[flang-commits] [flang] [NFC][flang] Added engineering option for triaging local-alloc-tbaa. (PR #149587)
Slava Zakharin via flang-commits
flang-commits at lists.llvm.org
Fri Jul 18 13:38:42 PDT 2025
https://github.com/vzakhari created https://github.com/llvm/llvm-project/pull/149587
I triaged a benchmark that showed inaccurate results, when local-alloc-tbaa
was enabled. It turned out to be not a real TBAA issue, but rather
TBAA affecting optimizations that affect FMA generation, which introduced
an expected accuracy variation. I would like to keep this threshold
control for future uses.
>From 7c79e23426968538f9932a71fb2d5b19af76e697 Mon Sep 17 00:00:00 2001
From: Slava Zakharin <szakharin at nvidia.com>
Date: Fri, 18 Jul 2025 10:19:19 -0700
Subject: [PATCH] [NFC][flang] Added engineering option for triaging
local-alloc-tbaa.
I triaged a benchmark that showed inaccurate results, when local-alloc-tbaa
was enabled. It turned out to be not a real TBAA issue, but rather
TBAA affecting optimizations that affect FMA generation, which introduced
an expected accuracy variation. I would like to keep this threshold
control for future uses.
---
.../lib/Optimizer/Transforms/AddAliasTags.cpp | 41 ++++++++++++++++---
.../Transforms/tbaa-local-alloc-threshold.fir | 23 +++++++++++
2 files changed, 59 insertions(+), 5 deletions(-)
create mode 100644 flang/test/Transforms/tbaa-local-alloc-threshold.fir
diff --git a/flang/lib/Optimizer/Transforms/AddAliasTags.cpp b/flang/lib/Optimizer/Transforms/AddAliasTags.cpp
index b27c1b26dedb3..85403ad257657 100644
--- a/flang/lib/Optimizer/Transforms/AddAliasTags.cpp
+++ b/flang/lib/Optimizer/Transforms/AddAliasTags.cpp
@@ -48,12 +48,21 @@ static llvm::cl::opt<bool>
llvm::cl::Hidden,
llvm::cl::desc("Add TBAA tags to local allocations."));
+// Engineering option to triage TBAA tags attachment for accesses
+// of allocatable entities.
+static llvm::cl::opt<unsigned> localAllocsThreshold(
+ "local-alloc-tbaa-threshold", llvm::cl::init(0), llvm::cl::ReallyHidden,
+ llvm::cl::desc("If present, stops generating TBAA tags for accesses of "
+ "local allocations after N accesses in a module"));
+
namespace {
/// Shared state per-module
class PassState {
public:
- PassState(mlir::DominanceInfo &domInfo) : domInfo(domInfo) {}
+ PassState(mlir::DominanceInfo &domInfo,
+ std::optional<unsigned> localAllocsThreshold)
+ : domInfo(domInfo), localAllocsThreshold(localAllocsThreshold) {}
/// memoised call to fir::AliasAnalysis::getSource
inline const fir::AliasAnalysis::Source &getSource(mlir::Value value) {
if (!analysisCache.contains(value))
@@ -84,6 +93,11 @@ class PassState {
// (e.g. !fir.ref<!fir.type<Derived{f:!fir.box<!fir.heap<f32>>}>>).
bool typeReferencesDescriptor(mlir::Type type);
+ // Returns true if we can attach a TBAA tag to an access of an allocatable
+ // entities. It checks if localAllocsThreshold allows the next tag
+ // attachment.
+ bool attachLocalAllocTag();
+
private:
mlir::DominanceInfo &domInfo;
fir::AliasAnalysis analysis;
@@ -103,6 +117,8 @@ class PassState {
// Local pass cache for derived types that contain descriptor
// member(s), to avoid the cost of isRecordWithDescriptorMember().
llvm::DenseSet<mlir::Type> typesContainingDescriptors;
+
+ std::optional<unsigned> localAllocsThreshold;
};
// Process fir.dummy_scope operations in the given func:
@@ -169,6 +185,19 @@ bool PassState::typeReferencesDescriptor(mlir::Type type) {
return false;
}
+bool PassState::attachLocalAllocTag() {
+ if (!localAllocsThreshold)
+ return true;
+ if (*localAllocsThreshold == 0) {
+ LLVM_DEBUG(llvm::dbgs().indent(2)
+ << "WARN: not assigning TBAA tag for an allocated entity access "
+ "due to the threshold\n");
+ return false;
+ }
+ --*localAllocsThreshold;
+ return true;
+}
+
class AddAliasTagsPass : public fir::impl::AddAliasTagsBase<AddAliasTagsPass> {
public:
void runOnOperation() override;
@@ -335,16 +364,16 @@ void AddAliasTagsPass::runOnAliasInterface(fir::FirAliasTagOpInterface op,
LLVM_DEBUG(llvm::dbgs().indent(2)
<< "WARN: unknown defining op for SourceKind::Allocate " << *op
<< "\n");
- } else if (source.isPointer()) {
+ } else if (source.isPointer() && state.attachLocalAllocTag()) {
LLVM_DEBUG(llvm::dbgs().indent(2)
<< "Found reference to allocation at " << *op << "\n");
tag = state.getFuncTreeWithScope(func, scopeOp).targetDataTree.getTag();
- } else if (name) {
+ } else if (name && state.attachLocalAllocTag()) {
LLVM_DEBUG(llvm::dbgs().indent(2) << "Found reference to allocation "
<< name << " at " << *op << "\n");
tag = state.getFuncTreeWithScope(func, scopeOp)
.allocatedDataTree.getTag(*name);
- } else {
+ } else if (state.attachLocalAllocTag()) {
LLVM_DEBUG(llvm::dbgs().indent(2)
<< "WARN: couldn't find a name for allocation " << *op
<< "\n");
@@ -372,7 +401,9 @@ void AddAliasTagsPass::runOnOperation() {
// thinks the pass operates on), then the real work of the pass is done in
// runOnAliasInterface
auto &domInfo = getAnalysis<mlir::DominanceInfo>();
- PassState state(domInfo);
+ PassState state(domInfo, localAllocsThreshold.getPosition()
+ ? std::optional<unsigned>(localAllocsThreshold)
+ : std::nullopt);
mlir::ModuleOp mod = getOperation();
mod.walk(
diff --git a/flang/test/Transforms/tbaa-local-alloc-threshold.fir b/flang/test/Transforms/tbaa-local-alloc-threshold.fir
new file mode 100644
index 0000000000000..27c19a6e23095
--- /dev/null
+++ b/flang/test/Transforms/tbaa-local-alloc-threshold.fir
@@ -0,0 +1,23 @@
+// Check that -local-alloc-tbaa-threshold option limits
+// the attachment of TBAA tags to accesses of locally allocated entities.
+// RUN: fir-opt --fir-add-alias-tags -local-alloc-tbaa-threshold=2 %s | FileCheck %s --check-prefixes=ALL,COUNT2
+// RUN: fir-opt --fir-add-alias-tags -local-alloc-tbaa-threshold=1 %s | FileCheck %s --check-prefixes=ALL,COUNT1
+// RUN: fir-opt --fir-add-alias-tags -local-alloc-tbaa-threshold=0 %s | FileCheck %s --check-prefixes=ALL,COUNT0
+
+// ALL-LABEL: func.func @_QPtest() {
+// COUNT2: fir.load{{.*}}{tbaa =
+// COUNT2: fir.store{{.*}}{tbaa =
+// COUNT1: fir.load{{.*}}{tbaa =
+// COUNT1-NOT: fir.store{{.*}}{tbaa =
+// COUNT0-NOT: fir.load{{.*}}{tbaa =
+// COUNT0-NOT: fir.store{{.*}}{tbaa =
+func.func @_QPtest() {
+ %0 = fir.dummy_scope : !fir.dscope
+ %1 = fir.alloca f32 {bindc_name = "x", uniq_name = "_QFtestEx"}
+ %2 = fir.declare %1 {uniq_name = "_QFtestEx"} : (!fir.ref<f32>) -> !fir.ref<f32>
+ %3 = fir.alloca f32 {bindc_name = "y", uniq_name = "_QFtestEy"}
+ %4 = fir.declare %3 {uniq_name = "_QFtestEy"} : (!fir.ref<f32>) -> !fir.ref<f32>
+ %5 = fir.load %4 : !fir.ref<f32>
+ fir.store %5 to %2 : !fir.ref<f32>
+ return
+}
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