[LoopPeel] Handle constants when updating exit values when peeling last.

Account for constant values when updating exit values after peeling an
iteration from the end. This can happen if the inner loop gets unrolled
and simplified.

Fixes llvm#140442.

Author: fhahn

llvm/lib/Transforms/Utils/LoopPeel.cpp

@@ -1214,7 +1214,7 @@ bool llvm::peelLoop(Loop *L, unsigned PeelCount, bool PeelLast, LoopInfo *LI,
     // Now adjust users of the original exit values by replacing them with the
     // exit value from the peeled iteration.
     for (const auto &[P, E] : ExitValues)
-      P->replaceAllUsesWith(VMap.lookup(E));
+      P->replaceAllUsesWith(isa<Constant>(E) ? E : &*VMap.lookup(E));
     formLCSSA(*L, DT, LI, SE);
   } else {
     // Now adjust the phi nodes in the loop header to get their initial values

llvm/test/Transforms/LoopUnroll/unroll-and-peel-last-iteration.ll

@@ -0,0 +1,79 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -p loop-unroll -S %s | FileCheck %s
+
+; Test case for https://github.com/llvm/llvm-project/issues/140442.
+define i32 @peel_last_iter_of_outer_lcssa_phi_with_constant_after_unrolling_inner() {
+; CHECK-LABEL: define i32 @peel_last_iter_of_outer_lcssa_phi_with_constant_after_unrolling_inner() {
+; CHECK-NEXT:  [[ENTRY:.*]]:
+; CHECK-NEXT:    br label %[[OUTER_HEADER:.*]]
+; CHECK:       [[OUTER_HEADER]]:
+; CHECK-NEXT:    [[IV:%.*]] = phi i16 [ 0, %[[ENTRY]] ], [ [[IV_NEXT:%.*]], %[[INNER_LATCH:.*]] ]
+; CHECK-NEXT:    br label %[[INNER_HEADER:.*]]
+; CHECK:       [[INNER_HEADER]]:
+; CHECK-NEXT:    br i1 false, label %[[THEN:.*]], label %[[INNER_LATCH]]
+; CHECK:       [[THEN]]:
+; CHECK-NEXT:    call void @foo(i32 0)
+; CHECK-NEXT:    br label %[[INNER_LATCH]]
+; CHECK:       [[INNER_LATCH]]:
+; CHECK-NEXT:    [[IV_NEXT]] = add nuw nsw i16 [[IV]], 1
+; CHECK-NEXT:    [[EC:%.*]] = icmp eq i16 [[IV_NEXT]], 999
+; CHECK-NEXT:    br i1 [[EC]], label %[[EXIT_PEEL_BEGIN:.*]], label %[[OUTER_HEADER]], !llvm.loop [[LOOP0:![0-9]+]]
+; CHECK:       [[EXIT_PEEL_BEGIN]]:
+; CHECK-NEXT:    [[IV_NEXT_LCSSA:%.*]] = phi i16 [ [[IV_NEXT]], %[[INNER_LATCH]] ]
+; CHECK-NEXT:    [[DOTLCSSA_LCSSA:%.*]] = phi i32 [ 1, %[[INNER_LATCH]] ]
+; CHECK-NEXT:    br label %[[OUTER_HEADER_PEEL:.*]]
+; CHECK:       [[OUTER_HEADER_PEEL]]:
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i16 [[IV_NEXT_LCSSA]], 999
+; CHECK-NEXT:    br label %[[INNER_HEADER_PEEL:.*]]
+; CHECK:       [[INNER_HEADER_PEEL]]:
+; CHECK-NEXT:    br i1 [[TMP0]], label %[[THEN_PEEL:.*]], label %[[INNER_LATCH_PEEL:.*]]
+; CHECK:       [[THEN_PEEL]]:
+; CHECK-NEXT:    call void @foo(i32 0)
+; CHECK-NEXT:    br label %[[INNER_LATCH_PEEL]]
+; CHECK:       [[INNER_LATCH_PEEL]]:
+; CHECK-NEXT:    [[IV_NEXT_PEEL:%.*]] = add i16 [[IV_NEXT_LCSSA]], 1
+; CHECK-NEXT:    [[EC_PEEL:%.*]] = icmp eq i16 [[IV_NEXT_PEEL]], 1000
+; CHECK-NEXT:    br i1 [[EC_PEEL]], label %[[EXIT_PEEL_NEXT:.*]], label %[[EXIT_PEEL_NEXT]]
+; CHECK:       [[EXIT_PEEL_NEXT]]:
+; CHECK-NEXT:    br label %[[OUTER_HEADER_PEEL_NEXT:.*]]
+; CHECK:       [[OUTER_HEADER_PEEL_NEXT]]:
+; CHECK-NEXT:    br label %[[EXIT:.*]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret i32 1
+;
+entry:
+  br label %outer.header
+
+outer.header:
+  %iv = phi i16 [ 0, %entry ], [ %iv.next, %outer.latch ]
+  %0 = icmp eq i16 %iv, 999
+  br label %inner.header
+
+inner.header:
+  %inner.iv = phi i32 [ 0, %outer.header ], [ %inner.iv.next, %inner.latch ]
+  %1 = add i32 %inner.iv, 1
+  br i1 %0, label %then, label %inner.latch
+
+then:
+  call void @foo(i32 %inner.iv)
+  br label %inner.latch
+
+inner.latch:
+  %inner.iv.next = add i32 %inner.iv, 1
+  %inner.ec = icmp eq i32 %inner.iv.next, 1
+  br i1 %inner.ec, label %outer.latch, label %inner.header
+
+outer.latch:
+  %iv.next = add i16 %iv, 1
+  %ec = icmp eq i16 %iv.next, 1000
+  br i1 %ec, label %exit, label %outer.header
+
+exit:
+  ret i32 %1
+}
+
+declare void @foo(i32)
+;.
+; CHECK: [[LOOP0]] = distinct !{[[LOOP0]], [[META1:![0-9]+]]}
+; CHECK: [[META1]] = !{!"llvm.loop.peeled.count", i32 1}
+;.