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cxxrtl: make ROMs writable, document memory::operator[].

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There is no practical benefit from using `const memory` for ROMs;
it uses an std::vector internally, which prevents contemporary
compilers from constant-propagating ROM contents. (It is not clear
whether they are permitted to do so.)

However, there is a major benefit from using non-const `memory` for
ROMs, which is the ability to dynamically fill the ROM for each
individual simulation.
This commit is contained in:
whitequark 2020-04-16 16:45:02 +00:00
parent 9043632dcc
commit 06c0338f2c
2 changed files with 6 additions and 4 deletions
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3
backends/cxxrtl/cxxrtl.cc
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@ -1166,8 +1166,7 @@ struct CxxrtlWorker {
});
dump_attrs(memory);
f << indent << (writable_memories[memory] ? "" : "const ")
<< "memory<" << memory->width << "> " << mangle(memory)
f << indent << "memory<" << memory->width << "> " << mangle(memory)
<< " { " << memory->size << "u";
if (init_cells.empty()) {
f << " };\n";

7
backends/cxxrtl/cxxrtl.h
View file

@ -604,12 +604,15 @@ struct memory {
auto _ = {std::move(std::begin(init.data), std::end(init.data), data.begin() + init.offset)...};
}
value<Width> &operator [](size_t index) {
// An operator for direct memory reads. May be used at any time during the simulation.
const value<Width> &operator [](size_t index) const {
assert(index < data.size());
return data[index];
}
const value<Width> &operator [](size_t index) const {
// An operator for direct memory writes. May only be used before the simulation is started. If used
// after the simulation is started, the design may malfunction.
value<Width> &operator [](size_t index) {
assert(index < data.size());
return data[index];
}