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/*
* Copyright 2015 WebAssembly Community Group participants
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
//
// Misc optimizations that are useful for and/or are only valid for
// emscripten output.
//
#include <wasm.h>
#include <pass.h>
namespace wasm {
struct PostEmscripten : public WalkerPass<WasmWalker<PostEmscripten>> {
// When we have a Load from a local value (typically a GetLocal) plus a constant offset,
// we may be able to fold it in.
// The semantics of the Add are to wrap, while wasm offset semantics purposefully do
// not wrap. So this is not always safe to do. For example, a load may depend on
// wrapping via
// (2^32 - 10) + 100 => wrap and load from address 90
// Without wrapping, we get something too large, and an error. *However*, for
// asm2wasm output coming from Emscripten, we allocate the lowest 1024 for mapped
// globals. Mapped globals are simple types (i32, float or double), always
// accessed directly by a single constant. Therefore if we see (..) + K where
// K is less then 1024, then if it wraps, it wraps into [0, 1024) which is at best
// a mapped global, but it can't be because they are accessed directly (at worst,
// it's 0 or an unused section of memory that was reserved for mapped globlas).
// Thus it is ok to optimize such small constants into Load offsets.
void visitLoad(Load *curr) {
if (curr->offset) return;
auto add = curr->ptr->dyn_cast<Binary>();
if (!add || add->op != Add) return;
assert(add->type == i32);
auto c = add->right->dyn_cast<Const>();
if (!c) {
c = add->left->dyn_cast<Const>();
if (c) {
// if one is a const, it's ok to swap
add->left = add->right;
add->right = c;
}
}
if (!c) return;
auto value = c->value.geti32();
if (value >= 0 && value < 1024) {
// foldable, by the above logic
curr->ptr = add->left;
curr->offset = value;
}
}
};
static RegisterPass<PostEmscripten> registerPass("post-emscripten", "miscellaneous optimizations for Emscripten-generated code");
} // namespace wasm
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