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[lld][WebAssembly] Delay creation of internal __wasm_memory_init function 

This also allows for its creation to be conditional so it is completely
elided when not needed.

Differential Revision: https://reviews.llvm.org/D93035
This commit is contained in:
Sam Clegg 2020-12-10 07:40:48 -08:00
parent 47e7ecdd7d
commit 199497086e
8 changed files with 168 additions and 166 deletions
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8
lld/test/wasm/data-segment-merging.ll
View File

@ -98,9 +98,9 @@
; PASSIVE-MERGE-NEXT: - Index: 0
; PASSIVE-MERGE-NEXT: Name: __wasm_call_ctors
; PASSIVE-MERGE-NEXT: - Index: 1
; PASSIVE-MERGE-NEXT: Name: __wasm_init_memory
; PASSIVE-MERGE-NEXT: - Index: 2
; PASSIVE-MERGE-NEXT: Name: __wasm_init_tls
; PASSIVE-MERGE-NEXT: - Index: 2
; PASSIVE-MERGE-NEXT: Name: __wasm_init_memory
; RUN: wasm-ld -no-gc-sections --no-entry --shared-memory --max-memory=131072 -no-merge-data-segments -o %t.separate.passive.wasm %t.passive.o
; RUN: obj2yaml %t.separate.passive.wasm | FileCheck %s --check-prefix=PASSIVE-SEPARATE
@ -135,6 +135,6 @@
; PASSIVE-SEPARATE-NEXT: - Index: 0
; PASSIVE-SEPARATE-NEXT: Name: __wasm_call_ctors
; PASSIVE-SEPARATE-NEXT: - Index: 1
; PASSIVE-SEPARATE-NEXT: Name: __wasm_init_memory
; PASSIVE-SEPARATE-NEXT: - Index: 2
; PASSIVE-SEPARATE-NEXT: Name: __wasm_init_tls
; PASSIVE-SEPARATE-NEXT: - Index: 2
; PASSIVE-SEPARATE-NEXT: Name: __wasm_init_memory

25
lld/test/wasm/data-segments.ll
View File

@ -64,7 +64,7 @@
; ACTIVE-NEXT: Name: __wasm_call_ctors
; PASSIVE-LABEL: - Type: START
; PASSIVE-NEXT: StartFunction: 1
; PASSIVE-NEXT: StartFunction: 2
; PASSIVE-LABEL: - Type: DATACOUNT
; PASSIVE-NEXT: Count: 2
; PASSIVE-LABEL: - Type: CODE
@ -74,12 +74,11 @@
; PASSIVE-NEXT: Body: 0B
; PASSIVE-NEXT: - Index: 1
; PASSIVE-NEXT: Locals: []
; PASSIVE-NEXT: Body: 0B
; PASSIVE-NEXT: - Index: 2
; PASSIVE-NEXT: Locals: []
; PASSIVE32-NEXT: Body: 41B4D60041004101FE480200044041B4D6004101427FFE0102001A054180084100410DFC08000041900841004114FC08010041B4D6004102FE17020041B4D600417FFE0002001A0BFC0900FC09010B
; PASSIVE64-NEXT: Body: 42B4D60041004101FE480200044042B4D6004101427FFE0102001A054280084100410DFC08000042900841004114FC08010042B4D6004102FE17020042B4D600417FFE0002001A0BFC0900FC09010B
; PASSIVE-NEXT: - Index: 2
; PASSIVE-NEXT: Locals: []
; PASSIVE-NEXT: Body: 0B
; PASSIVE-NEXT: - Type: DATA
; PASSIVE-NEXT: Segments:
; PASSIVE-NEXT: - SectionOffset: 3
@ -94,12 +93,12 @@
; PASSIVE-NEXT: - Index: 0
; PASSIVE-NEXT: Name: __wasm_call_ctors
; PASSIVE-NEXT: - Index: 1
; PASSIVE-NEXT: Name: __wasm_init_memory
; PASSIVE-NEXT: - Index: 2
; PASSIVE-NEXT: Name: __wasm_init_tls
; PASSIVE-NEXT: - Index: 2
; PASSIVE-NEXT: Name: __wasm_init_memory
; PASSIVE-PIC: - Type: START
; PASSIVE-PIC-NEXT: StartFunction: 2
; PASSIVE-PIC-NEXT: StartFunction: 3
; PASSIVE-PIC-NEXT: - Type: DATACOUNT
; PASSIVE-PIC-NEXT: Count: 1
; PASSIVE-PIC-NEXT: - Type: CODE
@ -111,15 +110,15 @@
; PASSIVE-PIC-NEXT: Locals: []
; PASSIVE-PIC-NEXT: Body: 0B
; PASSIVE-PIC-NEXT: - Index: 2
; PASSIVE-PIC-NEXT: Locals: []
; PASSIVE-PIC-NEXT: Body: 0B
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Locals:
; PASSIVE32-PIC-NEXT: - Type: I32
; PASSIVE64-PIC-NEXT: - Type: I64
; PASSIVE-PIC-NEXT: Count: 1
; PASSIVE32-PIC-NEXT: Body: 230141B4CE006A2100200041004101FE480200044020004101427FFE0102001A05410023016A410041B1CE00FC08000020004102FE1702002000417FFE0002001A0BFC09000B
; PASSIVE64-PIC-NEXT: Body: 230142B4CE006A2100200041004101FE480200044020004101427FFE0102001A05420023016A410041B1CE00FC08000020004102FE1702002000417FFE0002001A0BFC09000B
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Locals: []
; PASSIVE-PIC-NEXT: Body: 0B
; PASSIVE-PIC-NEXT: - Type: DATA
; PASSIVE-PIC-NEXT: Segments:
; PASSIVE-PIC-NEXT: - SectionOffset: 4
@ -133,6 +132,6 @@
; PASSIVE-PIC-NEXT: - Index: 1
; PASSIVE-PIC-NEXT: Name: __wasm_apply_relocs
; PASSIVE-PIC-NEXT: - Index: 2
; PASSIVE-PIC-NEXT: Name: __wasm_init_memory
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Name: __wasm_init_tls
; PASSIVE-PIC-NEXT: - Index: 3
; PASSIVE-PIC-NEXT: Name: __wasm_init_memory

2
lld/test/wasm/no-tls.s
View File

@ -28,7 +28,7 @@ _start:
# CHECK-NEXT: Mutable: true
# CHECK-NEXT: InitExpr:
# CHECK-NEXT: Opcode: I32_CONST
# CHECK-NEXT: Value: 66576
# CHECK-NEXT: Value: 66560
# __tls_base
# CHECK-NEXT: - Index: 1

14
lld/test/wasm/tls.s
View File

@ -83,7 +83,7 @@ tls3:
# CHECK-NEXT: Mutable: true
# CHECK-NEXT: InitExpr:
# CHECK-NEXT: Opcode: I32_CONST
# CHECK-NEXT: Value: 66592
# CHECK-NEXT: Value: 66576
# __tls_base
# CHECK-NEXT: - Index: 1
@ -112,8 +112,8 @@ tls3:
# CHECK: - Type: CODE
# CHECK-NEXT: Functions:
# Skip __wasm_call_ctors and __wasm_init_memory
# CHECK: - Index: 2
# Skip __wasm_call_ctors
# CHECK: - Index: 1
# CHECK-NEXT: Locals: []
# CHECK-NEXT: Body: 2000240120004100410CFC0800000B
@ -126,7 +126,7 @@ tls3:
# memory.init 1, 0
# end
# CHECK-NEXT: - Index: 3
# CHECK-NEXT: - Index: 2
# CHECK-NEXT: Locals: []
# CHECK-NEXT: Body: 2381808080004180808080006A0B
@ -136,7 +136,7 @@ tls3:
# i32.add
# end
# CHECK-NEXT: - Index: 4
# CHECK-NEXT: - Index: 3
# CHECK-NEXT: Locals: []
# CHECK-NEXT: Body: 2381808080004184808080006A0B
@ -146,7 +146,7 @@ tls3:
# i32.add
# end
# CHECK-NEXT: - Index: 5
# CHECK-NEXT: - Index: 4
# CHECK-NEXT: Locals: []
# CHECK-NEXT: Body: 2381808080004188808080006A0B
@ -156,7 +156,7 @@ tls3:
# i32.add
# end
# CHECK-NEXT: - Index: 6
# CHECK-NEXT: - Index: 5
# CHECK-NEXT: Locals: []
# CHECK-NEXT: Body: 2383808080000B

9
lld/wasm/Driver.cpp
View File

@ -646,15 +646,6 @@ static void createSyntheticSymbols() {
}
if (config->sharedMemory) {
// Passive segments are used to avoid memory being reinitialized on each
// thread's instantiation. These passive segments are initialized and
// dropped in __wasm_init_memory, which is registered as the start function
WasmSym::initMemory = symtab->addSyntheticFunction(
"__wasm_init_memory", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_init_memory"));
WasmSym::initMemoryFlag = symtab->addSyntheticDataSymbol(
"__wasm_init_memory_flag", WASM_SYMBOL_VISIBILITY_HIDDEN);
assert(WasmSym::initMemoryFlag);
WasmSym::tlsBase = createGlobalVariable("__tls_base", true);
WasmSym::tlsSize = createGlobalVariable("__tls_size", false);
WasmSym::tlsAlign = createGlobalVariable("__tls_align", false);

3
lld/wasm/MarkLive.cpp
View File

@ -101,9 +101,6 @@ void MarkLive::run() {
if (WasmSym::applyRelocs)
enqueue(WasmSym::applyRelocs);
if (WasmSym::initMemory)
enqueue(WasmSym::initMemory);
// Enqueue constructors in objects explicitly live from the command-line.
for (const ObjFile *obj : symtab->objectFiles)
if (obj->isLive())

7
lld/wasm/SyntheticSections.h
View File

@ -239,14 +239,9 @@ public:
class StartSection : public SyntheticSection {
public:
StartSection(bool hasInitializedSegments)
: SyntheticSection(llvm::wasm::WASM_SEC_START),
hasInitializedSegments(hasInitializedSegments) {}
StartSection() : SyntheticSection(llvm::wasm::WASM_SEC_START) {}
bool isNeeded() const override;
void writeBody() override;
protected:
bool hasInitializedSegments;
};
class ElemSection : public SyntheticSection {

266
lld/wasm/Writer.cpp
View File

@ -58,6 +58,7 @@ private:
bool needsPassiveInitialization(const OutputSegment *segment);
bool hasPassiveInitializedSegments();
void createSyntheticInitFunctions();
void createInitMemoryFunction();
void createApplyRelocationsFunction();
void createCallCtorsFunction();
@ -865,14 +866,81 @@ bool Writer::hasPassiveInitializedSegments() {
}) != segments.end();
}
void Writer::createSyntheticInitFunctions() {
// Passive segments are used to avoid memory being reinitialized on each
// thread's instantiation. These passive segments are initialized and
// dropped in __wasm_init_memory, which is registered as the start function
if (config->sharedMemory && hasPassiveInitializedSegments()) {
static WasmSignature nullSignature = {{}, {}};
WasmSym::initMemory = symtab->addSyntheticFunction(
"__wasm_init_memory", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_init_memory"));
WasmSym::initMemory->markLive();
WasmSym::initMemoryFlag = symtab->addSyntheticDataSymbol(
"__wasm_init_memory_flag", WASM_SYMBOL_VISIBILITY_HIDDEN);
WasmSym::initMemoryFlag->markLive();
}
}
void Writer::createInitMemoryFunction() {
LLVM_DEBUG(dbgs() << "createInitMemoryFunction\n");
assert(WasmSym::initMemory);
assert(WasmSym::initMemoryFlag);
assert(hasPassiveInitializedSegments());
uint64_t flagAddress = WasmSym::initMemoryFlag->getVirtualAddress();
bool is64 = config->is64.getValueOr(false);
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
// Initialize memory in a thread-safe manner. The thread that successfully
// increments the flag from 0 to 1 is is responsible for performing the
// memory initialization. Other threads go sleep on the flag until the
// first thread finishing initializing memory, increments the flag to 2,
// and wakes all the other threads. Once the flag has been set to 2,
// subsequently started threads will skip the sleep. All threads
// unconditionally drop their passive data segments once memory has been
// initialized. The generated code is as follows:
//
// (func $__wasm_init_memory
// (if
// (i32.atomic.rmw.cmpxchg align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 0)
// (i32.const 1)
// )
// (then
// (drop
// (i32.atomic.wait align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 1)
// (i32.const -1)
// )
// )
// )
// (else
// ( ... initialize data segments ... )
// (i32.atomic.store align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 2)
// )
// (drop
// (i32.atomic.notify align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const -1u)
// )
// )
// )
// )
// ( ... drop data segments ... )
// )
//
// When we are building with PIC, calculate the flag location using:
//
// (global.get $__memory_base)
// (i32.const $__init_memory_flag)
// (i32.const 1)
// With PIC code we cache the flag address in local 0
if (config->isPic) {
writeUleb128(os, 1, "num local decls");
@ -888,134 +956,84 @@ void Writer::createInitMemoryFunction() {
writeUleb128(os, 0, "num locals");
}
if (hasPassiveInitializedSegments()) {
// Initialize memory in a thread-safe manner. The thread that successfully
// increments the flag from 0 to 1 is is responsible for performing the
// memory initialization. Other threads go sleep on the flag until the
// first thread finishing initializing memory, increments the flag to 2,
// and wakes all the other threads. Once the flag has been set to 2,
// subsequently started threads will skip the sleep. All threads
// unconditionally drop their passive data segments once memory has been
// initialized. The generated code is as follows:
//
// (func $__wasm_init_memory
// (if
// (i32.atomic.rmw.cmpxchg align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 0)
// (i32.const 1)
// )
// (then
// (drop
// (i32.atomic.wait align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 1)
// (i32.const -1)
// )
// )
// )
// (else
// ( ... initialize data segments ... )
// (i32.atomic.store align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const 2)
// )
// (drop
// (i32.atomic.notify align=2 offset=0
// (i32.const $__init_memory_flag)
// (i32.const -1u)
// )
// )
// )
// )
// ( ... drop data segments ... )
// )
//
// When we are building with PIC, calculate the flag location using:
//
// (global.get $__memory_base)
// (i32.const $__init_memory_flag)
// (i32.const 1)
auto writeGetFlagAddress = [&]() {
if (config->isPic) {
writeU8(os, WASM_OPCODE_LOCAL_GET, "local.get");
writeUleb128(os, 0, "local 0");
} else {
writePtrConst(os, flagAddress, is64, "flag address");
}
};
// Atomically check whether this is the main thread.
writeGetFlagAddress();
writeI32Const(os, 0, "expected flag value");
writeI32Const(os, 1, "flag value");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_RMW_CMPXCHG, "i32.atomic.rmw.cmpxchg");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_IF, "IF");
writeU8(os, WASM_TYPE_NORESULT, "blocktype");
// Did not increment 0, so wait for main thread to initialize memory
writeGetFlagAddress();
writeI32Const(os, 1, "expected flag value");
writeI64Const(os, -1, "timeout");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_ATOMIC_WAIT, "i32.atomic.wait");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_DROP, "drop");
writeU8(os, WASM_OPCODE_ELSE, "ELSE");
// Did increment 0, so conditionally initialize passive data segments
for (const OutputSegment *s : segments) {
if (needsPassiveInitialization(s)) {
// destination address
writePtrConst(os, s->startVA, is64, "destination address");
if (config->isPic) {
writeU8(os, WASM_OPCODE_GLOBAL_GET, "GLOBAL_GET");
writeUleb128(os, WasmSym::memoryBase->getGlobalIndex(),
"memory_base");
writeU8(os, WASM_OPCODE_I32_ADD, "i32.add");
}
// source segment offset
writeI32Const(os, 0, "segment offset");
// memory region size
writeI32Const(os, s->size, "memory region size");
// memory.init instruction
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_MEMORY_INIT, "memory.init");
writeUleb128(os, s->index, "segment index immediate");
writeU8(os, 0, "memory index immediate");
}
auto writeGetFlagAddress = [&]() {
if (config->isPic) {
writeU8(os, WASM_OPCODE_LOCAL_GET, "local.get");
writeUleb128(os, 0, "local 0");
} else {
writePtrConst(os, flagAddress, is64, "flag address");
}
};
// Set flag to 2 to mark end of initialization
writeGetFlagAddress();
writeI32Const(os, 2, "flag value");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_ATOMIC_STORE, "i32.atomic.store");
writeMemArg(os, 2, 0);
// Atomically check whether this is the main thread.
writeGetFlagAddress();
writeI32Const(os, 0, "expected flag value");
writeI32Const(os, 1, "flag value");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_RMW_CMPXCHG, "i32.atomic.rmw.cmpxchg");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_IF, "IF");
writeU8(os, WASM_TYPE_NORESULT, "blocktype");
// Notify any waiters that memory initialization is complete
writeGetFlagAddress();
writeI32Const(os, -1, "number of waiters");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_ATOMIC_NOTIFY, "atomic.notify");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_DROP, "drop");
// Did not increment 0, so wait for main thread to initialize memory
writeGetFlagAddress();
writeI32Const(os, 1, "expected flag value");
writeI64Const(os, -1, "timeout");
writeU8(os, WASM_OPCODE_END, "END");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_ATOMIC_WAIT, "i32.atomic.wait");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_DROP, "drop");
// Unconditionally drop passive data segments
for (const OutputSegment *s : segments) {
if (needsPassiveInitialization(s)) {
// data.drop instruction
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_DATA_DROP, "data.drop");
writeUleb128(os, s->index, "segment index immediate");
writeU8(os, WASM_OPCODE_ELSE, "ELSE");
// Did increment 0, so conditionally initialize passive data segments
for (const OutputSegment *s : segments) {
if (needsPassiveInitialization(s)) {
// destination address
writePtrConst(os, s->startVA, is64, "destination address");
if (config->isPic) {
writeU8(os, WASM_OPCODE_GLOBAL_GET, "GLOBAL_GET");
writeUleb128(os, WasmSym::memoryBase->getGlobalIndex(),
"memory_base");
writeU8(os, WASM_OPCODE_I32_ADD, "i32.add");
}
// source segment offset
writeI32Const(os, 0, "segment offset");
// memory region size
writeI32Const(os, s->size, "memory region size");
// memory.init instruction
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_MEMORY_INIT, "memory.init");
writeUleb128(os, s->index, "segment index immediate");
writeU8(os, 0, "memory index immediate");
}
}
// Set flag to 2 to mark end of initialization
writeGetFlagAddress();
writeI32Const(os, 2, "flag value");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_I32_ATOMIC_STORE, "i32.atomic.store");
writeMemArg(os, 2, 0);
// Notify any waiters that memory initialization is complete
writeGetFlagAddress();
writeI32Const(os, -1, "number of waiters");
writeU8(os, WASM_OPCODE_ATOMICS_PREFIX, "atomics prefix");
writeUleb128(os, WASM_OPCODE_ATOMIC_NOTIFY, "atomic.notify");
writeMemArg(os, 2, 0);
writeU8(os, WASM_OPCODE_DROP, "drop");
writeU8(os, WASM_OPCODE_END, "END");
// Unconditionally drop passive data segments
for (const OutputSegment *s : segments) {
if (needsPassiveInitialization(s)) {
// data.drop instruction
writeU8(os, WASM_OPCODE_MISC_PREFIX, "bulk-memory prefix");
writeUleb128(os, WASM_OPCODE_DATA_DROP, "data.drop");
writeUleb128(os, s->index, "segment index immediate");
}
}
writeU8(os, WASM_OPCODE_END, "END");
@ -1206,7 +1224,7 @@ void Writer::createSyntheticSections() {
out.eventSec = make<EventSection>();
out.globalSec = make<GlobalSection>();
out.exportSec = make<ExportSection>();
out.startSec = make<StartSection>(hasPassiveInitializedSegments());
out.startSec = make<StartSection>();
out.elemSec = make<ElemSection>();
out.dataCountSec = make<DataCountSection>(segments);
out.linkingSec = make<LinkingSection>(initFunctions, segments);
@ -1235,6 +1253,8 @@ void Writer::run() {
populateProducers();
log("-- calculateImports");
calculateImports();
log("-- createSyntheticInitFunctions");
createSyntheticInitFunctions();
log("-- layoutMemory");
layoutMemory();