[PATCH] D56305: [AArch64] Support reserving arbitrary general purpose registers

Eli Friedman via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Tue Feb 12 14:36:25 PST 2019

efriedma 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");
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.

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