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[LLVM][MemCpyOpt] Unify alias tags if we optimize allocas #129537

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Apr 10, 2025
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[LLVM][MemCpyOpt] Unify alias tags if we optimize allocas #129537

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4c49512
[LLVM][MemCpyOpt] Unify alias tags if we optimize allocas
DominikAdamski Feb 24, 2025
5e04c39
Simplified test
DominikAdamski Mar 3, 2025
dddcc8a
Fix style
DominikAdamski Mar 18, 2025
ae4139a
Fix style
DominikAdamski Mar 18, 2025
3ab8a71
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DominikAdamski Mar 18, 2025
0824f68
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DominikAdamski Mar 21, 2025
b9cb648
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DominikAdamski Apr 7, 2025
7fc13ac
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DominikAdamski Apr 8, 2025
2f00aa7
Remove alias.scope
DominikAdamski Apr 8, 2025
92584ea
Merge branch 'main' into fix_memcpy_tbaa_tags
DominikAdamski Apr 8, 2025
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22 changes: 18 additions & 4 deletions llvm/lib/Transforms/Scalar/MemCpyOptimizer.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -1516,6 +1516,8 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
SmallVector<Instruction *, 4> LifetimeMarkers;
SmallSet<Instruction *, 4> NoAliasInstrs;
bool SrcNotDom = false;
SmallSet<Instruction *, 4> SrcAllocaInstUsers;
SmallSet<Instruction *, 4> DestAllocaInstUsers;

// Recursively track the user and check whether modified alias exist.
auto IsDereferenceableOrNull = [](Value *V, const DataLayout &DL) -> bool {
Expand All @@ -1524,8 +1526,8 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
};

auto CaptureTrackingWithModRef =
[&](Instruction *AI,
function_ref<bool(Instruction *)> ModRefCallback) -> bool {
[&](Instruction *AI, function_ref<bool(Instruction *)> ModRefCallback,
SmallSet<Instruction *, 4> &AllocaInstUsersWithTBAA) -> bool {
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@nikic nikic Apr 6, 2025

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No need for the argument, the variable is captured.

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@DominikAdamski DominikAdamski Apr 7, 2025

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Done

SmallVector<Instruction *, 8> Worklist;
Worklist.push_back(AI);
unsigned MaxUsesToExplore = getDefaultMaxUsesToExploreForCaptureTracking();
Expand Down Expand Up @@ -1569,6 +1571,8 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
continue;
}
}
if (UI != Store && UI->hasMetadata(LLVMContext::MD_tbaa))
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@nikic nikic Mar 18, 2025

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What about MD_tbaa_struct?

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@DominikAdamski DominikAdamski Mar 21, 2025

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I added MD_tbaa_struct .

AllocaInstUsersWithTBAA.insert(UI);
if (UI->hasMetadata(LLVMContext::MD_noalias))
NoAliasInstrs.insert(UI);
if (!ModRefCallback(UI))
Expand Down Expand Up @@ -1621,7 +1625,8 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
return true;
};

if (!CaptureTrackingWithModRef(DestAlloca, DestModRefCallback))
if (!CaptureTrackingWithModRef(DestAlloca, DestModRefCallback,
DestAllocaInstUsers))
return false;
// Bailout if Dest may have any ModRef before Store.
if (!ReachabilityWorklist.empty() &&
Expand All @@ -1647,7 +1652,8 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
return true;
};

if (!CaptureTrackingWithModRef(SrcAlloca, SrcModRefCallback))
if (!CaptureTrackingWithModRef(SrcAlloca, SrcModRefCallback,
SrcAllocaInstUsers))
return false;

// We can do the transformation. First, move the SrcAlloca to the start of the
Expand Down Expand Up @@ -1681,6 +1687,14 @@ bool MemCpyOptPass::performStackMoveOptzn(Instruction *Load, Instruction *Store,
for (Instruction *I : NoAliasInstrs)
I->setMetadata(LLVMContext::MD_noalias, nullptr);

// If we merge two allocas we need to uniform alias tags as well
if (!SrcAllocaInstUsers.empty()) {
MDNode *mergeTBAA =
(*SrcAllocaInstUsers.begin())->getMetadata(LLVMContext::MD_tbaa);
for (Instruction *It : DestAllocaInstUsers)
It->setMetadata(LLVMContext::MD_tbaa, mergeTBAA);
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@nikic nikic Mar 18, 2025

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This doesn't look safe? You are just assigning one tag to all the accesses now. There may be accesses with different tags, and replacing them with a different one would be a miscompile.

The easy fix here would be to just drop all the tbaa data like is done for noalias metadata. Otherwise you need to do by-offset analysis of accesses and merge the tbaa tags for specific offsets.

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@nikic nikic Mar 18, 2025

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In any case, we should have a test case where multiple tbaa tags are involved at different offsets.

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@DominikAdamski DominikAdamski Mar 21, 2025

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I dropped tbaa metadata and I added another test.

}

LLVM_DEBUG(dbgs() << "Stack Move: Performed staack-move optimization\n");
NumStackMove++;
return true;
Expand Down
73 changes: 73 additions & 0 deletions llvm/test/Transforms/MemCpyOpt/memcpy-tbaa.ll
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@nikic nikic Mar 18, 2025

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This test still doesn't look minimal. Wouldn't it be sufficient to have a store with one tbaa tag, a memcpy and a load with another tag?

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@DominikAdamski DominikAdamski Mar 21, 2025

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I removed unnecessary instructions.

Original file line number Diff line number Diff line change
@@ -0,0 +1,73 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
; RUN: opt < %s -passes=memcpyopt,dse -S -verify-memoryssa | FileCheck %s

define void @test() local_unnamed_addr {
; CHECK-LABEL: define void @test() local_unnamed_addr {
; CHECK-NEXT: [[TEST_ARRAY_B:%.*]] = alloca [31 x float], align 4
; CHECK-NEXT: br label %[[INIT_LOOP:.*]]
; CHECK: [[INIT_LOOP]]:
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ 0, [[TMP0:%.*]] ], [ [[INDVARS_IV_NEXT:%.*]], %[[INIT_LOOP]] ]
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add nuw nsw i64 [[INDVARS_IV]], 1
; CHECK-NEXT: [[TMP1:%.*]] = getelementptr float, ptr [[TEST_ARRAY_B]], i64 [[INDVARS_IV]]
; CHECK-NEXT: store float 0x3E6AA51880000000, ptr [[TMP1]], align 4, !tbaa [[TBAA0:![0-9]+]]
; CHECK-NEXT: [[EXITCOND_NOT:%.*]] = icmp eq i64 [[INDVARS_IV_NEXT]], 32
; CHECK-NEXT: br i1 [[EXITCOND_NOT]], label %[[DOTPREHEADER55_PREHEADER:.*]], label %[[INIT_LOOP]]
; CHECK: [[_PREHEADER55_PREHEADER:.*:]]
; CHECK-NEXT: br label %[[LOOP:.*]]
; CHECK: [[LOOP]]:
; CHECK-NEXT: [[INDVARS_IV73:%.*]] = phi i64 [ 0, %[[DOTPREHEADER55_PREHEADER]] ], [ [[INDVARS_IV_NEXT74:%.*]], %[[LOOP]] ]
; CHECK-NEXT: [[INDVARS_IV_NEXT74]] = add nuw nsw i64 [[INDVARS_IV73]], 1
; CHECK-NEXT: [[TMP2:%.*]] = getelementptr float, ptr [[TEST_ARRAY_B]], i64 [[INDVARS_IV73]]
; CHECK-NEXT: [[TMP3:%.*]] = load float, ptr [[TMP2]], align 4, !tbaa [[TBAA0]]
; CHECK-NEXT: [[EXITCOND76_NOT:%.*]] = icmp eq i64 [[INDVARS_IV_NEXT74]], 32
; CHECK-NEXT: br i1 [[EXITCOND76_NOT]], label %[[LOOP]], [[DOT_CRIT_EDGE56:label %.*]]
; CHECK: [[__CRIT_EDGE56:.*:]]
; CHECK-NEXT: ret void
;
%test_array_a = alloca [31 x float], align 4
%test_array_b = alloca [31 x float], align 4
br label %init_loop

init_loop:
%indvars.iv = phi i64 [ 0, %0 ], [ %indvars.iv.next, %init_loop ]
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%1 = getelementptr float, ptr %test_array_b, i64 %indvars.iv
store float 0x3E6AA51880000000, ptr %1, align 4, !tbaa !12
%exitcond.not = icmp eq i64 %indvars.iv.next, 32
br i1 %exitcond.not, label %.preheader55.preheader, label %init_loop

.preheader55.preheader:
call void @llvm.memcpy.p0.p0.i64(ptr noundef nonnull align 4 dereferenceable(124) %test_array_a, ptr noundef nonnull align 4 dereferenceable(124) %test_array_b, i64 124, i1 false)
br label %loop

loop: ; preds = %.preheader, %211
%indvars.iv73 = phi i64 [ 0, %.preheader55.preheader ], [ %indvars.iv.next74, %loop ]
%indvars.iv.next74 = add nuw nsw i64 %indvars.iv73, 1
%2 = getelementptr float, ptr %test_array_a, i64 %indvars.iv73
%3 = load float, ptr %2, align 4, !tbaa !31
%exitcond76.not = icmp eq i64 %indvars.iv.next74, 32
br i1 %exitcond76.not, label %loop, label %._crit_edge56

._crit_edge56: ; preds = %loop, %._crit_edge
ret void
}

; Function Attrs: nocallback nofree nounwind willreturn memory(argmem: readwrite)
declare void @llvm.memcpy.p0.p0.i64(ptr noalias writeonly captures(none), ptr noalias readonly captures(none), i64, i1 immarg)

!7 = !{!"any data access", !8, i64 0}
!8 = !{!"any access", !9, i64 0}
!9 = !{!"Flang function root test"}
!12 = !{!13, !13, i64 0}
!13 = !{!"allocated data/test_array_a", !14, i64 0}
!14 = !{!"allocated data", !7, i64 0}
!31 = !{!32, !32, i64 0}
!32 = !{!"allocated data/test_array_b", !14, i64 0}
;.
; CHECK: [[TBAA0]] = !{[[META1:![0-9]+]], [[META1]], i64 0}
; CHECK: [[META1]] = !{!"allocated data/test_array_a", [[META2:![0-9]+]], i64 0}
; CHECK: [[META2]] = !{!"allocated data", [[META3:![0-9]+]], i64 0}
; CHECK: [[META3]] = !{!"any data access", [[META4:![0-9]+]], i64 0}
; CHECK: [[META4]] = !{!"any access", [[META5:![0-9]+]], i64 0}
; CHECK: [[META5]] = !{!"Flang function root test"}
;.