[LoopSink] Do not try to sink phi nodes.

[fhahn]

Summary

Skip phi nodes in the preheader. They may not be considered loop
invariant by the assertion below.

[reames]

LGTM - Not sure this is the right deep fix, but it's at least reasonable.

llvm/lib/Transforms/Scalar/LoopSink.cpp

static bool sinkLoopInvariantInstructions(Loop &L, AAResults &AA, LoopInfo &LI,
   llvm::stable_sort(ColdLoopBBs, [&](BasicBlock *A, BasicBlock *B) {
     return BFI.getBlockFreq(A) < BFI.getBlockFreq(B);
   });
 
   // Traverse preheader's instructions in reverse order becaue if A depends
   // on B (A appears after B), A needs to be sinked first before B can be
   // sinked.
   for (Instruction &I : llvm::make_early_inc_range(llvm::reverse(*Preheader))) {
+    if (isa<PHINode>(&I))
+      continue;
     // No need to check for instruction's operands are loop invariant.
     assert(L.hasLoopInvariantOperands(&I) &&
            "Insts in a loop's preheader should have loop invariant operands!");
     if (!canSinkOrHoistInst(I, &AA, &DT, &L, CurAST, MSSAU.get(), false,
                             LICMFlags.get()))
       continue;
     if (sinkInstruction(L, I, ColdLoopBBs, LoopBlockNumber, LI, DT, BFI,
                         MSSAU.get()))

llvm/test/Transforms/LICM/loop-sink-phi-in-preheader.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -S -passes=loop-sink %s | FileCheck %s
+
+define void @preheader_of_inner_has_phi() !prof !0 {
+; CHECK-LABEL: @preheader_of_inner_has_phi(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    br label [[OUTER_HEADER:%.*]]
+; CHECK:       outer.header:
+; CHECK-NEXT:    [[PH_1:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[PH_2:%.*]], [[OUTER_LATCH:%.*]] ]
+; CHECK-NEXT:    br label [[INNER:%.*]]
+; CHECK:       inner:
+; CHECK-NEXT:    [[PH_2]] = phi i32 [ 0, [[OUTER_HEADER]] ], [ 0, [[INNER]] ]
+; CHECK-NEXT:    br i1 false, label [[OUTER_LATCH]], label [[INNER]], !prof [[PROF1:![0-9]+]]
+; CHECK:       outer.latch:
+; CHECK-NEXT:    br label [[OUTER_HEADER]]
+;
+entry:
+  br label %outer.header
+
+outer.header:
+  %ph.1 = phi i32 [ 0, %entry ], [ %ph.2, %outer.latch ]
+  br label %inner
+
+inner:
+  %ph.2 = phi i32 [ 0, %outer.header ], [ 0, %inner ]
+  br i1 false, label %outer.latch, label %inner, !prof !1
+
+outer.latch:
+  br label %outer.header
+}
+
+!0 = !{!"function_entry_count", i64 549102}
+!1 = !{!"branch_weights", i32 4027913, i32 1}