[Mlir-commits] [mlir] [MLIR][LLVM] Fix memory explosion when converting global variable bodies in ModuleTranslation (PR #82708)
llvmlistbot at llvm.org
llvmlistbot at llvm.org
Thu Feb 22 15:47:01 PST 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-mlir-llvm
Author: Xiang Li (python3kgae)
<details>
<summary>Changes</summary>
There is memory explosion when converting the body or initializer region of a large global variable, e.g. a constant array.
For example, when translating a constant array of 100000 strings:
llvm.mlir.global internal constant @<!-- -->cats_strings() {addr_space = 0 : i32, alignment = 16 : i64} : !llvm.array<100000 x ptr<i8>> {
%0 = llvm.mlir.undef : !llvm.array<100000 x ptr<i8>>
%1 = llvm.mlir.addressof @<!-- -->om_1 : !llvm.ptr<array<1 x i8>>
%2 = llvm.getelementptr %1[0, 0] : (!llvm.ptr<array<1 x i8>>) -> !llvm.ptr<i8>
%3 = llvm.insertvalue %2, %0[0] : !llvm.array<100000 x ptr<i8>>
%4 = llvm.mlir.addressof @<!-- -->om_2 : !llvm.ptr<array<1 x i8>>
%5 = llvm.getelementptr %4[0, 0] : (!llvm.ptr<array<1 x i8>>) -> !llvm.ptr<i8>
%6 = llvm.insertvalue %5, %3[1] : !llvm.array<100000 x ptr<i8>>
%7 = llvm.mlir.addressof @<!-- -->om_3 : !llvm.ptr<array<1 x i8>>
%8 = llvm.getelementptr %7[0, 0] : (!llvm.ptr<array<1 x i8>>) -> !llvm.ptr<i8>
%9 = llvm.insertvalue %8, %6[2] : !llvm.array<100000 x ptr<i8>>
%10 = llvm.mlir.addressof @<!-- -->om_4 : !llvm.ptr<array<1 x i8>>
%11 = llvm.getelementptr %10[0, 0] : (!llvm.ptr<array<1 x i8>>) -> !llvm.ptr<i8>
%12 = llvm.insertvalue %11, %9[3] : !llvm.array<100000 x ptr<i8>>
... (ignore the remaining part)
}
where @<!-- -->om_1, @<!-- -->om_2, ... are string global constants.
Each time an operation is converted to LLVM, a new constant is created. When it comes to llvm.insertvalue, a new constant array of 100000 elements is created and the old constant array (input) is not destroyed. This causes memory explosion. We observed that, on a system with 128 GB memory, the translation of 100000 elements got killed due to using up all the memory. On a system with 64 GB, 65536 elements was enough to cause the translation killed.
There is a previous patch (https://reviews.llvm.org/D148487) which fix this issue but was reverted for https://github.com/llvm/llvm-project/issues/62802
The old patch checks generated constants and destroyed them if there is no use. But the check of use for the constant is too early, which cause the constant be removed before use.
This new patch added a map was added a map to save expected use count for a constant. Then decrease when reach each use.
And only erase the constant when the use count reach to zero
With new patch, the repro in https://github.com/llvm/llvm-project/issues/62802 finished correctly.
---
Full diff: https://github.com/llvm/llvm-project/pull/82708.diff
2 Files Affected:
- (modified) mlir/lib/Target/LLVMIR/ModuleTranslation.cpp (+64)
- (added) mlir/test/Target/LLVMIR/erase-dangling-constants.mlir (+72)
``````````diff
diff --git a/mlir/lib/Target/LLVMIR/ModuleTranslation.cpp b/mlir/lib/Target/LLVMIR/ModuleTranslation.cpp
index ee8fffd959c883..64c37b1d5fa961 100644
--- a/mlir/lib/Target/LLVMIR/ModuleTranslation.cpp
+++ b/mlir/lib/Target/LLVMIR/ModuleTranslation.cpp
@@ -51,11 +51,15 @@
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Verifier.h"
+#include "llvm/Support/Debug.h"
+#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/ModuleUtils.h"
#include <optional>
+#define DEBUG_TYPE "llvm-dialect-to-llvm-ir"
+
using namespace mlir;
using namespace mlir::LLVM;
using namespace mlir::LLVM::detail;
@@ -1042,17 +1046,77 @@ LogicalResult ModuleTranslation::convertGlobals() {
for (auto op : getModuleBody(mlirModule).getOps<LLVM::GlobalOp>()) {
if (Block *initializer = op.getInitializerBlock()) {
llvm::IRBuilder<> builder(llvmModule->getContext());
+
+ int numConstantsHit = 0;
+ int numConstantsErased = 0;
+ DenseMap<llvm::ConstantAggregate *, int> constantAggregateUseMap;
+
for (auto &op : initializer->without_terminator()) {
if (failed(convertOperation(op, builder)) ||
!isa<llvm::Constant>(lookupValue(op.getResult(0))))
return emitError(op.getLoc(), "unemittable constant value");
+
+ // When emitting an LLVM constant, a new constant is created and the old
+ // constant may become dangling and take space. We should remove the
+ // dangling constants to avoid memory explosion especially for constant
+ // arrays whose number of elements is large.
+ // Because multiple operations may refer to the same constant, we need
+ // to count the number of uses of each constant array and remove it only
+ // when the count becomes zero.
+ if (op.getNumResults() == 1) {
+ Value result = op.getResult(0);
+ auto cst = dyn_cast<llvm::ConstantAggregate>(lookupValue(result));
+ if (!cst)
+ continue;
+ numConstantsHit++;
+ auto iter = constantAggregateUseMap.find(cst);
+ int numUsers = std::distance(result.use_begin(), result.use_end());
+ if (iter == constantAggregateUseMap.end())
+ constantAggregateUseMap.try_emplace(cst, numUsers);
+ else
+ iter->second += numUsers;
+ }
+ for (Value v : op.getOperands()) {
+ auto cst = dyn_cast<llvm::ConstantAggregate>(lookupValue(v));
+ if (!cst)
+ continue;
+ auto iter = constantAggregateUseMap.find(cst);
+ assert(iter != constantAggregateUseMap.end() && "constant not found");
+ iter->second--;
+ if (iter->second == 0) {
+ cst->removeDeadConstantUsers();
+ if (cst->user_empty()) {
+ cst->destroyConstant();
+ numConstantsErased++;
+ }
+ constantAggregateUseMap.erase(iter);
+ }
+ }
}
+
ReturnOp ret = cast<ReturnOp>(initializer->getTerminator());
llvm::Constant *cst =
cast<llvm::Constant>(lookupValue(ret.getOperand(0)));
auto *global = cast<llvm::GlobalVariable>(lookupGlobal(op));
if (!shouldDropGlobalInitializer(global->getLinkage(), cst))
global->setInitializer(cst);
+
+ // Try to remove the dangling constants again after all operations are
+ // converted.
+ for (auto it : constantAggregateUseMap) {
+ auto cst = it.first;
+ cst->removeDeadConstantUsers();
+ if (cst->user_empty()) {
+ cst->destroyConstant();
+ numConstantsErased++;
+ }
+ }
+
+ LLVM_DEBUG(llvm::dbgs()
+ << "Convert initializer for " << op.getName() << "\n";
+ llvm::dbgs() << numConstantsHit << " new constants hit\n";
+ llvm::dbgs()
+ << numConstantsErased << " dangling constants erased\n";);
}
}
diff --git a/mlir/test/Target/LLVMIR/erase-dangling-constants.mlir b/mlir/test/Target/LLVMIR/erase-dangling-constants.mlir
new file mode 100644
index 00000000000000..b3b5d540ae88fc
--- /dev/null
+++ b/mlir/test/Target/LLVMIR/erase-dangling-constants.mlir
@@ -0,0 +1,72 @@
+// RUN: mlir-translate -mlir-to-llvmir %s -debug-only=llvm-dialect-to-llvm-ir 2>&1 | FileCheck %s
+
+// CHECK: Convert initializer for dup_const
+// CHECK: 6 new constants hit
+// CHECK: 3 dangling constants erased
+// CHECK: Convert initializer for unique_const
+// CHECK: 6 new constants hit
+// CHECK: 5 dangling constants erased
+
+
+// CHECK:@dup_const = global { [2 x double], [2 x double], [2 x double] } { [2 x double] [double 3.612250e-02, double 5.119230e-02], [2 x double] [double 3.612250e-02, double 5.119230e-02], [2 x double] [double 3.612250e-02, double 5.119230e-02] }
+
+llvm.mlir.global @dup_const() : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)> {
+ %c0 = llvm.mlir.constant(3.612250e-02 : f64) : f64
+ %c1 = llvm.mlir.constant(5.119230e-02 : f64) : f64
+
+ %empty0 = llvm.mlir.undef : !llvm.array<2 x f64>
+ %a00 = llvm.insertvalue %c0, %empty0[0] : !llvm.array<2 x f64>
+
+ %empty1 = llvm.mlir.undef : !llvm.array<2 x f64>
+ %a10 = llvm.insertvalue %c0, %empty1[0] : !llvm.array<2 x f64>
+
+ %empty2 = llvm.mlir.undef : !llvm.array<2 x f64>
+ %a20 = llvm.insertvalue %c0, %empty2[0] : !llvm.array<2 x f64>
+
+// NOTE: a00, a10, a20 are all same ConstantAggregate which not used at this point.
+// should not delete it before all of the uses of the ConstantAggregate finished.
+
+ %a01 = llvm.insertvalue %c1, %a00[1] : !llvm.array<2 x f64>
+ %a11 = llvm.insertvalue %c1, %a10[1] : !llvm.array<2 x f64>
+ %a21 = llvm.insertvalue %c1, %a20[1] : !llvm.array<2 x f64>
+ %empty_r = llvm.mlir.undef : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+ %r0 = llvm.insertvalue %a01, %empty_r[0] : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+ %r1 = llvm.insertvalue %a11, %r0[1] : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+ %r2 = llvm.insertvalue %a21, %r1[2] : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+
+ llvm.return %r2 : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+ }
+
+// CHECK:@unique_const = global { [2 x double], [2 x double], [2 x double] } { [2 x double] [double 3.612250e-02, double 5.119230e-02], [2 x double] [double 3.312250e-02, double 5.219230e-02], [2 x double] [double 3.412250e-02, double 5.419230e-02] }
+
+llvm.mlir.global @unique_const() : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)> {
+ %c0 = llvm.mlir.constant(3.612250e-02 : f64) : f64
+ %c1 = llvm.mlir.constant(5.119230e-02 : f64) : f64
+
+ %c2 = llvm.mlir.constant(3.312250e-02 : f64) : f64
+ %c3 = llvm.mlir.constant(5.219230e-02 : f64) : f64
+
+ %c4 = llvm.mlir.constant(3.412250e-02 : f64) : f64
+ %c5 = llvm.mlir.constant(5.419230e-02 : f64) : f64
+
+ %2 = llvm.mlir.undef : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+
+ %3 = llvm.mlir.undef : !llvm.array<2 x f64>
+
+ %4 = llvm.insertvalue %c0, %3[0] : !llvm.array<2 x f64>
+ %5 = llvm.insertvalue %c1, %4[1] : !llvm.array<2 x f64>
+
+ %6 = llvm.insertvalue %5, %2[0] : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+
+ %7 = llvm.insertvalue %c2, %3[0] : !llvm.array<2 x f64>
+ %8 = llvm.insertvalue %c3, %7[1] : !llvm.array<2 x f64>
+
+ %9 = llvm.insertvalue %8, %6[1] : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+
+ %10 = llvm.insertvalue %c4, %3[0] : !llvm.array<2 x f64>
+ %11 = llvm.insertvalue %c5, %10[1] : !llvm.array<2 x f64>
+
+ %12 = llvm.insertvalue %11, %9[2] : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+
+ llvm.return %12 : !llvm.struct<(array<2 x f64>, array<2 x f64>, array<2 x f64>)>
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/82708
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