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SDAG: Avoid a large, usually empty SmallVector in a recursive function 

This SmallVector is using up 128 bytes on the stack every time despite
almost always being empty[1], and since this function can recurse quite
deeply that adds up to a lot of overhead. We've seen this run afoul of
ulimits in some cases with ASAN on.

Replacing the SmallVector with a std::vector trades an occasional heap
allocation for vastly less stack usage.

[1]: I gathered some stats on an internal test suite and the vector
was non-empty in only 45,000 of 10,000,000 calls to this function.

llvm-svn: 288441
This commit is contained in:
Justin Bogner 2016-12-02 00:11:01 +00:00
parent 8986361fa1
commit 35c5e58f8c
1 changed files with 2 additions and 2 deletions
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4
llvm/lib/CodeGen/SelectionDAG/LegalizeTypes.cpp
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@ -484,7 +484,7 @@ SDNode *DAGTypeLegalizer::AnalyzeNewNode(SDNode *N) {
// updated after all operands have been analyzed. Since this is rare,
// the code tries to minimize overhead in the non-morphing case.
SmallVector<SDValue, 8> NewOps;
std::vector<SDValue> NewOps;
unsigned NumProcessed = 0;
for (unsigned i = 0, e = N->getNumOperands(); i != e; ++i) {
SDValue OrigOp = N->getOperand(i);
@ -500,7 +500,7 @@ SDNode *DAGTypeLegalizer::AnalyzeNewNode(SDNode *N) {
NewOps.push_back(Op);
} else if (Op != OrigOp) {
// This is the first operand to change - add all operands so far.
NewOps.append(N->op_begin(), N->op_begin() + i);
NewOps.insert(NewOps.end(), N->op_begin(), N->op_begin() + i);
NewOps.push_back(Op);
}
}