void optimizeJmps(Vunit& unit) {
auto isEmpty = [&](Vlabel b, Vinstr::Opcode op) {
auto& code = unit.blocks[b].code;
return code.size() == 1 && op == code[0].op;
};
bool changed = false;
bool ever_changed = false;
jit::vector<int> npreds(unit.blocks.size(), 0);
do {
if (changed) {
fill(npreds.begin(), npreds.end(), 0);
}
changed = false;
PostorderWalker{unit}.dfs([&](Vlabel b) {
for (auto s : succs(unit.blocks[b])) {
npreds[s]++;
}
});
// give roots an extra predecessor to prevent cloning them.
for (auto b : unit.roots) {
npreds[b]++;
}
PostorderWalker{unit}.dfs([&](Vlabel b) {
auto& block = unit.blocks[b];
auto& code = block.code;
assert(!code.empty());
if (code.back().op == Vinstr::jcc) {
auto ss = succs(block);
if (ss[0] == ss[1]) {
// both edges have same target, change to jmp
code.back() = jmp{ss[0]};
changed = true;
}
}
if (code.back().op == Vinstr::jmp) {
auto& s = code.back().jmp_.target;
if (isEmpty(s, Vinstr::jmp)) {
// skip over s
s = unit.blocks[s].code.back().jmp_.target;
changed = true;
} else if (npreds[s] == 1 || isEmpty(s, Vinstr::jcc)) {
// overwrite jmp with copy of s
auto& code2 = unit.blocks[s].code;
code.pop_back();
code.insert(code.end(), code2.begin(), code2.end());
changed = true;
}
} else {
for (auto& s : succs(block)) {
if (isEmpty(s, Vinstr::jmp)) {
// skip over s
s = unit.blocks[s].code.back().jmp_.target;
changed = true;
}
}
}
});
ever_changed |= changed;
} while (changed);
if (ever_changed) {
printUnit(kVasmJumpsLevel, "after vasm-jumps", unit);
}
}
void optimizeJmps(Vunit& unit) {
auto isEmpty = [&](Vlabel b, Vinstr::Opcode op) {
auto& code = unit.blocks[b].code;
return code.size() == 1 && op == code[0].op;
};
bool changed = false;
bool ever_changed = false;
// The number of incoming edges from (reachable) predecessors for each block.
// It is maintained as an upper bound of the actual value during the
// transformation.
jit::vector<int> npreds(unit.blocks.size(), 0);
do {
if (changed) {
std::fill(begin(npreds), end(npreds), 0);
}
changed = false;
PostorderWalker{unit} .dfs([&](Vlabel b) {
for (auto s : succs(unit.blocks[b])) {
npreds[s]++;
}
});
// give entry an extra predecessor to prevent cloning it.
npreds[unit.entry]++;
PostorderWalker{unit} .dfs([&](Vlabel b) {
auto& block = unit.blocks[b];
auto& code = block.code;
assertx(!code.empty());
if (code.back().op == Vinstr::jcc) {
auto ss = succs(block);
if (ss[0] == ss[1]) {
// both edges have same target, change to jmp
code.back() = jmp{ss[0]};
--npreds[ss[0]];
changed = true;
} else {
auto jcc_i = code.back().jcc_;
if (isEmpty(jcc_i.targets[0], Vinstr::fallback)) {
jcc_i = jcc{ccNegate(jcc_i.cc), jcc_i.sf,
{jcc_i.targets[1], jcc_i.targets[0]}};
}
if (isEmpty(jcc_i.targets[1], Vinstr::fallback)) {
// replace jcc with fallbackcc and jmp
const auto& fb_i = unit.blocks[jcc_i.targets[1]].code[0].fallback_;
const auto t0 = jcc_i.targets[0];
const auto jcc_origin = code.back().origin;
code.pop_back();
code.emplace_back(
fallbackcc{jcc_i.cc, jcc_i.sf, fb_i.dest, fb_i.trflags});
code.back().origin = jcc_origin;
code.emplace_back(jmp{t0});
code.back().origin = jcc_origin;
changed = true;
}
}
}
for (auto& s : succs(block)) {
if (isEmpty(s, Vinstr::jmp)) {
// skip over s
--npreds[s];
s = unit.blocks[s].code.back().jmp_.target;
++npreds[s];
changed = true;
}
}
if (code.back().op == Vinstr::jmp) {
auto s = code.back().jmp_.target;
if (npreds[s] == 1 || isEmpty(s, Vinstr::jcc)) {
// overwrite jmp with copy of s
auto& code2 = unit.blocks[s].code;
code.pop_back();
code.insert(code.end(), code2.begin(), code2.end());
if (--npreds[s]) {
for (auto ss : succs(block)) {
++npreds[ss];
}
}
changed = true;
}
}
});
ever_changed |= changed;
} while (changed);
if (ever_changed) {
printUnit(kVasmJumpsLevel, "after vasm-jumps", unit);
}
}