[PredicateInfo] Reserve adjacent LN_Last defs for the same phi use (#164577)

This patch fixes a missed optimization issue: predicate infos might be
lost in phi-use scenarios.

Due to the existence of and-chains, a phi-use might be associated with
multiple LN_Last predicate infos.
E.g.,
```cpp
// TWO LN_Last Predicate Info defs:
// 1. a >= 1
// 2. a < 2
if ( a < 1 || a >= 2) {
  a = 1;
}  
// PHI use of `a`
use(a)
```
However, previously, `popStackUntilDFSScope` reserved only ONE LN_Last
def for a phi use (i.e., reserve only one of `a >= 1` / `a < 2`),
although there might be multiple LN_Last defs for the same phi use.


This patch reserves the adjacent LN_Last defs if they are designated for
the same phi use.

Author: Camsyn

llvm/lib/Transforms/Utils/PredicateInfo.cpp

@@ -260,9 +260,16 @@ bool PredicateInfoBuilder::stackIsInScope(const ValueDFSStack &Stack,
   // next to the defs they must go with so that we can know it's time to pop
   // the stack when we hit the end of the phi uses for a given def.
   const ValueDFS &Top = *Stack.back().V;
-  if (Top.LocalNum == LN_Last && Top.PInfo) {
-    if (!VDUse.U)
-      return false;
+  assert(Top.PInfo && "RenameStack should only contain predicate infos (defs)");
+  if (Top.LocalNum == LN_Last) {
+    if (!VDUse.U) {
+      assert(VDUse.PInfo && "A non-use VDUse should have a predicate info");
+      // We should reserve adjacent LN_Last defs for the same phi use.
+      return VDUse.LocalNum == LN_Last &&
+             // If the two phi defs have the same edge, they must be designated
+             // for the same succ BB.
+             getBlockEdge(Top.PInfo) == getBlockEdge(VDUse.PInfo);
+    }
     auto *PHI = dyn_cast<PHINode>(VDUse.U->getUser());
     if (!PHI)
       return false;

llvm/test/Transforms/SCCP/conditions-ranges.ll

@@ -1547,3 +1547,28 @@ bb2:
   call void @use(i1 %c4)
   ret void
 }
+
+define i1 @and_predicate_dominating_phi(i32 %x) {
+; CHECK-LABEL: @and_predicate_dominating_phi(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[XGE1:%.*]] = icmp uge i32 [[X:%.*]], 1
+; CHECK-NEXT:    [[XLT2:%.*]] = icmp ult i32 [[X]], 2
+; CHECK-NEXT:    [[AND:%.*]] = and i1 [[XGE1]], [[XLT2]]
+; CHECK-NEXT:    br i1 [[AND]], label [[PHI:%.*]], label [[NOPE:%.*]]
+; CHECK:       nope:
+; CHECK-NEXT:    br label [[PHI]]
+; CHECK:       phi:
+; CHECK-NEXT:    ret i1 true
+;
+entry:
+  %xge1 = icmp uge i32 %x, 1
+  %xlt2 = icmp ult i32 %x, 2
+  %and = and i1 %xge1, %xlt2
+  br i1 %and, label %phi, label %nope
+nope:
+  br label %phi
+phi:
+  %res = phi i32 [ %x, %entry ], [ 1, %nope ]
+  %ret = icmp uge i32 %res, 1
+  ret i1 %ret
+}

llvm/test/Transforms/Util/PredicateInfo/testandor.ll

@@ -994,3 +994,30 @@ define void @test_assume_deep_and_tree(i1 %a1) {
   call void @foo(i1 %a15)
   ret void
 }
+
+define i32 @test_and_with_phinode(i32 %x) {
+; CHECK-LABEL: @test_and_with_phinode(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[XGE1:%.*]] = icmp uge i32 [[X:%.*]], 1
+; CHECK-NEXT:    [[XLT2:%.*]] = icmp ult i32 [[X]], 2
+; CHECK-NEXT:    [[AND:%.*]] = and i1 [[XGE1]], [[XLT2]]
+; CHECK:         [[X_0_1:%.*]] = bitcast i32 [[X]] to i32
+; CHECK:         [[X_0_2:%.*]] = bitcast i32 [[X_0_1]] to i32
+; CHECK-NEXT:    br i1 [[AND]], label [[PHI:%.*]], label [[NOPE:%.*]]
+; CHECK:       nope:
+; CHECK-NEXT:    br label [[PHI]]
+; CHECK:       phi:
+; CHECK-NEXT:    [[RES:%.*]] = phi i32 [ [[X_0_2]], [[ENTRY:%.*]] ], [ 1, [[NOPE]] ]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+entry:
+  %xge1 = icmp uge i32 %x, 1
+  %xlt2 = icmp ult i32 %x, 2
+  %and = and i1 %xge1, %xlt2
+  br i1 %and, label %phi, label %nope
+nope:
+  br label %phi
+phi:
+  %res = phi i32 [ %x, %entry ], [ 1, %nope ]
+  ret i32 %res
+}