[llvm] [LoopVersioning] Add a check to see if the input loop is in LCSSA form (PR #116443)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Vedant Paranjape (VedantParanjape)

<details>
<summary>Changes</summary>

Loop Optimizations expect the input loop to be in LCSSA form. But it seems that LoopVersioning doesn't have any check to see if the loop is actually in LCSSA form. As a result, if we give it a loop which is not in LCSSA form but still correct semantically, the resulting transformation fails to pass through verifier pass with the following error.

Instruction does not dominate all uses!
%inc = add nsw i16 undef, 1
store i16 %inc, ptr @<!-- -->c, align 1

As the loop is not in LCSSA form, LoopVersioning's transformations leads to invalid IR! As some instructions do not dominate all their uses.

This patch checks if a loop is in LCSSA form, if not it will call formLCSSARecursively on the loop before passing it to LoopVersioning.

Fixes: #<!-- -->36998

---
Full diff: https://github.com/llvm/llvm-project/pull/116443.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/LoopVersioning.cpp (+4) 
- (added) llvm/test/Transforms/LoopVersioning/crash-36998.ll (+57) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/LoopVersioning.cpp b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
index 8f8c40a4e73be1..b437ddb4b0a300 100644
--- a/llvm/lib/Transforms/Utils/LoopVersioning.cpp
+++ b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
@@ -26,6 +26,7 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
+#include "llvm/Transforms/Utils/LoopUtils.h"
 #include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
 
 using namespace llvm;
@@ -278,6 +279,9 @@ bool runImpl(LoopInfo *LI, LoopAccessInfoManager &LAIs, DominatorTree *DT,
     if (!LAI.hasConvergentOp() &&
         (LAI.getNumRuntimePointerChecks() ||
          !LAI.getPSE().getPredicate().isAlwaysTrue())) {
+      if (!L->isLCSSAForm(*DT))
+        llvm::formLCSSARecursively(*L, *DT, LI, SE);
+
       LoopVersioning LVer(LAI, LAI.getRuntimePointerChecking()->getChecks(), L,
                           LI, DT, SE);
       LVer.versionLoop();
diff --git a/llvm/test/Transforms/LoopVersioning/crash-36998.ll b/llvm/test/Transforms/LoopVersioning/crash-36998.ll
new file mode 100644
index 00000000000000..53bcb5f310c001
--- /dev/null
+++ b/llvm/test/Transforms/LoopVersioning/crash-36998.ll
@@ -0,0 +1,57 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -passes=loop-versioning -aa-pipeline='' -S < %s | FileCheck %s
+target triple = "x86_64-unknown-linux-gnu"
+
+ at a = external global i16, align 1
+ at b = external global i16, align 1
+ at c = external global i16, align 1
+
+define void @f2() {
+; CHECK-LABEL: define void @f2() {
+; CHECK-NEXT:  [[FOR_BODY_LVER_CHECK:.*:]]
+; CHECK-NEXT:    [[BOUND0:%.*]] = icmp ult ptr @b, getelementptr inbounds nuw (i8, ptr @a, i64 2)
+; CHECK-NEXT:    [[BOUND1:%.*]] = icmp ult ptr @a, getelementptr inbounds nuw (i8, ptr @b, i64 2)
+; CHECK-NEXT:    [[FOUND_CONFLICT:%.*]] = and i1 [[BOUND0]], [[BOUND1]]
+; CHECK-NEXT:    br i1 [[FOUND_CONFLICT]], label %[[FOR_BODY_PH_LVER_ORIG:.*]], label %[[FOR_BODY_PH:.*]]
+; CHECK:       [[FOR_BODY_PH_LVER_ORIG]]:
+; CHECK-NEXT:    br label %[[FOR_BODY_LVER_ORIG:.*]]
+; CHECK:       [[FOR_BODY_LVER_ORIG]]:
+; CHECK-NEXT:    [[TMP0:%.*]] = load i16, ptr @a, align 1
+; CHECK-NEXT:    store i16 [[TMP0]], ptr @b, align 1
+; CHECK-NEXT:    [[INC_LVER_ORIG:%.*]] = add nsw i16 undef, 1
+; CHECK-NEXT:    br i1 false, label %[[FOR_BODY_LVER_ORIG]], label %[[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT:.*]]
+; CHECK:       [[FOR_BODY_PH]]:
+; CHECK-NEXT:    br label %[[FOR_BODY:.*]]
+; CHECK:       [[FOR_BODY]]:
+; CHECK-NEXT:    [[TMP1:%.*]] = load i16, ptr @a, align 1, !alias.scope [[META0:![0-9]+]]
+; CHECK-NEXT:    store i16 [[TMP1]], ptr @b, align 1, !alias.scope [[META3:![0-9]+]], !noalias [[META0]]
+; CHECK-NEXT:    [[INC:%.*]] = add nsw i16 undef, 1
+; CHECK-NEXT:    br i1 false, label %[[FOR_BODY]], label %[[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT1:.*]]
+; CHECK:       [[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT]]:
+; CHECK-NEXT:    [[INC_LCSSA_PH:%.*]] = phi i16 [ [[INC_LVER_ORIG]], %[[FOR_BODY_LVER_ORIG]] ]
+; CHECK-NEXT:    [[SPLIT2_PH:%.*]] = phi i16 [ [[INC_LVER_ORIG]], %[[FOR_BODY_LVER_ORIG]] ]
+; CHECK-NEXT:    br label %[[FOR_COND_FOR_END_CRIT_EDGE:.*]]
+; CHECK:       [[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT1]]:
+; CHECK-NEXT:    [[INC_LCSSA_PH2:%.*]] = phi i16 [ [[INC]], %[[FOR_BODY]] ]
+; CHECK-NEXT:    [[SPLIT2_PH3:%.*]] = phi i16 [ [[INC]], %[[FOR_BODY]] ]
+; CHECK-NEXT:    br label %[[FOR_COND_FOR_END_CRIT_EDGE]]
+; CHECK:       [[FOR_COND_FOR_END_CRIT_EDGE]]:
+; CHECK-NEXT:    [[INC_LCSSA:%.*]] = phi i16 [ [[INC_LCSSA_PH]], %[[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT]] ], [ [[INC_LCSSA_PH2]], %[[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT1]] ]
+; CHECK-NEXT:    [[SPLIT2:%.*]] = phi i16 [ [[SPLIT2_PH]], %[[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT]] ], [ [[SPLIT2_PH3]], %[[FOR_COND_FOR_END_CRIT_EDGE_LOOPEXIT1]] ]
+; CHECK-NEXT:    store i16 [[INC_LCSSA]], ptr @c, align 1
+; CHECK-NEXT:    ret void
+;
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %0 = load i16, ptr @a, align 1
+  store i16 %0, ptr @b, align 1
+  %inc = add nsw i16 undef, 1
+  br i1 false, label %for.body, label %for.cond.for.end_crit_edge
+
+for.cond.for.end_crit_edge:                       ; preds = %for.body
+  %split2 = phi i16 [ %inc, %for.body ]
+  store i16 %inc, ptr @c, align 1
+  ret void
+}
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/116443