phosek added inline comments.
================
Comment at: clang/lib/Driver/ToolChains/Arch/AArch64.cpp:318
+ if (Args.hasArg(options::OPT_ffixed_x0))
+ Features.push_back("+reserve-x0");
----------------
efriedma wrote:
> trong wrote:
> > trong wrote:
> > > phosek wrote:
> > > > trong wrote:
> > > > > What happens (should happen) if we reserve x0 and compile a function
> > > > > with a return value?
> > > > Clang throws `error: AArch64 doesn't support function calls if any of
> > > > the argument registers is reserved.` as for any other argument register.
> > > Yes, if x0 is reserved, it can't be used to pass arguments. But what
> > > happens if x0 is used to return a value? For example:
> > > ```
> > > int foo() {
> > > return 1;
> > > }
> > > ```
> > > It would be helpful if compiling `foo()` with x0 reserved threw an error.
> > OTOH, [[ https://godbolt.org/z/jWW09Y | gcc ]] doesn't complain, so maybe
> > we're OK here.
> IIRC gcc never reports any errors for its version of these flags; that
> doesn't mean we should accept constructs which don't have defined behavior.
> I'd prefer to print an error in any situation where we would implicitly use a
> reserved register.
I've looked into it, but there are several places where LLVM assumes and uses
x0, so I think it's safer for now to disallow reserving x0 just like we do with
x19.
Repository:
rC Clang
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D56305/new/
https://reviews.llvm.org/D56305
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