================
@@ -1144,6 +1322,27 @@ RValue CodeGenFunction::EmitAtomicExpr(AtomicExpr *E) {
LValue AtomicVal = MakeAddrLValue(Ptr, AtomicTy);
AtomicInfo Atomics(*this, AtomicVal);
+ // A `_BitInt(N)` read-modify-write whose value width has padding bits, or
+ // whose size forces a libcall, cannot use a single atomicrmw: the op would
+ // carry into / compare the padding bits, and no arbitrary-width
+ // __atomic_fetch_* libcall exists. Emit a compare-exchange loop instead.
+ // Bitwise and/or/xor are exact even with padding, so only the wide case
needs
+ // the loop for them. load/store/exchange/compare_exchange keep their paths.
+ if (MemTy->isBitIntType()) {
+ llvm::AtomicRMWInst::BinOp BinOp;
+ bool RMWReturnsNew;
+ if (classifyBitIntRMW(E->getOp(), MemTy->isSignedIntegerType(), BinOp,
+ RMWReturnsNew)) {
+ bool WideOrNonPow2 = (Size & (Size - 1)) != 0 || Size > 16;
+ bool Bitwise = BinOp == llvm::AtomicRMWInst::And ||
+ BinOp == llvm::AtomicRMWInst::Or ||
+ BinOp == llvm::AtomicRMWInst::Xor;
+ if (WideOrNonPow2 || (hasBitIntPadding(MemTy, getContext()) && !Bitwise))
----------------
huixie90 wrote:
The current libc++ approach of CAS for types with padding is that we maintain
the invariant that at all times atomic will have zeros in their paddings and
all operations will need to clear the incoming paddings first (see
__builtin_clear_padding) https://github.com/llvm/llvm-project/pull/76180/changes
I think the __buitin_clear_padding already handles _BitInt
instead of emitting a loop, could we just leave as it is and let the libc++
code to handle it?
https://github.com/llvm/llvm-project/pull/204815
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