[SCEV] Fix isImpliedViaMerge() with values from previous iteration (PR56242)

When trying to prove an implied condition on a phi by proving it
for all incoming values, we need to be careful about values coming
from a backedge, as these may refer to a previous loop iteration.
A variant of this issue was fixed in D101829, but the dominance
condition used there isn't quite right: It checks that the value
dominates the incoming block, which doesn't exclude backedges
(values defined in a loop will usually dominate the loop latch,
which is the incoming block of the backedge).

Instead, we should be checking for domination of the phi block.
Any values defined inside the loop will not dominate the loop
header phi.

Fixes #56242.

Differential Revision: https://reviews.llvm.org/D128640

Author: nikic

llvm/lib/Analysis/ScalarEvolution.cpp

@@ -11812,7 +11812,7 @@ bool ScalarEvolution::isImpliedViaMerge(ICmpInst::Predicate Pred,
       const SCEV *L = getSCEV(LPhi->getIncomingValueForBlock(IncBB));
       // Make sure L does not refer to a value from a potentially previous
       // iteration of a loop.
-      if (!properlyDominates(L, IncBB))
+      if (!properlyDominates(L, LBB))
         return false;
       if (!ProvedEasily(L, RHS))
         return false;

llvm/test/Transforms/IRCE/decrementing-loop.ll

@@ -210,17 +210,16 @@ exit:
   ret void
 }
 
-; TODO: we need to be more careful when trying to look through phi nodes in
-; cycles, because the condition to prove may reference the previous value of
-; the phi. So we currently fail to optimize this case.
 ; Check that we can figure out that IV is non-negative via implication through
 ; two Phi nodes, one being AddRec.
 define void @test_05(i32* %a, i32* %a_len_ptr, i1 %cond) {
 
 ; CHECK-LABEL: test_05
-; CHECK: entry:
-; CHECK:   br label %merge
-; CHECK-NOT: mainloop
+; CHECK:       mainloop:
+; CHECK-NEXT:    br label %loop
+; CHECK:       loop:
+; CHECK:         br i1 true, label %in.bounds, label %out.of.bounds
+; CHECK:       loop.preloop:
 
  entry:
   %len.a = load i32, i32* %a_len_ptr, !range !0

llvm/test/Transforms/IndVarSimplify/pr56242.ll

@@ -9,12 +9,14 @@ define void @test(ptr %arr) {
 ; CHECK-NEXT:    br label [[LOOP_HEADER:%.*]]
 ; CHECK:       loop.header:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i64 [ [[IV_INC:%.*]], [[LOOP_LATCH:%.*]] ], [ 0, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[PREV:%.*]] = phi i32 [ [[V:%.*]], [[LOOP_LATCH]] ], [ 0, [[ENTRY]] ]
 ; CHECK-NEXT:    [[PTR:%.*]] = getelementptr inbounds i32, ptr [[ARR:%.*]], i64 [[IV]]
-; CHECK-NEXT:    [[V:%.*]] = load i32, ptr [[PTR]], align 4
+; CHECK-NEXT:    [[V]] = load i32, ptr [[PTR]], align 4
 ; CHECK-NEXT:    [[CMP1:%.*]] = icmp sgt i32 [[V]], 0
 ; CHECK-NEXT:    br i1 [[CMP1]], label [[IF:%.*]], label [[LOOP_LATCH]]
 ; CHECK:       if:
-; CHECK-NEXT:    call void @use(i1 false)
+; CHECK-NEXT:    [[CMP2:%.*]] = icmp slt i32 [[PREV]], 0
+; CHECK-NEXT:    call void @use(i1 [[CMP2]])
 ; CHECK-NEXT:    br label [[LOOP_LATCH]]
 ; CHECK:       loop.latch:
 ; CHECK-NEXT:    [[IV_INC]] = add nuw nsw i64 [[IV]], 1