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[lld][WebAssembly] Perform data relocations during start function

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We already perform memory initialization and apply global relocations
during start.  It makes sense to performs data relocations too.  I think
the reason we were not doing this already is solely historical.

Differential Revision: https://reviews.llvm.org/D117412
This commit is contained in:
Sam Clegg
2022-01-15 15:33:02 -08:00
parent 64de0064f3
commit ec47dba1c8
3 changed files with 60 additions and 16 deletions
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49
lld/wasm/Writer.cpp
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@@ -1022,7 +1022,16 @@ void Writer::createSyntheticInitFunctions() {
}
}
if (WasmSym::applyGlobalRelocs && WasmSym::initMemory) {
int startCount = 0;
if (WasmSym::applyGlobalRelocs)
startCount++;
if (WasmSym::WasmSym::initMemory || WasmSym::applyDataRelocs)
startCount++;
// If there is only one start function we can just use that function
// itself as the Wasm start function, otherwise we need to synthesize
// a new function to call them in sequence.
if (startCount > 1) {
WasmSym::startFunction = symtab->addSyntheticFunction(
"__wasm_start", WASM_SYMBOL_VISIBILITY_HIDDEN,
make<SyntheticFunction>(nullSignature, "__wasm_start"));
@@ -1179,6 +1188,14 @@ void Writer::createInitMemoryFunction() {
}
}
// Memory init is now complete. Apply data relocation if there
// are any.
if (WasmSym::applyDataRelocs) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyDataRelocs->getFunctionIndex(),
"function index");
}
if (config->sharedMemory) {
// Set flag to 2 to mark end of initialization
writeGetFlagAddress();
@@ -1231,17 +1248,28 @@ void Writer::createInitMemoryFunction() {
}
void Writer::createStartFunction() {
// If the start function exists when we have more than one function to call.
if (WasmSym::startFunction) {
std::string bodyContent;
{
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::initMemory->getFunctionIndex(),
"function index");
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyGlobalRelocs->getFunctionIndex(),
"function index");
if (WasmSym::initMemory) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::initMemory->getFunctionIndex(),
"function index");
} else if (WasmSym::applyDataRelocs) {
// When initMemory is present it calls applyDataRelocs. If not,
// we must call it directly.
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyDataRelocs->getFunctionIndex(),
"function index");
}
if (WasmSym::applyGlobalRelocs) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyGlobalRelocs->getFunctionIndex(),
"function index");
}
writeU8(os, WASM_OPCODE_END, "END");
}
createFunction(WasmSym::startFunction, bodyContent);
@@ -1249,6 +1277,8 @@ void Writer::createStartFunction() {
WasmSym::startFunction = WasmSym::initMemory;
} else if (WasmSym::applyGlobalRelocs) {
WasmSym::startFunction = WasmSym::applyGlobalRelocs;
} else if (WasmSym::applyDataRelocs) {
WasmSym::startFunction = WasmSym::applyDataRelocs;
}
}
@@ -1311,8 +1341,7 @@ void Writer::createCallCtorsFunction() {
// If __wasm_call_ctors isn't referenced, there aren't any ctors, and we
// aren't calling `__wasm_apply_data_relocs` for Emscripten-style PIC, don't
// define the `__wasm_call_ctors` function.
if (!WasmSym::callCtors->isLive() && !WasmSym::applyDataRelocs &&
initFunctions.empty())
if (!WasmSym::callCtors->isLive() && initFunctions.empty())
return;
// First write the body's contents to a string.
@@ -1321,7 +1350,7 @@ void Writer::createCallCtorsFunction() {
raw_string_ostream os(bodyContent);
writeUleb128(os, 0, "num locals");
if (WasmSym::applyDataRelocs) {
if (WasmSym::applyDataRelocs && !WasmSym::initMemory) {
writeU8(os, WASM_OPCODE_CALL, "CALL");
writeUleb128(os, WasmSym::applyDataRelocs->getFunctionIndex(),
"function index");