[llvm] [MSSA] Don't require clone creation to succeed (PR #76819)
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Wed Jan 3 06:20:40 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-analysis
Author: Nikita Popov (nikic)
<details>
<summary>Changes</summary>
Sometimes, we create a MemoryAccess for an instruction, which is later simplified (e.g. via devirtualization) such that the new instruction has no memory effects anymore.
If we later clone the instruction (e.g. during unswitching), then MSSA will not create a MemoryAccess for the new instruction, triggering an assert.
Disable the assertion (by passing CreationMustSucceed=false) and adjust getDefiningAccessForClone() to work correctly in that case.
This PR implements the alternative suggestion by @<!-- -->alinas from https://github.com/llvm/llvm-project/pull/76142.
---
Full diff: https://github.com/llvm/llvm-project/pull/76819.diff
2 Files Affected:
- (modified) llvm/lib/Analysis/MemorySSAUpdater.cpp (+4-13)
- (added) llvm/test/Transforms/SimpleLoopUnswitch/memssa-readnone-access.ll (+117)
``````````diff
diff --git a/llvm/lib/Analysis/MemorySSAUpdater.cpp b/llvm/lib/Analysis/MemorySSAUpdater.cpp
index 9ad60f774e9f86..e87ae7d71fffe2 100644
--- a/llvm/lib/Analysis/MemorySSAUpdater.cpp
+++ b/llvm/lib/Analysis/MemorySSAUpdater.cpp
@@ -568,7 +568,6 @@ static MemoryAccess *onlySingleValue(MemoryPhi *MP) {
static MemoryAccess *getNewDefiningAccessForClone(MemoryAccess *MA,
const ValueToValueMapTy &VMap,
PhiToDefMap &MPhiMap,
- bool CloneWasSimplified,
MemorySSA *MSSA) {
MemoryAccess *InsnDefining = MA;
if (MemoryDef *DefMUD = dyn_cast<MemoryDef>(InsnDefining)) {
@@ -578,18 +577,10 @@ static MemoryAccess *getNewDefiningAccessForClone(MemoryAccess *MA,
if (Instruction *NewDefMUDI =
cast_or_null<Instruction>(VMap.lookup(DefMUDI))) {
InsnDefining = MSSA->getMemoryAccess(NewDefMUDI);
- if (!CloneWasSimplified)
- assert(InsnDefining && "Defining instruction cannot be nullptr.");
- else if (!InsnDefining || isa<MemoryUse>(InsnDefining)) {
+ if (!InsnDefining || isa<MemoryUse>(InsnDefining)) {
// The clone was simplified, it's no longer a MemoryDef, look up.
- auto DefIt = DefMUD->getDefsIterator();
- // Since simplified clones only occur in single block cloning, a
- // previous definition must exist, otherwise NewDefMUDI would not
- // have been found in VMap.
- assert(DefIt != MSSA->getBlockDefs(DefMUD->getBlock())->begin() &&
- "Previous def must exist");
InsnDefining = getNewDefiningAccessForClone(
- &*(--DefIt), VMap, MPhiMap, CloneWasSimplified, MSSA);
+ DefMUD->getDefiningAccess(), VMap, MPhiMap, MSSA);
}
}
}
@@ -624,9 +615,9 @@ void MemorySSAUpdater::cloneUsesAndDefs(BasicBlock *BB, BasicBlock *NewBB,
MemoryAccess *NewUseOrDef = MSSA->createDefinedAccess(
NewInsn,
getNewDefiningAccessForClone(MUD->getDefiningAccess(), VMap,
- MPhiMap, CloneWasSimplified, MSSA),
+ MPhiMap, MSSA),
/*Template=*/CloneWasSimplified ? nullptr : MUD,
- /*CreationMustSucceed=*/CloneWasSimplified ? false : true);
+ /*CreationMustSucceed=*/false);
if (NewUseOrDef)
MSSA->insertIntoListsForBlock(NewUseOrDef, NewBB, MemorySSA::End);
}
diff --git a/llvm/test/Transforms/SimpleLoopUnswitch/memssa-readnone-access.ll b/llvm/test/Transforms/SimpleLoopUnswitch/memssa-readnone-access.ll
new file mode 100644
index 00000000000000..2aaf777683e116
--- /dev/null
+++ b/llvm/test/Transforms/SimpleLoopUnswitch/memssa-readnone-access.ll
@@ -0,0 +1,117 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -S -passes="loop-mssa(loop-instsimplify,simple-loop-unswitch<nontrivial>)" < %s | FileCheck %s
+
+ at vtable = constant ptr @foo
+
+declare void @foo() memory(none)
+declare void @bar()
+
+; The call becomes known readnone after simplification, but still have a
+; MemoryAccess. Make sure this does not lead to an assertion failure.
+define void @test(i1 %c) {
+; CHECK-LABEL: define void @test(
+; CHECK-SAME: i1 [[C:%.*]]) {
+; CHECK-NEXT: [[C_FR:%.*]] = freeze i1 [[C]]
+; CHECK-NEXT: br i1 [[C_FR]], label [[DOTSPLIT_US:%.*]], label [[DOTSPLIT:%.*]]
+; CHECK: .split.us:
+; CHECK-NEXT: br label [[LOOP_US:%.*]]
+; CHECK: loop.us:
+; CHECK-NEXT: call void @foo()
+; CHECK-NEXT: br label [[EXIT_SPLIT_US:%.*]]
+; CHECK: exit.split.us:
+; CHECK-NEXT: br label [[EXIT:%.*]]
+; CHECK: .split:
+; CHECK-NEXT: br label [[LOOP:%.*]]
+; CHECK: loop:
+; CHECK-NEXT: call void @foo()
+; CHECK-NEXT: br label [[LOOP]]
+; CHECK: exit:
+; CHECK-NEXT: ret void
+;
+ br label %loop
+
+loop:
+ %fn = load ptr, ptr @vtable, align 8
+ call void %fn()
+ br i1 %c, label %exit, label %loop
+
+exit:
+ ret void
+}
+
+; Variant with another access after the call.
+define void @test2(i1 %c, ptr %p) {
+; CHECK-LABEL: define void @test2(
+; CHECK-SAME: i1 [[C:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT: [[C_FR:%.*]] = freeze i1 [[C]]
+; CHECK-NEXT: br i1 [[C_FR]], label [[DOTSPLIT_US:%.*]], label [[DOTSPLIT:%.*]]
+; CHECK: .split.us:
+; CHECK-NEXT: br label [[LOOP_US:%.*]]
+; CHECK: loop.us:
+; CHECK-NEXT: call void @foo()
+; CHECK-NEXT: call void @bar()
+; CHECK-NEXT: br label [[EXIT_SPLIT_US:%.*]]
+; CHECK: exit.split.us:
+; CHECK-NEXT: br label [[EXIT:%.*]]
+; CHECK: .split:
+; CHECK-NEXT: br label [[LOOP:%.*]]
+; CHECK: loop:
+; CHECK-NEXT: call void @foo()
+; CHECK-NEXT: call void @bar()
+; CHECK-NEXT: br label [[LOOP]]
+; CHECK: exit:
+; CHECK-NEXT: ret void
+;
+ br label %loop
+
+loop:
+ %fn = load ptr, ptr @vtable, align 8
+ call void %fn()
+ call void @bar()
+ br i1 %c, label %exit, label %loop
+
+exit:
+ ret void
+}
+
+; Variant with another access after the call and no access before the call.
+define void @test3(i1 %c, ptr %p) {
+; CHECK-LABEL: define void @test3(
+; CHECK-SAME: i1 [[C:%.*]], ptr [[P:%.*]]) {
+; CHECK-NEXT: [[C_FR:%.*]] = freeze i1 [[C]]
+; CHECK-NEXT: br i1 [[C_FR]], label [[DOTSPLIT_US:%.*]], label [[DOTSPLIT:%.*]]
+; CHECK: .split.us:
+; CHECK-NEXT: br label [[LOOP_US:%.*]]
+; CHECK: loop.us:
+; CHECK-NEXT: br label [[SPLIT_US:%.*]]
+; CHECK: split.us:
+; CHECK-NEXT: call void @foo()
+; CHECK-NEXT: call void @bar()
+; CHECK-NEXT: br label [[EXIT_SPLIT_US:%.*]]
+; CHECK: exit.split.us:
+; CHECK-NEXT: br label [[EXIT:%.*]]
+; CHECK: .split:
+; CHECK-NEXT: br label [[LOOP:%.*]]
+; CHECK: loop:
+; CHECK-NEXT: br label [[SPLIT:%.*]]
+; CHECK: split:
+; CHECK-NEXT: call void @foo()
+; CHECK-NEXT: call void @bar()
+; CHECK-NEXT: br label [[LOOP]]
+; CHECK: exit:
+; CHECK-NEXT: ret void
+;
+ br label %loop
+
+loop:
+ %fn = load ptr, ptr @vtable, align 8
+ br label %split
+
+split:
+ call void %fn()
+ call void @bar()
+ br i1 %c, label %exit, label %loop
+
+exit:
+ ret void
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/76819
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