[llvm] 7e14161 - [LoopPeel] Handle constants when updating exit values when peeling last.

[Florian Hahn via llvm-commits]

Author: Florian Hahn
Date: 2025-05-18T10:17:21+01:00
New Revision: 7e14161f49b32387988cf9d937bbfaa27d0fbdd5

URL: https://github.com/llvm/llvm-project/commit/7e14161f49b32387988cf9d937bbfaa27d0fbdd5
DIFF: https://github.com/llvm/llvm-project/commit/7e14161f49b32387988cf9d937bbfaa27d0fbdd5.diff

LOG: [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 https://github.com/llvm/llvm-project/issues/140442.

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

Modified: 
    llvm/lib/Transforms/Utils/LoopPeel.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/LoopPeel.cpp b/llvm/lib/Transforms/Utils/LoopPeel.cpp
index c5c637ab6e202..646354bba9df6 100644
--- a/llvm/lib/Transforms/Utils/LoopPeel.cpp
+++ b/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

diff  --git a/llvm/test/Transforms/LoopUnroll/unroll-and-peel-last-iteration.ll b/llvm/test/Transforms/LoopUnroll/unroll-and-peel-last-iteration.ll
new file mode 100644
index 0000000000000..f57fb2d9b7057
--- /dev/null
+++ b/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}
+;.