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bpf: avoid load from read-only sections

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If users tried to have a structure decl/init code like below
   struct test_t t = { .memeber1 = 45 };
It is very likely that compiler will generate a readonly section
to hold up the init values for variable t. Later load of t members,
e.g., t.member1 will result in a read from readonly section.

BPF program cannot handle relocation. This will force users to
write:
  struct test_t t = {};
  t.member1 = 45;
This is just inconvenient and unintuitive.

This patch addresses this issue by implementing BPF PreprocessISelDAG.
For any load from a global constant structure or an global array of
constant struct, it attempts to
translate it into a constant directly. The traversal of the
constant struct and other constant data structures are similar
to where the assembler emits read-only sections.

Four different unit test cases are also added to cover
different scenarios.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 305560
This commit is contained in:
Yonghong Song 2017-06-16 15:41:16 +00:00
parent 0e4184c6b7
commit a6c7bc9945
5 changed files with 420 additions and 7 deletions
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240
lib/Target/BPF/BPFISelDAGToDAG.cpp
View File

@ -22,11 +22,14 @@
#include "llvm/CodeGen/MachineInstrBuilder.h"
#include "llvm/CodeGen/MachineRegisterInfo.h"
#include "llvm/CodeGen/SelectionDAGISel.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/Endian.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetMachine.h"
using namespace llvm;
#define DEBUG_TYPE "bpf-isel"
@ -42,6 +45,8 @@ public:
return "BPF DAG->DAG Pattern Instruction Selection";
}
void PreprocessISelDAG() override;
private:
// Include the pieces autogenerated from the target description.
#include "BPFGenDAGISel.inc"
@ -51,15 +56,31 @@ private:
// Complex Pattern for address selection.
bool SelectAddr(SDValue Addr, SDValue &Base, SDValue &Offset);
bool SelectFIAddr(SDValue Addr, SDValue &Base, SDValue &Offset);
// Find constants from a constant structure
typedef std::vector<unsigned char> val_vec_type;
bool fillGenericConstant(const DataLayout &DL, const Constant *CV,
val_vec_type &Vals, uint64_t Offset);
bool fillConstantDataArray(const DataLayout &DL, const ConstantDataArray *CDA,
val_vec_type &Vals, int Offset);
bool fillConstantArray(const DataLayout &DL, const ConstantArray *CA,
val_vec_type &Vals, int Offset);
bool fillConstantStruct(const DataLayout &DL, const ConstantStruct *CS,
val_vec_type &Vals, int Offset);
bool getConstantFieldValue(const GlobalAddressSDNode *Node, uint64_t Offset,
uint64_t Size, unsigned char *ByteSeq);
// Mapping from ConstantStruct global value to corresponding byte-list values
std::map<const void *, val_vec_type> cs_vals_;
};
}
} // namespace
// ComplexPattern used on BPF Load/Store instructions
bool BPFDAGToDAGISel::SelectAddr(SDValue Addr, SDValue &Base, SDValue &Offset) {
// if Address is FI, get the TargetFrameIndex.
SDLoc DL(Addr);
if (FrameIndexSDNode *FIN = dyn_cast<FrameIndexSDNode>(Addr)) {
Base = CurDAG->getTargetFrameIndex(FIN->getIndex(), MVT::i64);
Base = CurDAG->getTargetFrameIndex(FIN->getIndex(), MVT::i64);
Offset = CurDAG->getTargetConstant(0, DL, MVT::i64);
return true;
}
@ -85,13 +106,14 @@ bool BPFDAGToDAGISel::SelectAddr(SDValue Addr, SDValue &Base, SDValue &Offset) {
}
}
Base = Addr;
Base = Addr;
Offset = CurDAG->getTargetConstant(0, DL, MVT::i64);
return true;
}
// ComplexPattern used on BPF FI instruction
bool BPFDAGToDAGISel::SelectFIAddr(SDValue Addr, SDValue &Base, SDValue &Offset) {
bool BPFDAGToDAGISel::SelectFIAddr(SDValue Addr, SDValue &Base,
SDValue &Offset) {
SDLoc DL(Addr);
if (!CurDAG->isBaseWithConstantOffset(Addr))
@ -102,8 +124,7 @@ bool BPFDAGToDAGISel::SelectFIAddr(SDValue Addr, SDValue &Base, SDValue &Offset)
if (isInt<16>(CN->getSExtValue())) {
// If the first operand is a FI, get the TargetFI Node
if (FrameIndexSDNode *FIN =
dyn_cast<FrameIndexSDNode>(Addr.getOperand(0)))
if (FrameIndexSDNode *FIN = dyn_cast<FrameIndexSDNode>(Addr.getOperand(0)))
Base = CurDAG->getTargetFrameIndex(FIN->getIndex(), MVT::i64);
else
return false;
@ -129,7 +150,8 @@ void BPFDAGToDAGISel::Select(SDNode *Node) {
// tablegen selection should be handled here.
switch (Opcode) {
default: break;
default:
break;
case ISD::SDIV: {
DebugLoc Empty;
const DebugLoc &DL = Node->getDebugLoc();
@ -181,6 +203,210 @@ void BPFDAGToDAGISel::Select(SDNode *Node) {
SelectCode(Node);
}
void BPFDAGToDAGISel::PreprocessISelDAG() {
// Iterate through all nodes, only interested in loads from ConstantStruct
// ConstantArray should have converted by IR->DAG processing
for (SelectionDAG::allnodes_iterator I = CurDAG->allnodes_begin(),
E = CurDAG->allnodes_end();
I != E;) {
SDNode *Node = &*I++;
unsigned Opcode = Node->getOpcode();
if (Opcode != ISD::LOAD)
continue;
unsigned char new_val[8]; // hold up the constant values replacing loads.
bool to_replace = false;
SDLoc DL(Node);
const LoadSDNode *LD = cast<LoadSDNode>(Node);
uint64_t size = LD->getMemOperand()->getSize();
if (!size || size > 8 || (size & (size - 1)))
continue;
SDNode *LDAddrNode = LD->getOperand(1).getNode();
// Match LDAddr against either global_addr or (global_addr + offset)
unsigned opcode = LDAddrNode->getOpcode();
if (opcode == ISD::ADD) {
SDValue OP1 = LDAddrNode->getOperand(0);
SDValue OP2 = LDAddrNode->getOperand(1);
// We want to find the pattern global_addr + offset
SDNode *OP1N = OP1.getNode();
if (OP1N->getOpcode() <= ISD::BUILTIN_OP_END ||
OP1N->getNumOperands() == 0)
continue;
DEBUG(dbgs() << "Check candidate load: "; LD->dump(); dbgs() << '\n');
const GlobalAddressSDNode *GADN =
dyn_cast<GlobalAddressSDNode>(OP1N->getOperand(0).getNode());
const ConstantSDNode *CDN = dyn_cast<ConstantSDNode>(OP2.getNode());
if (GADN && CDN)
to_replace =
getConstantFieldValue(GADN, CDN->getZExtValue(), size, new_val);
} else if (LDAddrNode->getOpcode() > ISD::BUILTIN_OP_END &&
LDAddrNode->getNumOperands() > 0) {
DEBUG(dbgs() << "Check candidate load: "; LD->dump(); dbgs() << '\n');
SDValue OP1 = LDAddrNode->getOperand(0);
if (const GlobalAddressSDNode *GADN =
dyn_cast<GlobalAddressSDNode>(OP1.getNode()))
to_replace = getConstantFieldValue(GADN, 0, size, new_val);
}
if (!to_replace)
continue;
// replacing the old with a new value
uint64_t val;
if (size == 1)
val = *(uint8_t *)new_val;
else if (size == 2)
val = *(uint16_t *)new_val;
else if (size == 4)
val = *(uint32_t *)new_val;
else {
val = *(uint64_t *)new_val;
}
DEBUG(dbgs() << "Replacing load of size " << size << " with constant "
<< val << '\n');
SDValue NVal = CurDAG->getConstant(val, DL, MVT::i64);
// After replacement, the current node is dead, we need to
// go backward one step to make iterator still work
I--;
SDValue From[] = {SDValue(Node, 0), SDValue(Node, 1)};
SDValue To[] = {NVal, NVal};
CurDAG->ReplaceAllUsesOfValuesWith(From, To, 2);
I++;
// It is safe to delete node now
CurDAG->DeleteNode(Node);
}
}
bool BPFDAGToDAGISel::getConstantFieldValue(const GlobalAddressSDNode *Node,
uint64_t Offset, uint64_t Size,
unsigned char *ByteSeq) {
const GlobalVariable *V = dyn_cast<GlobalVariable>(Node->getGlobal());
if (!V || !V->hasInitializer())
return false;
const Constant *Init = V->getInitializer();
const DataLayout &DL = CurDAG->getDataLayout();
val_vec_type TmpVal;
auto it = cs_vals_.find(static_cast<const void *>(Init));
if (it != cs_vals_.end()) {
TmpVal = it->second;
} else {
uint64_t total_size = 0;
if (const ConstantStruct *CS = dyn_cast<ConstantStruct>(Init))
total_size =
DL.getStructLayout(cast<StructType>(CS->getType()))->getSizeInBytes();
else if (const ConstantArray *CA = dyn_cast<ConstantArray>(Init))
total_size = DL.getTypeAllocSize(CA->getType()->getElementType()) *
CA->getNumOperands();
else
return false;
val_vec_type Vals(total_size, 0);
if (fillGenericConstant(DL, Init, Vals, 0) == false)
return false;
cs_vals_[static_cast<const void *>(Init)] = Vals;
TmpVal = std::move(Vals);
}
// test whether host endianness matches target
uint8_t test_buf[2];
uint16_t test_val = 0x2345;
if (DL.isLittleEndian())
support::endian::write16le(test_buf, test_val);
else
support::endian::write16be(test_buf, test_val);
bool endian_match = *(uint16_t *)test_buf == test_val;
for (uint64_t i = Offset, j = 0; i < Offset + Size; i++, j++)
ByteSeq[j] = endian_match ? TmpVal[i] : TmpVal[Offset + Size - 1 - j];
return true;
}
bool BPFDAGToDAGISel::fillGenericConstant(const DataLayout &DL,
const Constant *CV,
val_vec_type &Vals, uint64_t Offset) {
uint64_t Size = DL.getTypeAllocSize(CV->getType());
if (isa<ConstantAggregateZero>(CV) || isa<UndefValue>(CV))
return true; // already done
if (const ConstantInt *CI = dyn_cast<ConstantInt>(CV)) {
uint64_t val = CI->getZExtValue();
DEBUG(dbgs() << "Byte array at offset " << Offset << " with value " << val
<< '\n');
if (Size > 8 || (Size & (Size - 1)))
return false;
// Store based on target endian
for (uint64_t i = 0; i < Size; ++i) {
Vals[Offset + i] = DL.isLittleEndian()
? ((val >> (i * 8)) & 0xFF)
: ((val >> ((Size - i - 1) * 8)) & 0xFF);
}
return true;
}
if (const ConstantDataArray *CDA = dyn_cast<ConstantDataArray>(CV))
return fillConstantDataArray(DL, CDA, Vals, Offset);
if (const ConstantArray *CA = dyn_cast<ConstantArray>(CV))
return fillConstantArray(DL, CA, Vals, Offset);
if (const ConstantStruct *CVS = dyn_cast<ConstantStruct>(CV))
return fillConstantStruct(DL, CVS, Vals, Offset);
return false;
}
bool BPFDAGToDAGISel::fillConstantDataArray(const DataLayout &DL,
const ConstantDataArray *CDA,
val_vec_type &Vals, int Offset) {
for (unsigned i = 0, e = CDA->getNumElements(); i != e; ++i) {
if (fillGenericConstant(DL, CDA->getElementAsConstant(i), Vals, Offset) ==
false)
return false;
Offset += DL.getTypeAllocSize(CDA->getElementAsConstant(i)->getType());
}
return true;
}
bool BPFDAGToDAGISel::fillConstantArray(const DataLayout &DL,
const ConstantArray *CA,
val_vec_type &Vals, int Offset) {
for (unsigned i = 0, e = CA->getNumOperands(); i != e; ++i) {
if (fillGenericConstant(DL, CA->getOperand(i), Vals, Offset) == false)
return false;
Offset += DL.getTypeAllocSize(CA->getOperand(i)->getType());
}
return true;
}
bool BPFDAGToDAGISel::fillConstantStruct(const DataLayout &DL,
const ConstantStruct *CS,
val_vec_type &Vals, int Offset) {
const StructLayout *Layout = DL.getStructLayout(CS->getType());
for (unsigned i = 0, e = CS->getNumOperands(); i != e; ++i) {
const Constant *Field = CS->getOperand(i);
uint64_t SizeSoFar = Layout->getElementOffset(i);
if (fillGenericConstant(DL, Field, Vals, Offset + SizeSoFar) == false)
return false;
}
return true;
}
FunctionPass *llvm::createBPFISelDag(BPFTargetMachine &TM) {
return new BPFDAGToDAGISel(TM);
}

52
test/CodeGen/BPF/rodata_1.ll Normal file
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@ -0,0 +1,52 @@
; RUN: llc < %s -march=bpfel -verify-machineinstrs | FileCheck %s
; RUN: llc < %s -march=bpfeb -verify-machineinstrs | FileCheck %s
; Source code:
; struct test_t1 {
; char a, b, c;
; };
; struct test_t2 {
; int a, b, c, d, e;
; };
;
; struct test_t1 g1;
; struct test_t2 g2;
; int test()
; {
; struct test_t1 t1 = {.c = 1};
; struct test_t2 t2 = {.c = 1};
; g1 = t1;
; g2 = t2;
; return 0;
; }
%struct.test_t1 = type { i8, i8, i8 }
%struct.test_t2 = type { i32, i32, i32, i32, i32 }
@test.t1 = private unnamed_addr constant %struct.test_t1 { i8 0, i8 0, i8 1 }, align 1
@test.t2 = private unnamed_addr constant %struct.test_t2 { i32 0, i32 0, i32 1, i32 0, i32 0 }, align 4
@g1 = common local_unnamed_addr global %struct.test_t1 zeroinitializer, align 1
@g2 = common local_unnamed_addr global %struct.test_t2 zeroinitializer, align 4
; Function Attrs: nounwind
define i32 @test() local_unnamed_addr #0 {
; CHECK-LABEL: test:
entry:
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* getelementptr inbounds (%struct.test_t1, %struct.test_t1* @g1, i64 0, i32 0), i8* getelementptr inbounds (%struct.test_t1, %struct.test_t1* @test.t1, i64 0, i32 0), i64 3, i32 1, i1 false)
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* bitcast (%struct.test_t2* @g2 to i8*), i8* bitcast (%struct.test_t2* @test.t2 to i8*), i64 20, i32 4, i1 false)
; CHECK: r1 = <MCOperand Expr:(g1)>ll
; CHECK: r2 = 0
; CHECK: *(u8 *)(r1 + 1) = r2
; CHECK: r3 = 1
; CHECK: *(u8 *)(r1 + 2) = r3
; CHECK: r1 = <MCOperand Expr:(g2)>ll
; CHECK: *(u32 *)(r1 + 8) = r3
ret i32 0
}
; CHECK: .section .rodata,"a",@progbits
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i32, i1) #1
attributes #0 = { nounwind }
attributes #1 = { argmemonly nounwind }

51
test/CodeGen/BPF/rodata_2.ll Normal file
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@ -0,0 +1,51 @@
; RUN: llc < %s -march=bpfel -verify-machineinstrs | FileCheck %s
; RUN: llc < %s -march=bpfeb -verify-machineinstrs | FileCheck %s
; Source code:
; struct test_t1 {
; char a;
; int b;
; };
; struct test_t2 {
; char a, b;
; struct test_t1 c[2];
; int d[2];
; int e;
; };
; struct test_t2 g;
; int test()
; {
; struct test_t2 t2 = {.c = {{}, {.b = 1}}, .d = {2, 3}};
; g = t2;
; return 0;
; }
%struct.test_t2 = type { i8, i8, [2 x %struct.test_t1], [2 x i32], i32 }
%struct.test_t1 = type { i8, i32 }
@test.t2 = private unnamed_addr constant %struct.test_t2 { i8 0, i8 0, [2 x %struct.test_t1] [%struct.test_t1 zeroinitializer, %struct.test_t1 { i8 0, i32 1 }], [2 x i32] [i32 2, i32 3], i32 0 }, align 4
@g = common local_unnamed_addr global %struct.test_t2 zeroinitializer, align 4
; Function Attrs: nounwind
define i32 @test() local_unnamed_addr #0 {
; CHECK-LABEL: test:
entry:
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* getelementptr inbounds (%struct.test_t2, %struct.test_t2* @g, i64 0, i32 0), i8* getelementptr inbounds (%struct.test_t2, %struct.test_t2* @test.t2, i64 0, i32 0), i64 32, i32 4, i1 false)
; CHECK: r1 = <MCOperand Expr:(g)>ll
; CHECK: r2 = 0
; CHECK: *(u32 *)(r1 + 28) = r2
; CHECK: r3 = 3
; CHECK: *(u32 *)(r1 + 24) = r3
; CHECK: r3 = 2
; CHECK: *(u32 *)(r1 + 20) = r3
; CHECK: r3 = 1
; CHECK: *(u32 *)(r1 + 16) = r3
ret i32 0
}
; CHECK: .section .rodata.cst32,"aM",@progbits,32
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i32, i1) #1
attributes #0 = { nounwind }
attributes #1 = { argmemonly nounwind }

41
test/CodeGen/BPF/rodata_3.ll Normal file
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@ -0,0 +1,41 @@
; REQUIRES: x86_64-linux
; RUN: llc < %s -march=bpfel -verify-machineinstrs | FileCheck --check-prefix=CHECK-EL %s
; RUN: llc < %s -march=bpfeb -verify-machineinstrs | FileCheck --check-prefix=CHECK-EB %s
;
; This test requires little-endian host, so we specific x86_64-linux here.
; Source code:
; struct test_t1 {
; char a;
; int b, c, d;
; };
;
; struct test_t1 g;
; int test()
; {
; struct test_t1 t1 = {.a = 1};
; g = t1;
; return 0;
; }
%struct.test_t1 = type { i8, i32, i32, i32 }
@test.t1 = private unnamed_addr constant %struct.test_t1 { i8 1, i32 0, i32 0, i32 0 }, align 4
@g = common local_unnamed_addr global %struct.test_t1 zeroinitializer, align 4
; Function Attrs: nounwind
define i32 @test() local_unnamed_addr #0 {
entry:
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* getelementptr inbounds (%struct.test_t1, %struct.test_t1* @g, i64 0, i32 0), i8* getelementptr inbounds (%struct.test_t1, %struct.test_t1* @test.t1, i64 0, i32 0), i64 16, i32 4, i1 false)
; CHECK-EL: r2 = 1
; CHECK-EL: *(u32 *)(r1 + 0) = r2
; CHECK-EB: r2 = 16777216
; CHECK-EB: *(u32 *)(r1 + 0) = r2
ret i32 0
}
; CHECK-EL: .section .rodata.cst16,"aM",@progbits,16
; CHECK-EB: .section .rodata.cst16,"aM",@progbits,16
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i32, i1) #1
attributes #0 = { nounwind }
attributes #1 = { argmemonly nounwind }

43
test/CodeGen/BPF/rodata_4.ll Normal file
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@ -0,0 +1,43 @@
; RUN: llc < %s -march=bpfel -verify-machineinstrs | FileCheck %s
; RUN: llc < %s -march=bpfeb -verify-machineinstrs | FileCheck %s
; Source code:
; struct test_t1
; {
; short a;
; short b;
; char c;
; };
;
; struct test_t1 g;
; int test()
; {
; struct test_t1 t1[] = {{50, 500, 5}, {60, 600, 6}, {70, 700, 7}, {80, 800, 8} };
;
; g = t1[1];
; return 0;
; }
%struct.test_t1 = type { i16, i16, i8 }
@test.t1 = private unnamed_addr constant [4 x %struct.test_t1] [%struct.test_t1 { i16 50, i16 500, i8 5 }, %struct.test_t1 { i16 60, i16 600, i8 6 }, %struct.test_t1 { i16 70, i16 700, i8 7 }, %struct.test_t1 { i16 80, i16 800, i8 8 }], align 2
@g = common local_unnamed_addr global %struct.test_t1 zeroinitializer, align 2
; Function Attrs: nounwind
define i32 @test() local_unnamed_addr #0 {
; CHECK-LABEL: test:
entry:
tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* bitcast (%struct.test_t1* @g to i8*), i8* bitcast (%struct.test_t1* getelementptr inbounds ([4 x %struct.test_t1], [4 x %struct.test_t1]* @test.t1, i64 0, i64 1) to i8*), i64 6, i32 2, i1 false)
; CHECK: r2 = 600
; CHECK: *(u16 *)(r1 + 2) = r2
; CHECK: r2 = 60
; CHECK: *(u16 *)(r1 + 0) = r2
ret i32 0
}
; CHECK .section .rodata,"a",@progbits
; Function Attrs: argmemonly nounwind
declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i32, i1) #1
attributes #0 = { nounwind }
attributes #1 = { argmemonly nounwind }