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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>
#include <wasm-builder.h>
#include <ir/localize.h>
#include <asmjs/shared-constants.h>
namespace wasm {
struct PostEmscripten : public WalkerPass<PostWalker<PostEmscripten>> {
bool isFunctionParallel() override { return true; }
Pass* create() override { return new 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.
#define SAFE_MAX 1024
void optimizeMemoryAccess(Expression*& ptr, Address& offset) {
while (1) {
auto* add = ptr->dynCast<Binary>();
if (!add) break;
if (add->op != AddInt32) break;
auto* left = add->left->dynCast<Const>();
auto* right = add->right->dynCast<Const>();
// note: in optimized code, we shouldn't see an add of two constants, so don't worry about that much
// (precompute would optimize that)
if (left) {
auto value = left->value.geti32();
if (value >= 0 && value < SAFE_MAX) {
offset = offset + value;
ptr = add->right;
continue;
}
}
if (right) {
auto value = right->value.geti32();
if (value >= 0 && value < SAFE_MAX) {
offset = offset + value;
ptr = add->left;
continue;
}
}
break;
}
// finally ptr may be a const, but it isn't worth folding that in (we still have a const); in fact,
// it's better to do the opposite for gzip purposes as well as for readability.
auto* last = ptr->dynCast<Const>();
if (last) {
last->value = Literal(int32_t(last->value.geti32() + offset));
offset = 0;
}
}
void visitLoad(Load* curr) {
optimizeMemoryAccess(curr->ptr, curr->offset);
}
void visitStore(Store* curr) {
optimizeMemoryAccess(curr->ptr, curr->offset);
}
void visitCallImport(CallImport* curr) {
// special asm.js imports can be optimized
auto* import = getModule()->getImport(curr->target);
if (import->module == GLOBAL_MATH) {
if (import->base == POW) {
if (auto* exponent = curr->operands[1]->dynCast<Const>()) {
if (exponent->value == Literal(double(2.0))) {
// This is just a square operation, do a multiply
Localizer localizer(curr->operands[0], getFunction(), getModule());
Builder builder(*getModule());
replaceCurrent(builder.makeBinary(MulFloat64, localizer.expr, builder.makeGetLocal(localizer.index, localizer.expr->type)));
} else if (exponent->value == Literal(double(0.5))) {
// This is just a square root operation
replaceCurrent(Builder(*getModule()).makeUnary(SqrtFloat64, curr->operands[0]));
}
}
}
}
}
};
Pass *createPostEmscriptenPass() {
return new PostEmscripten();
}
} // namespace wasm
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