Emit memory attribute for readonly non-pure inline asm

[nikic]

I came across this comment:

rust/compiler/rustc_codegen_llvm/src/asm.rs

Line 344 in 2f4dfc7

// LLVM doesn't have an attribute to represent ReadOnly + SideEffect

This does exist nowadays in the form of memory(read, inaccessiblemem: readwrite).

[folkertdev]

I believe that particular combination can't actually be created with the public API yet. At least, it doesn't occur in the LLVM test suite (neither does memory(read, argmem: none, inaccessiblemem: readwrite). Playing around with the other public functions in https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/Support/ModRef.h doesn't appear to give the right combination.

[nikic]

Not sure what you mean by public API here. We do need an extra case in

rust/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

Lines 552 to 556 in e10aa88

	enum class LLVMRustMemoryEffects {
	None,
	ReadOnly,
	InaccessibleMemOnly,
	};

that produces MemoryEffects::readOnly() | MemoryEffects::inaccessibleMemOnly().