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nontemporal_store: make sure that the intrinsic is truly just a hint

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This commit is contained in:
Ralf Jung 2024-07-24 20:55:43 +02:00
parent 29e924841f
commit 28e0907111
5 changed files with 49 additions and 15 deletions
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30
compiler/rustc_codegen_llvm/src/builder.rs
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@ -727,13 +727,29 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
llvm::LLVMSetVolatile(store, llvm::True);
}
if flags.contains(MemFlags::NONTEMPORAL) {
// According to LLVM [1] building a nontemporal store must
// *always* point to a metadata value of the integer 1.
//
// [1]: https://llvm.org/docs/LangRef.html#store-instruction
let one = self.cx.const_i32(1);
let node = llvm::LLVMMDNodeInContext(self.cx.llcx, &one, 1);
llvm::LLVMSetMetadata(store, llvm::MD_nontemporal as c_uint, node);
// Make sure that the current target architectures supports "sane" non-temporal
// stores, i.e., non-temporal stores that are equivalent to regular stores except
// for performance. LLVM doesn't seem to care about this, and will happily treat
// `!nontemporal` stores as-if they were normal stores (for reordering optimizations
// etc) even on x86, despite later lowering them to MOVNT which do *not* behave like
// regular stores but require special fences.
// So we keep a list of architectures where `!nontemporal` is known to be truly just
// a hint, and use regular stores everywhere else.
// (In the future, we could alternatively ensure that an sfence gets emitted after a sequence of movnt
// before any kind of synchronizing operation. But it's not clear how to do that with LLVM.)
const WELL_BEHAVED_NONTEMPORAL_ARCHS: &[&str] = &["aarch64", "arm"];
let use_nontemporal =
WELL_BEHAVED_NONTEMPORAL_ARCHS.contains(&&*self.cx.tcx.sess.target.arch);
if use_nontemporal {
// According to LLVM [1] building a nontemporal store must
// *always* point to a metadata value of the integer 1.
//
// [1]: https://llvm.org/docs/LangRef.html#store-instruction
let one = self.cx.const_i32(1);
let node = llvm::LLVMMDNodeInContext(self.cx.llcx, &one, 1);
llvm::LLVMSetMetadata(store, llvm::MD_nontemporal as c_uint, node);
}
}
store
}