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[ImplicitNullChecks] Hoist trivial depdendencies if possible

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When trying to convert a loading instruction into a FAULTING_LOAD, we
sometimes face code like this:

  if %R10 is not null:
    %R9<def> = MOV32ri Immediate
    %R9<def, tied> = AND32rm %R9, 0x20(%R10)
  else:
    goto TRAP

In these cases we would like to use the AND32rm instruction as the
faulting operation by hoisting the "depedency" def-ing %R9 also above
the control flow, transforming the program into:

  %R9<def> = MOV32ri Immediate
  %R9<def, tied> = FAULTING_LOAD_OP(AND32rm %R9, 0x20(%R10), FailPath: TRAP)

This change teaches ImplicitNullChecks to do the above, when safe.

llvm-svn: 273501
This commit is contained in:
Sanjoy Das 2016-06-22 22:16:51 +00:00
parent 6c4a0b7a05
commit 6cd19fcfb8
2 changed files with 425 additions and 20 deletions
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179
lib/CodeGen/ImplicitNullChecks.cpp
View File

@ -28,6 +28,7 @@
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/CodeGen/Passes.h"
#include "llvm/CodeGen/MachineFunction.h"
#include "llvm/CodeGen/MachineMemOperand.h"
@ -75,14 +76,19 @@ class ImplicitNullChecks : public MachineFunctionPass {
// The block branched to if the pointer is null.
MachineBasicBlock *NullSucc;
// If this is non-null, then MemOperation has a dependency on on this
// instruction; and it needs to be hoisted to execute before MemOperation.
MachineInstr *OnlyDependency;
public:
explicit NullCheck(MachineInstr *memOperation, MachineInstr *checkOperation,
MachineBasicBlock *checkBlock,
MachineBasicBlock *notNullSucc,
MachineBasicBlock *nullSucc)
MachineBasicBlock *nullSucc,
MachineInstr *onlyDependency)
: MemOperation(memOperation), CheckOperation(checkOperation),
CheckBlock(checkBlock), NotNullSucc(notNullSucc), NullSucc(nullSucc) {
}
CheckBlock(checkBlock), NotNullSucc(notNullSucc), NullSucc(nullSucc),
OnlyDependency(onlyDependency) {}
MachineInstr *getMemOperation() const { return MemOperation; }
@ -93,10 +99,13 @@ class ImplicitNullChecks : public MachineFunctionPass {
MachineBasicBlock *getNotNullSucc() const { return NotNullSucc; }
MachineBasicBlock *getNullSucc() const { return NullSucc; }
MachineInstr *getOnlyDependency() const { return OnlyDependency; }
};
const TargetInstrInfo *TII = nullptr;
const TargetRegisterInfo *TRI = nullptr;
AliasAnalysis *AA = nullptr;
MachineModuleInfo *MMI = nullptr;
bool analyzeBlockForNullChecks(MachineBasicBlock &MBB,
@ -113,6 +122,10 @@ public:
}
bool runOnMachineFunction(MachineFunction &MF) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AAResultsWrapperPass>();
MachineFunctionPass::getAnalysisUsage(AU);
}
MachineFunctionProperties getRequiredProperties() const override {
return MachineFunctionProperties().set(
@ -126,14 +139,22 @@ public:
/// machine instruction can be re-ordered from after the machine instructions
/// seen so far to before them.
class HazardDetector {
DenseSet<unsigned> RegDefs;
static MachineInstr *getUnknownMI() {
return DenseMapInfo<MachineInstr *>::getTombstoneKey();
}
// Maps physical registers to the instruction defining them. If there has
// been more than one def of an specific register, that register is mapped to
// getUnknownMI().
DenseMap<unsigned, MachineInstr *> RegDefs;
DenseSet<unsigned> RegUses;
const TargetRegisterInfo &TRI;
bool hasSeenClobber;
AliasAnalysis &AA;
public:
explicit HazardDetector(const TargetRegisterInfo &TRI) :
TRI(TRI), hasSeenClobber(false) {}
explicit HazardDetector(const TargetRegisterInfo &TRI, AliasAnalysis &AA)
: TRI(TRI), hasSeenClobber(false), AA(AA) {}
/// \brief Make a note of \p MI for later queries to isSafeToHoist.
///
@ -141,8 +162,10 @@ public:
void rememberInstruction(MachineInstr *MI);
/// \brief Return true if it is safe to hoist \p MI from after all the
/// instructions seen so far (via rememberInstruction) to before it.
bool isSafeToHoist(MachineInstr *MI);
/// instructions seen so far (via rememberInstruction) to before it. If \p MI
/// has one and only one transitive dependency, set \p Dependency to that
/// instruction. If there are more dependencies, return false.
bool isSafeToHoist(MachineInstr *MI, MachineInstr *&Dependency);
/// \brief Return true if this instance of HazardDetector has been clobbered
/// (i.e. has no more useful information).
@ -181,15 +204,23 @@ void HazardDetector::rememberInstruction(MachineInstr *MI) {
if (!MO.isReg() || !MO.getReg())
continue;
if (MO.isDef())
RegDefs.insert(MO.getReg());
else
if (MO.isDef()) {
auto It = RegDefs.find(MO.getReg());
if (It == RegDefs.end())
RegDefs.insert({MO.getReg(), MI});
else {
assert(It->second && "Found null MI?");
It->second = getUnknownMI();
}
} else
RegUses.insert(MO.getReg());
}
}
bool HazardDetector::isSafeToHoist(MachineInstr *MI) {
bool HazardDetector::isSafeToHoist(MachineInstr *MI,
MachineInstr *&Dependency) {
assert(!isClobbered() && "isSafeToHoist cannot do anything useful!");
Dependency = nullptr;
// Right now we don't want to worry about LLVM's memory model. This can be
// made more precise later.
@ -199,9 +230,54 @@ bool HazardDetector::isSafeToHoist(MachineInstr *MI) {
for (auto &MO : MI->operands()) {
if (MO.isReg() && MO.getReg()) {
for (unsigned Reg : RegDefs)
if (TRI.regsOverlap(Reg, MO.getReg()))
return false; // We found a write-after-write or read-after-write
for (auto &RegDef : RegDefs) {
unsigned Reg = RegDef.first;
MachineInstr *MI = RegDef.second;
if (!TRI.regsOverlap(Reg, MO.getReg()))
continue;
// We found a write-after-write or read-after-write, see if the
// instruction causing this dependency can be hoisted too.
if (MI == getUnknownMI())
// We don't have precise dependency information.
return false;
if (Dependency) {
if (Dependency == MI)
continue;
// We already have one dependency, and we can track only one.
return false;
}
// Now check if MI is actually a dependency that can be hoisted.
// We don't want to track transitive dependencies. We already know that
// MI is the only instruction that defines Reg, but we need to be sure
// that it does not use any registers that have been defined (trivially
// checked below by ensuring that there are no register uses), and that
// it is the only def for every register it defines (otherwise we could
// violate a write after write hazard).
auto IsMIOperandSafe = [&](MachineOperand &MO) {
if (!MO.isReg() || !MO.getReg())
return true;
if (MO.isUse())
return false;
assert((!MO.isDef() || RegDefs.count(MO.getReg())) &&
"All defs must be tracked in RegDefs by now!");
return !MO.isDef() || RegDefs.find(MO.getReg())->second == MI;
};
if (!all_of(MI->operands(), IsMIOperandSafe))
return false;
// Now check for speculation safety:
bool SawStore = true;
if (!MI->isSafeToMove(&AA, SawStore) || MI->mayLoad())
return false;
Dependency = MI;
}
if (MO.isDef())
for (unsigned Reg : RegUses)
@ -217,6 +293,7 @@ bool ImplicitNullChecks::runOnMachineFunction(MachineFunction &MF) {
TII = MF.getSubtarget().getInstrInfo();
TRI = MF.getRegInfo().getTargetRegisterInfo();
MMI = &MF.getMMI();
AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
SmallVector<NullCheck, 16> NullCheckList;
@ -229,6 +306,16 @@ bool ImplicitNullChecks::runOnMachineFunction(MachineFunction &MF) {
return !NullCheckList.empty();
}
// Return true if any register aliasing \p Reg is live-in into \p MBB.
static bool AnyAliasLiveIn(const TargetRegisterInfo *TRI,
MachineBasicBlock *MBB, unsigned Reg) {
for (MCRegAliasIterator AR(Reg, TRI, /*IncludeSelf*/ true); AR.isValid();
++AR)
if (MBB->isLiveIn(*AR))
return true;
return false;
}
/// Analyze MBB to check if its terminating branch can be turned into an
/// implicit null check. If yes, append a description of the said null check to
/// NullCheckList and return true, else return false.
@ -330,20 +417,56 @@ bool ImplicitNullChecks::analyzeBlockForNullChecks(
unsigned PointerReg = MBP.LHS.getReg();
HazardDetector HD(*TRI);
HazardDetector HD(*TRI, *AA);
for (auto MII = NotNullSucc->begin(), MIE = NotNullSucc->end(); MII != MIE;
++MII) {
MachineInstr *MI = &*MII;
unsigned BaseReg;
int64_t Offset;
MachineInstr *Dependency = nullptr;
if (TII->getMemOpBaseRegImmOfs(MI, BaseReg, Offset, TRI))
if (MI->mayLoad() && !MI->isPredicable() && BaseReg == PointerReg &&
Offset < PageSize && MI->getDesc().getNumDefs() <= 1 &&
HD.isSafeToHoist(MI)) {
NullCheckList.emplace_back(MI, MBP.ConditionDef, &MBB, NotNullSucc,
NullSucc);
return true;
HD.isSafeToHoist(MI, Dependency)) {
auto DependencyOperandIsOk = [&](MachineOperand &MO) {
assert(!(MO.isReg() && MO.isUse()) &&
"No transitive dependendencies please!");
if (!MO.isReg() || !MO.getReg() || !MO.isDef())
return true;
// Make sure that we won't clobber any live ins to the sibling block
// by hoisting Dependency. For instance, we can't hoist INST to
// before the null check (even if it safe, and does not violate any
// dependencies in the non_null_block) if %rdx is live in to
// _null_block.
//
// test %rcx, %rcx
// je _null_block
// _non_null_block:
// %rdx<def> = INST
// ...
if (AnyAliasLiveIn(TRI, NullSucc, MO.getReg()))
return false;
// Make sure Dependency isn't re-defining the base register. Then we
// won't get the memory operation on the address we want.
if (TRI->regsOverlap(MO.getReg(), BaseReg))
return false;
return true;
};
bool DependencyOperandsAreOk =
!Dependency ||
all_of(Dependency->operands(), DependencyOperandIsOk);
if (DependencyOperandsAreOk) {
NullCheckList.emplace_back(MI, MBP.ConditionDef, &MBB, NotNullSucc,
NullSucc, Dependency);
return true;
}
}
HD.rememberInstruction(MI);
@ -399,6 +522,11 @@ void ImplicitNullChecks::rewriteNullChecks(
(void)BranchesRemoved;
assert(BranchesRemoved > 0 && "expected at least one branch!");
if (auto *DepMI = NC.getOnlyDependency()) {
DepMI->removeFromParent();
NC.getCheckBlock()->insert(NC.getCheckBlock()->end(), DepMI);
}
// Insert a faulting load where the conditional branch was originally. We
// check earlier ensures that this bit of code motion is legal. We do not
// touch the successors list for any basic block since we haven't changed
@ -418,6 +546,16 @@ void ImplicitNullChecks::rewriteNullChecks(
continue;
MBB->addLiveIn(Reg);
}
if (auto *DepMI = NC.getOnlyDependency()) {
for (auto &MO : DepMI->operands()) {
if (!MO.isReg() || !MO.getReg() || !MO.isDef())
continue;
if (!NC.getNotNullSucc()->isLiveIn(MO.getReg()))
NC.getNotNullSucc()->addLiveIn(MO.getReg());
}
}
NC.getMemOperation()->eraseFromParent();
NC.getCheckOperation()->eraseFromParent();
@ -433,5 +571,6 @@ char ImplicitNullChecks::ID = 0;
char &llvm::ImplicitNullChecksID = ImplicitNullChecks::ID;
INITIALIZE_PASS_BEGIN(ImplicitNullChecks, "implicit-null-checks",
"Implicit null checks", false, false)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(ImplicitNullChecks, "implicit-null-checks",
"Implicit null checks", false, false)

266
test/CodeGen/X86/implicit-null-checks.mir Normal file
View File

@ -0,0 +1,266 @@
# RUN: llc -run-pass implicit-null-checks -mtriple=x86_64-apple-macosx -o /dev/null %s 2>&1 | FileCheck %s
--- |
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-apple-macosx"
;; Positive test
define i32 @imp_null_check_with_bitwise_op_0(i32* %x, i32 %val) {
entry:
br i1 undef, label %is_null, label %not_null, !make.implicit !0
is_null:
ret i32 42
not_null:
br i1 undef, label %ret_100, label %ret_200
ret_100:
ret i32 100
ret_200:
ret i32 200
}
;; Negative test. The regalloc is such that we cannot hoist the
;; instruction materializing 2200000 into %eax
define i32 @imp_null_check_with_bitwise_op_1(i32* %x, i32 %val, i32* %ptr) {
entry:
br i1 undef, label %is_null, label %not_null, !make.implicit !0
is_null:
ret i32 undef
not_null:
br i1 undef, label %ret_100, label %ret_200
ret_100:
ret i32 100
ret_200:
ret i32 200
}
;; Negative test: IR is identical to
;; @imp_null_check_with_bitwise_op_0 but MIR differs.
define i32 @imp_null_check_with_bitwise_op_2(i32* %x, i32 %val) {
entry:
br i1 undef, label %is_null, label %not_null, !make.implicit !0
is_null:
ret i32 42
not_null:
br i1 undef, label %ret_100, label %ret_200
ret_100:
ret i32 100
ret_200:
ret i32 200
}
;; Negative test: IR is identical to
;; @imp_null_check_with_bitwise_op_0 but MIR differs.
define i32 @imp_null_check_with_bitwise_op_3(i32* %x, i32 %val) {
entry:
br i1 undef, label %is_null, label %not_null, !make.implicit !0
is_null:
ret i32 42
not_null:
br i1 undef, label %ret_100, label %ret_200
ret_100:
ret i32 100
ret_200:
ret i32 200
}
!0 = !{}
...
---
name: imp_null_check_with_bitwise_op_0
# CHECK-LABEL: name: imp_null_check_with_bitwise_op_0
alignment: 4
allVRegsAllocated: true
tracksRegLiveness: true
tracksSubRegLiveness: false
liveins:
- { reg: '%rdi' }
- { reg: '%esi' }
# CHECK: bb.0.entry:
# CHECK: %eax = MOV32ri 2200000
# CHECK-NEXT: %eax = FAULTING_LOAD_OP %bb.3.is_null, 196, killed %eax, killed %rdi, 1, _, 0, _, implicit-def dead %eflags :: (load 4 from %ir.x)
# CHECK-NEXT: JMP_1 %bb.1.not_null
body: |
bb.0.entry:
successors: %bb.3.is_null, %bb.1.not_null
liveins: %esi, %rdi
TEST64rr %rdi, %rdi, implicit-def %eflags
JE_1 %bb.3.is_null, implicit %eflags
bb.1.not_null:
successors: %bb.4.ret_100, %bb.2.ret_200
liveins: %esi, %rdi
%eax = MOV32ri 2200000
%eax = AND32rm killed %eax, killed %rdi, 1, _, 0, _, implicit-def dead %eflags :: (load 4 from %ir.x)
CMP32rr killed %eax, killed %esi, implicit-def %eflags
JE_1 %bb.4.ret_100, implicit %eflags
bb.2.ret_200:
%eax = MOV32ri 200
RET 0, %eax
bb.3.is_null:
%eax = MOV32ri 42
RET 0, %eax
bb.4.ret_100:
%eax = MOV32ri 100
RET 0, %eax
...
---
name: imp_null_check_with_bitwise_op_1
alignment: 4
allVRegsAllocated: true
isSSA: false
tracksRegLiveness: true
tracksSubRegLiveness: false
liveins:
- { reg: '%rdi' }
- { reg: '%esi' }
- { reg: '%rdx' }
# CHECK: bb.0.entry:
# CHECK: %eax = MOV32rm killed %rdx, 1, _, 0, _ :: (volatile load 4 from %ir.ptr)
# CHECK-NEXT: TEST64rr %rdi, %rdi, implicit-def %eflags
# CHECK-NEXT: JE_1 %bb.3.is_null, implicit %eflags
body: |
bb.0.entry:
successors: %bb.3.is_null, %bb.1.not_null
liveins: %esi, %rdi, %rdx
%eax = MOV32rm killed %rdx, 1, _, 0, _ :: (volatile load 4 from %ir.ptr)
TEST64rr %rdi, %rdi, implicit-def %eflags
JE_1 %bb.3.is_null, implicit %eflags
bb.1.not_null:
successors: %bb.4.ret_100, %bb.2.ret_200
liveins: %esi, %rdi
%eax = MOV32ri 2200000
%eax = AND32rm killed %eax, killed %rdi, 1, _, 0, _, implicit-def dead %eflags :: (load 4 from %ir.x)
CMP32rr killed %eax, killed %esi, implicit-def %eflags
JE_1 %bb.4.ret_100, implicit %eflags
bb.2.ret_200:
successors: %bb.3.is_null
%eax = MOV32ri 200
bb.3.is_null:
liveins: %eax, %ah, %al, %ax, %bh, %bl, %bp, %bpl, %bx, %eax, %ebp, %ebx, %rax, %rbp, %rbx, %r12, %r13, %r14, %r15, %r12b, %r13b, %r14b, %r15b, %r12d, %r13d, %r14d, %r15d, %r12w, %r13w, %r14w, %r15w
RET 0, %eax
bb.4.ret_100:
%eax = MOV32ri 100
RET 0, %eax
...
---
name: imp_null_check_with_bitwise_op_2
# CHECK-LABEL: name: imp_null_check_with_bitwise_op_2
alignment: 4
allVRegsAllocated: true
tracksRegLiveness: true
tracksSubRegLiveness: false
liveins:
- { reg: '%rdi' }
- { reg: '%esi' }
# CHECK: bb.0.entry:
# CHECK: TEST64rr %rdi, %rdi, implicit-def %eflags
# CHECK-NEXT: JE_1 %bb.3.is_null, implicit %eflags
body: |
bb.0.entry:
successors: %bb.3.is_null, %bb.1.not_null
liveins: %esi, %rdi
TEST64rr %rdi, %rdi, implicit-def %eflags
JE_1 %bb.3.is_null, implicit %eflags
bb.1.not_null:
successors: %bb.4.ret_100, %bb.2.ret_200
liveins: %esi, %rdi
%eax = MOV32ri 2200000
%eax = ADD32ri killed %eax, 100, implicit-def dead %eflags
%eax = AND32rm killed %eax, killed %rdi, 1, _, 0, _, implicit-def dead %eflags :: (load 4 from %ir.x)
CMP32rr killed %eax, killed %esi, implicit-def %eflags
JE_1 %bb.4.ret_100, implicit %eflags
bb.2.ret_200:
%eax = MOV32ri 200
RET 0, %eax
bb.3.is_null:
%eax = MOV32ri 42
RET 0, %eax
bb.4.ret_100:
%eax = MOV32ri 100
RET 0, %eax
...
---
name: imp_null_check_with_bitwise_op_3
# CHECK-LABEL: name: imp_null_check_with_bitwise_op_3
alignment: 4
allVRegsAllocated: true
tracksRegLiveness: true
tracksSubRegLiveness: false
liveins:
- { reg: '%rdi' }
- { reg: '%rsi' }
# CHECK: bb.0.entry:
# CHECK: TEST64rr %rdi, %rdi, implicit-def %eflags
# CHECK-NEXT: JE_1 %bb.3.is_null, implicit %eflags
body: |
bb.0.entry:
successors: %bb.3.is_null, %bb.1.not_null
liveins: %rsi, %rdi
TEST64rr %rdi, %rdi, implicit-def %eflags
JE_1 %bb.3.is_null, implicit %eflags
bb.1.not_null:
successors: %bb.4.ret_100, %bb.2.ret_200
liveins: %rsi, %rdi
%rdi = MOV64ri 5000
%rdi = AND64rm killed %rdi, killed %rdi, 1, _, 0, _, implicit-def dead %eflags :: (load 4 from %ir.x)
CMP64rr killed %rdi, killed %rsi, implicit-def %eflags
JE_1 %bb.4.ret_100, implicit %eflags
bb.2.ret_200:
%eax = MOV32ri 200
RET 0, %eax
bb.3.is_null:
%eax = MOV32ri 42
RET 0, %eax
bb.4.ret_100:
%eax = MOV32ri 100
RET 0, %eax
...