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

VedantParanjape · 2024-11-15T22:15:53Z

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

github-actions · 2024-11-15T22:16:16Z

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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: llvm#36998

aeubanks

I'm not familiar with the pass and will let @fhahn take a look, but does it need to be a function pass as opposed to a loop pass?

VedantParanjape · 2024-11-15T23:23:36Z

I'm familiar with the pass and will let @fhahn take a look, but does it need to be a function pass as opposed to a loop pass?

do you mean to say not familiar?

I am not super familiar with this pass as well, but I think because it uses LoopAccessInfo and LoopInfo and they cannot be queried per Loop? so it's a function pass.

aeubanks · 2024-11-15T23:24:41Z

sorry yeah, "not familiar"

github-actions · 2024-11-16T06:01:43Z

Thank you for submitting a Pull Request (PR) to the LLVM Project!

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If you wish to, you can add reviewers by using the "Reviewers" section on this page.

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llvmbot · 2024-11-16T06:01:57Z

@llvm/pr-subscribers-llvm-transforms

Author: Vedant Paranjape (VedantParanjape)

Changes

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 --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"
+
+@a = external global i16, align 1
+@b = external global i16, align 1
+@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

VedantParanjape · 2024-11-26T16:46:48Z

ping! @fhahn

VedantParanjape · 2024-12-06T05:02:24Z

ping !

aeubanks

seems reasonable

llvm/lib/Transforms/Utils/LoopVersioning.cpp

llvm/test/Transforms/LoopVersioning/crash-36998.ll

Co-authored-by: Florian Hahn <[email protected]>

github-actions · 2024-12-09T20:37:14Z

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff e887f8290df419ffd4e018b6f8afbaeb1912cf0e cce211b6e5c1ab8cdd3532a60274f58ff3a84ed6 --extensions cpp -- llvm/lib/Transforms/Utils/LoopVersioning.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/Transforms/Utils/LoopVersioning.cpp b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
index 5ee551e6f0..94594e18c2 100644
--- a/llvm/lib/Transforms/Utils/LoopVersioning.cpp
+++ b/llvm/lib/Transforms/Utils/LoopVersioning.cpp
@@ -280,7 +280,7 @@ bool runImpl(LoopInfo *LI, LoopAccessInfoManager &LAIs, DominatorTree *DT,
         (LAI.getNumRuntimePointerChecks() ||
          !LAI.getPSE().getPredicate().isAlwaysTrue())) {
       if (!L->isLCSSAForm(*DT))
-       formLCSSARecursively(*L, *DT, LI, SE);
+        formLCSSARecursively(*L, *DT, LI, SE);
 
       LoopVersioning LVer(LAI, LAI.getRuntimePointerChecking()->getChecks(), L,
                           LI, DT, SE);

github-actions · 2024-12-16T16:28:35Z

✅ With the latest revision this PR passed the undef deprecator.

github-actions · 2024-12-16T16:55:41Z

@VedantParanjape Congratulations on having your first Pull Request (PR) merged into the LLVM Project!

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VedantParanjape requested a review from fhahn November 15, 2024 22:16

VedantParanjape force-pushed the main branch from 28ff01f to bfc6152 Compare November 15, 2024 22:18

VedantParanjape added llvm:optimizations loopoptim labels Nov 15, 2024

VedantParanjape requested a review from aeubanks November 15, 2024 22:20

VedantParanjape mentioned this pull request Nov 15, 2024

"Instruction does not dominate all uses" with opt -disable-basicaa -loop-versioning #36998

Closed

aeubanks reviewed Nov 15, 2024

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llvmbot added the llvm:transforms label Nov 16, 2024

aeubanks approved these changes Dec 9, 2024

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fhahn reviewed Dec 9, 2024

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Update llvm/lib/Transforms/Utils/LoopVersioning.cpp

682c557

Co-authored-by: Florian Hahn <[email protected]>

fixed nitpick in the testcase

19f9ae2

remove undef from test

cce211b

VedantParanjape merged commit b21fa18 into llvm:main Dec 16, 2024
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