[lld][WebAssembly] Add an --initial-heap option
#75594
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It is beneficial to preallocate a certain number of pages in the linear memory (i. e. use the "minimum" field of WASM memories) so that fewer "memory.grow"s are needed at startup. So far, the way to do that has been to pass the "--initial-memory" option to the linker. It works, but has the very significant downside of requiring the user to know the size of static data beforehand, as it must not exceed the number of bytes passed-in as "--initial-memory". The new "--initial-heap" option avoids this downside by simply appending the specified number of pages to static data (and stack), regardless of how large they already are.
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@llvm/pr-subscribers-lld @llvm/pr-subscribers-lld-wasm Author: None (SingleAccretion) ChangesIt is beneficial to preallocate a certain number of pages in the linear memory (i. e. use the "minimum" field of WASM memories) so that fewer "memory.grow"s are needed at startup. So far, the way to do that has been to pass the "--initial-memory" option to the linker. It works, but has the very significant downside of requiring the user to know the size of static data beforehand, as it must not exceed the number of bytes passed-in as "--initial-memory". The new "--initial-heap" option avoids this downside by simply appending the specified number of pages to static data (and stack), regardless of how large they already are. Ref: emscripten-core/emscripten#20888. Full diff: https://github.com/llvm/llvm-project/pull/75594.diff 6 Files Affected:
diff --git a/lld/docs/WebAssembly.rst b/lld/docs/WebAssembly.rst
index dad3177e2c7dff..3f554de46d38a7 100644
--- a/lld/docs/WebAssembly.rst
+++ b/lld/docs/WebAssembly.rst
@@ -123,9 +123,13 @@ WebAssembly-specific options:
is not possible for undefined data symbols. Undefined data symbols will
still be reported as normal (in accordance with ``--unresolved-symbols``).
+.. option:: --initial-heap=<value>
+
+ Initial size of the heap. Default: zero.
+
.. option:: --initial-memory=<value>
- Initial size of the linear memory. Default: static data size.
+ Initial size of the linear memory. Default: the sum of stack, static data and heap sizes.
.. option:: --max-memory=<value>
diff --git a/lld/test/wasm/initial-heap.test b/lld/test/wasm/initial-heap.test
new file mode 100644
index 00000000000000..3e8bbd36535d30
--- /dev/null
+++ b/lld/test/wasm/initial-heap.test
@@ -0,0 +1,27 @@
+RUN: llvm-mc -filetype=obj -triple=wasm32-unknown-unknown %p/Inputs/start.s -o %t.o
+
+; The initial heap size will be added to the stack size
+RUN: wasm-ld %t.o -o %t1.wasm --stack-first -z stack-size=65536 --initial-heap=131072
+RUN: obj2yaml %t1.wasm | FileCheck %s --check-prefixes=CHECK,CHECK-2P
+
+; Also test that we can parse and process a large size correctly
+RUN: wasm-ld %t.o -o %t2.wasm --stack-first -z stack-size=65536 --initial-heap=4294901760
+RUN: obj2yaml %t2.wasm | FileCheck %s --check-prefixes=CHECK,CHECK-4G
+
+CHECK: - Type: MEMORY
+CHECK-NEXT: Memories:
+CHECK-2P-NEXT: Minimum: 0x3
+CHECK-4G-NEXT: Minimum: 0x10000
+
+; Test various error cases.
+RUN: not wasm-ld %t.o -o %t3.wasm --initial-heap=131073 2>&1 | FileCheck %s --check-prefix NOT-PAGE-MULTIPLE
+RUN: not wasm-ld %t.o -o %t4.wasm --stack-first -z stack-size=65536 --initial-heap=4295032832 2>&1 | FileCheck %s --check-prefix TOO-LARGE-BY-ITSELF
+RUN: not wasm-ld %t.o -o %t5.wasm --stack-first -z stack-size=131072 --initial-heap=4294901760 2>&1 | FileCheck %s --check-prefix TOO-LARGE-WITH-STACK
+RUN: not wasm-ld %t.o -o %t6.wasm --stack-first -z stack-size=65536 --initial-heap=131072 --initial-memory=131072 2>&1 | FileCheck %s --check-prefix INITIAL-MEMORY-TOO-SMALL
+RUN: not wasm-ld %t.o -o %t7.wasm --stack-first -z stack-size=65536 --initial-heap=131072 --max-memory=131072 2>&1 | FileCheck %s --check-prefix MAX-MEMORY-TOO-SMALL
+
+NOT-PAGE-MULTIPLE: initial heap must be 65536-byte aligned
+TOO-LARGE-BY-ITSELF: initial heap too large, cannot be greater than 4294901760
+TOO-LARGE-WITH-STACK: initial heap too large, cannot be greater than 4294836224
+INITIAL-MEMORY-TOO-SMALL: initial memory too small, 196608 bytes needed
+MAX-MEMORY-TOO-SMALL: maximum memory too small, 196608 bytes needed
diff --git a/lld/wasm/Config.h b/lld/wasm/Config.h
index d76d43852acffe..104d4704840149 100644
--- a/lld/wasm/Config.h
+++ b/lld/wasm/Config.h
@@ -68,6 +68,7 @@ struct Configuration {
bool isStatic = false;
bool trace;
uint64_t globalBase;
+ uint64_t initialHeap;
uint64_t initialMemory;
uint64_t maxMemory;
uint64_t zStackSize;
diff --git a/lld/wasm/Driver.cpp b/lld/wasm/Driver.cpp
index c68fe33a14e29d..a354260c605258 100644
--- a/lld/wasm/Driver.cpp
+++ b/lld/wasm/Driver.cpp
@@ -502,9 +502,10 @@ static void readConfigs(opt::InputArgList &args) {
errorHandler().verbose = args.hasArg(OPT_verbose);
LLVM_DEBUG(errorHandler().verbose = true);
- config->initialMemory = args::getInteger(args, OPT_initial_memory, 0);
- config->globalBase = args::getInteger(args, OPT_global_base, 0);
config->tableBase = args::getInteger(args, OPT_table_base, 0);
+ config->globalBase = args::getInteger(args, OPT_global_base, 0);
+ config->initialHeap = args::getInteger(args, OPT_initial_heap, 0);
+ config->initialMemory = args::getInteger(args, OPT_initial_memory, 0);
config->maxMemory = args::getInteger(args, OPT_max_memory, 0);
config->zStackSize =
args::getZOptionValue(args, OPT_z, "stack-size", WasmPageSize);
diff --git a/lld/wasm/Options.td b/lld/wasm/Options.td
index 2df6196d5e8ce6..95ebc202a45187 100644
--- a/lld/wasm/Options.td
+++ b/lld/wasm/Options.td
@@ -215,6 +215,9 @@ defm soname: Eq<"soname", "Set the module name in the generated name section">;
def import_table: FF<"import-table">,
HelpText<"Import function table from the environment">;
+def initial_heap: JJ<"initial-heap=">,
+ HelpText<"Initial size of the heap">;
+
def initial_memory: JJ<"initial-memory=">,
HelpText<"Initial size of the linear memory">;
diff --git a/lld/wasm/Writer.cpp b/lld/wasm/Writer.cpp
index 0576bf2907e49c..805018c58dccb4 100644
--- a/lld/wasm/Writer.cpp
+++ b/lld/wasm/Writer.cpp
@@ -439,6 +439,16 @@ void Writer::layoutMemory() {
maxMemorySetting = 1ULL << 34;
}
+ if (config->initialHeap != 0) {
+ if (config->initialHeap != alignTo(config->initialHeap, WasmPageSize))
+ error("initial heap must be " + Twine(WasmPageSize) + "-byte aligned");
+ uint64_t maxInitialHeap = maxMemorySetting - memoryPtr;
+ if (config->initialHeap > maxInitialHeap)
+ error("initial heap too large, cannot be greater than " +
+ Twine(maxInitialHeap));
+ memoryPtr += config->initialHeap;
+ }
+
if (config->initialMemory != 0) {
if (config->initialMemory != alignTo(config->initialMemory, WasmPageSize))
error("initial memory must be " + Twine(WasmPageSize) + "-byte aligned");
|
sbc100
approved these changes
Dec 15, 2023
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It is beneficial to preallocate a certain number of pages in the linear memory (i. e. use the "minimum" field of WASM memories) so that fewer "memory.grow"s are needed at startup.
So far, the way to do that has been to pass the "--initial-memory" option to the linker. It works, but has the very significant downside of requiring the user to know the size of static data beforehand, as it must not exceed the number of bytes passed-in as "--initial-memory".
The new "--initial-heap" option avoids this downside by simply appending the specified number of pages to static data (and stack), regardless of how large they already are.
Ref: emscripten-core/emscripten#20888.