icmp ne i64 %weird.iv.wide.lcssa, %weird.iv.wide.lcssa
+; CHECK-NEXT:    br i1 false, label %exiting_2, label %exit.loopexit
+; CHECK:       exiting_2:
+; CHECK-NEXT:    %bar_ret = call i1 @bar()
+; CHECK-NEXT:    br i1 %bar_ret, label %exit.loopexit, label %exiting_3
+; CHECK:       exiting_3:
+; CHECK-NEXT:    %baz_ret = call i1 @baz()
+; CHECK-NEXT:    %continue = icmp ne i32 %iv.next, %len
+; CHECK-NEXT:    %or.cond = select i1 %baz_ret, i1 %continue, i1 false
+; CHECK-NEXT:    br i1 %or.cond, label %loop, label %exit.loopexit
+; CHECK:       exit.loopexit:
+; CHECK-NEXT:    %val.ph = phi i1 [ %baz_ret, %exiting_3 ], [ %bar_ret, %exiting_2 ], [ %iv.wide.is_not_zero, %exiting_1 ], [ %result, %loop ]
+; CHECK-NEXT:    br label %exit
+; CHECK:       exit:
+; CHECK-NEXT:    %val = phi i1 [ false, %entry ], [ %val.ph, %exit.loopexit ]
+; CHECK-NEXT:    ret i1 %val
+;
+entry:
+  %length_not_zero = icmp ne i32 %len, 0
+  br i1 %length_not_zero, label %weird_loop, label %exit
+
+weird_loop:
+  %weird.iv = phi i32 [ 0, %entry ], [ %weird.iv.next, %weird_loop ]
+  %weird.iv.next = add i32 %weird.iv, 1
+  %weird.iv.wide = zext i32 %weird.iv to i64
+  %weird.cond = call i1 @bar()
+  br i1 %weird.cond, label %weird_loop, label %loop
+
+loop:
+  %iv = phi i32 [ %weird.iv, %weird_loop ], [ %iv.next, %latch ]
+  %iv.wide = phi i64 [ %weird.iv.wide, %weird_loop ], [ %iv.wide.next, %latch ]
+  %iv.next = add i32 %iv, 1
+  %iv.wide.next = add i64 %iv.wide, 1
+  %left_ptr = getelementptr inbounds i8, i8* %lhs, i32 %iv
+  %right_ptr = getelementptr inbounds i8, i8* %rhs, i32 %iv
+  %result = call i1 @foo(i8* %left_ptr, i8* %right_ptr)
+  br i1 %result, label %exiting_1, label %exit
+
+exiting_1:
+  %iv.wide.is_not_zero = icmp ne i64 %iv.wide, %weird.iv.wide
+  br i1 %iv.wide.is_not_zero, label %exiting_2, label %exit
+
+exiting_2:
+  %bar_ret = call i1 @bar()
+  br i1 %bar_ret, label %exit, label %exiting_3
+
+exiting_3:
+  %baz_ret = call i1 @baz()
+  br i1 %baz_ret, label %latch, label %exit
+
+latch:
+  %continue = icmp ne i32 %iv.next, %len
+  br i1 %continue, label %loop, label %exit
+
+exit:
+  %val = phi i1 [ %result, %loop ], [ %iv.wide.is_not_zero, %exiting_1 ],
+  [ %bar_ret, %exiting_2 ], [ %baz_ret, %exiting_3 ],
+  [ %baz_ret, %latch ], [ 0, %entry ]
+  ret i1 %val
+}