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[Transforms][GlobalSRA] huge array causes long compilation time and huge memory usage.

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Summary:
For artificial cases (huge array, few usages), Global SRA optimization creates
a lot of redundant data. It creates an instance of GlobalVariable for each array
element. For huge array, that means huge compilation time and huge memory usage.
Following example compiles for 10 minutes and requires 40GB of memory.

namespace {
  char LargeBuffer[64 * 1024 * 1024];
}

int main ( void ) {

    LargeBuffer[0] = 0;

    printf("\n ");

    return LargeBuffer[0] == 0;
}

The fix is to avoid Global SRA for large arrays.

Reviewers: craig.topper, rnk, efriedma, fhahn

Reviewed By: rnk

Subscribers: xbolva00, lebedev.ri, lkail, merge_guards_bot, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71993
This commit is contained in:
Alexey Lapshin 2019-12-25 00:38:09 +03:00
parent 4cccff07f7
commit 5b59e688d3
2 changed files with 138 additions and 65 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

142
lib/Transforms/IPO/GlobalOpt.cpp
View File

@ -433,6 +433,20 @@ static bool GlobalUsersSafeToSRA(GlobalValue *GV) {
return true;
}
static bool CanDoGlobalSRA(GlobalVariable *GV) {
Constant *Init = GV->getInitializer();
if (isa<StructType>(Init->getType())) {
// nothing to check
} else if (SequentialType *STy = dyn_cast<SequentialType>(Init->getType())) {
if (STy->getNumElements() > 16 && GV->hasNUsesOrMore(16))
return false; // It's not worth it.
} else
return false;
return GlobalUsersSafeToSRA(GV);
}
/// Copy over the debug info for a variable to its SRA replacements.
static void transferSRADebugInfo(GlobalVariable *GV, GlobalVariable *NGV,
uint64_t FragmentOffsetInBits,
@ -462,88 +476,94 @@ static void transferSRADebugInfo(GlobalVariable *GV, GlobalVariable *NGV,
/// insert so that the caller can reprocess it.
static GlobalVariable *SRAGlobal(GlobalVariable *GV, const DataLayout &DL) {
// Make sure this global only has simple uses that we can SRA.
if (!GlobalUsersSafeToSRA(GV))
if (!CanDoGlobalSRA(GV))
return nullptr;
assert(GV->hasLocalLinkage());
Constant *Init = GV->getInitializer();
Type *Ty = Init->getType();
std::vector<GlobalVariable *> NewGlobals;
Module::GlobalListType &Globals = GV->getParent()->getGlobalList();
std::map<unsigned, GlobalVariable *> NewGlobals;
// Get the alignment of the global, either explicit or target-specific.
unsigned StartAlignment = GV->getAlignment();
if (StartAlignment == 0)
StartAlignment = DL.getABITypeAlignment(GV->getType());
if (StructType *STy = dyn_cast<StructType>(Ty)) {
unsigned NumElements = STy->getNumElements();
NewGlobals.reserve(NumElements);
const StructLayout &Layout = *DL.getStructLayout(STy);
for (unsigned i = 0, e = NumElements; i != e; ++i) {
Constant *In = Init->getAggregateElement(i);
assert(In && "Couldn't get element of initializer?");
GlobalVariable *NGV = new GlobalVariable(STy->getElementType(i), false,
GlobalVariable::InternalLinkage,
In, GV->getName()+"."+Twine(i),
GV->getThreadLocalMode(),
GV->getType()->getAddressSpace());
NGV->setExternallyInitialized(GV->isExternallyInitialized());
NGV->copyAttributesFrom(GV);
Globals.push_back(NGV);
NewGlobals.push_back(NGV);
// Loop over all users and create replacement variables for used aggregate
// elements.
for (User *GEP : GV->users()) {
assert(((isa<ConstantExpr>(GEP) && cast<ConstantExpr>(GEP)->getOpcode() ==
Instruction::GetElementPtr) ||
isa<GetElementPtrInst>(GEP)) &&
"NonGEP CE's are not SRAable!");
// Ignore the 1th operand, which has to be zero or else the program is quite
// broken (undefined). Get the 2nd operand, which is the structure or array
// index.
unsigned ElementIdx = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
if (NewGlobals.count(ElementIdx) == 1)
continue; // we`ve already created replacement variable
assert(NewGlobals.count(ElementIdx) == 0);
Type *ElTy = nullptr;
if (StructType *STy = dyn_cast<StructType>(Ty))
ElTy = STy->getElementType(ElementIdx);
else if (SequentialType *STy = dyn_cast<SequentialType>(Ty))
ElTy = STy->getElementType();
assert(ElTy);
Constant *In = Init->getAggregateElement(ElementIdx);
assert(In && "Couldn't get element of initializer?");
GlobalVariable *NGV = new GlobalVariable(
ElTy, false, GlobalVariable::InternalLinkage, In,
GV->getName() + "." + Twine(ElementIdx), GV->getThreadLocalMode(),
GV->getType()->getAddressSpace());
NGV->setExternallyInitialized(GV->isExternallyInitialized());
NGV->copyAttributesFrom(GV);
NewGlobals.insert(std::make_pair(ElementIdx, NGV));
if (StructType *STy = dyn_cast<StructType>(Ty)) {
const StructLayout &Layout = *DL.getStructLayout(STy);
// Calculate the known alignment of the field. If the original aggregate
// had 256 byte alignment for example, something might depend on that:
// propagate info to each field.
uint64_t FieldOffset = Layout.getElementOffset(i);
uint64_t FieldOffset = Layout.getElementOffset(ElementIdx);
Align NewAlign(MinAlign(StartAlignment, FieldOffset));
if (NewAlign > Align(DL.getABITypeAlignment(STy->getElementType(i))))
if (NewAlign >
Align(DL.getABITypeAlignment(STy->getElementType(ElementIdx))))
NGV->setAlignment(NewAlign);
// Copy over the debug info for the variable.
uint64_t Size = DL.getTypeAllocSizeInBits(NGV->getValueType());
uint64_t FragmentOffsetInBits = Layout.getElementOffsetInBits(i);
transferSRADebugInfo(GV, NGV, FragmentOffsetInBits, Size, NumElements);
}
} else if (SequentialType *STy = dyn_cast<SequentialType>(Ty)) {
unsigned NumElements = STy->getNumElements();
if (NumElements > 16 && GV->hasNUsesOrMore(16))
return nullptr; // It's not worth it.
NewGlobals.reserve(NumElements);
auto ElTy = STy->getElementType();
uint64_t EltSize = DL.getTypeAllocSize(ElTy);
Align EltAlign(DL.getABITypeAlignment(ElTy));
uint64_t FragmentSizeInBits = DL.getTypeAllocSizeInBits(ElTy);
for (unsigned i = 0, e = NumElements; i != e; ++i) {
Constant *In = Init->getAggregateElement(i);
assert(In && "Couldn't get element of initializer?");
GlobalVariable *NGV = new GlobalVariable(STy->getElementType(), false,
GlobalVariable::InternalLinkage,
In, GV->getName()+"."+Twine(i),
GV->getThreadLocalMode(),
GV->getType()->getAddressSpace());
NGV->setExternallyInitialized(GV->isExternallyInitialized());
NGV->copyAttributesFrom(GV);
Globals.push_back(NGV);
NewGlobals.push_back(NGV);
uint64_t FragmentOffsetInBits = Layout.getElementOffsetInBits(ElementIdx);
transferSRADebugInfo(GV, NGV, FragmentOffsetInBits, Size,
STy->getNumElements());
} else if (SequentialType *STy = dyn_cast<SequentialType>(Ty)) {
uint64_t EltSize = DL.getTypeAllocSize(ElTy);
Align EltAlign(DL.getABITypeAlignment(ElTy));
uint64_t FragmentSizeInBits = DL.getTypeAllocSizeInBits(ElTy);
// Calculate the known alignment of the field. If the original aggregate
// had 256 byte alignment for example, something might depend on that:
// propagate info to each field.
Align NewAlign(MinAlign(StartAlignment, EltSize * i));
Align NewAlign(MinAlign(StartAlignment, EltSize * ElementIdx));
if (NewAlign > EltAlign)
NGV->setAlignment(NewAlign);
transferSRADebugInfo(GV, NGV, FragmentSizeInBits * i, FragmentSizeInBits,
NumElements);
transferSRADebugInfo(GV, NGV, FragmentSizeInBits * ElementIdx,
FragmentSizeInBits, STy->getNumElements());
}
}
if (NewGlobals.empty())
return nullptr;
Module::GlobalListType &Globals = GV->getParent()->getGlobalList();
for (auto NewGlobalVar : NewGlobals)
Globals.push_back(NewGlobalVar.second);
LLVM_DEBUG(dbgs() << "PERFORMING GLOBAL SRA ON: " << *GV << "\n");
Constant *NullInt =Constant::getNullValue(Type::getInt32Ty(GV->getContext()));
@ -559,11 +579,11 @@ static GlobalVariable *SRAGlobal(GlobalVariable *GV, const DataLayout &DL) {
// Ignore the 1th operand, which has to be zero or else the program is quite
// broken (undefined). Get the 2nd operand, which is the structure or array
// index.
unsigned Val = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
if (Val >= NewGlobals.size()) Val = 0; // Out of bound array access.
unsigned ElementIdx = cast<ConstantInt>(GEP->getOperand(2))->getZExtValue();
assert(NewGlobals.count(ElementIdx) == 1);
Value *NewPtr = NewGlobals[Val];
Type *NewTy = NewGlobals[Val]->getValueType();
Value *NewPtr = NewGlobals[ElementIdx];
Type *NewTy = NewGlobals[ElementIdx]->getValueType();
// Form a shorter GEP if needed.
if (GEP->getNumOperands() > 3) {
@ -581,7 +601,8 @@ static GlobalVariable *SRAGlobal(GlobalVariable *GV, const DataLayout &DL) {
for (unsigned i = 3, e = GEPI->getNumOperands(); i != e; ++i)
Idxs.push_back(GEPI->getOperand(i));
NewPtr = GetElementPtrInst::Create(
NewTy, NewPtr, Idxs, GEPI->getName() + "." + Twine(Val), GEPI);
NewTy, NewPtr, Idxs, GEPI->getName() + "." + Twine(ElementIdx),
GEPI);
}
}
GEP->replaceAllUsesWith(NewPtr);
@ -596,17 +617,8 @@ static GlobalVariable *SRAGlobal(GlobalVariable *GV, const DataLayout &DL) {
Globals.erase(GV);
++NumSRA;
// Loop over the new globals array deleting any globals that are obviously
// dead. This can arise due to scalarization of a structure or an array that
// has elements that are dead.
unsigned FirstGlobal = 0;
for (unsigned i = 0, e = NewGlobals.size(); i != e; ++i)
if (NewGlobals[i]->use_empty()) {
Globals.erase(NewGlobals[i]);
if (FirstGlobal == i) ++FirstGlobal;
}
return FirstGlobal != NewGlobals.size() ? NewGlobals[FirstGlobal] : nullptr;
assert(NewGlobals.size() > 0);
return NewGlobals.begin()->second;
}
/// Return true if all users of the specified value will trap if the value is

61
test/Transforms/GlobalOpt/long-compilation-global-sra.ll Normal file
View File

@ -0,0 +1,61 @@
; RUN: opt %s --O0 -globalopt -S -o -
; This is a regression test against very slow execution...
; In bad case it should fail by timeout.
; Hand-reduced from this example.
; clang++ -mllvm -disable-llvm-optzns
;#include <stdio.h>
;
;namespace {
; char LargeBuffer[64 * 1024 * 1024];
;}
;
;int main ( void ) {
;
; LargeBuffer[0] = 0;
;
; printf("");
;
; return LargeBuffer[0] == 0;
;}
; check that global array LargeBufferE was optimized out
; and local variable LargeBufferE.0 was used instead.
; CHECK-NOT: global
; CHECK: main()
; CHECK-NEXT: LargeBufferE.0
; CHECK-NOT: global
; ModuleID = 'test.cpp'
source_filename = "test.cpp"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
@LargeBufferE = internal global [67108864 x i8] zeroinitializer, align 16
@.str = private unnamed_addr constant [1 x i8] c"\00", align 1
; Function Attrs: norecurse uwtable
define dso_local i32 @main() #0 {
%1 = alloca i32, align 4
store i32 0, i32* %1, align 4
store i8 0, i8* getelementptr inbounds ([67108864 x i8], [67108864 x i8]* @LargeBufferE, i64 0, i64 0), align 16
%2 = call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([1 x i8], [1 x i8]* @.str, i64 0, i64 0))
%3 = load i8, i8* getelementptr inbounds ([67108864 x i8], [67108864 x i8]* @LargeBufferE, i64 0, i64 0), align 16
%4 = sext i8 %3 to i32
%5 = icmp eq i32 %4, 0
%6 = zext i1 %5 to i32
ret i32 %6
}
declare dso_local i32 @printf(i8*, ...) #0
attributes #0 = { norecurse uwtable }
!llvm.module.flags = !{!0}
!llvm.ident = !{!1}
!0 = !{i32 1, !"wchar_size", i32 4}
!1 = !{!"clang version 10.0.0 "}