[cfe-dev] reserved registers on inline asm clobber list silently ignored

[Reid Kleckner via cfe-dev]

On Thu, Jul 5, 2018 at 11:28 AM Friedman, Eli via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> On 7/5/2018 7:01 AM, Ties Stuij via cfe-dev wrote:
> > Hi all,
> >
> > I'm looking at an issue where when specifying a reserved register (fp,
> base pointer) on the clobber list of an asm() statement, we don't preserve
> the original value across the asm code. Practically the register in the
> clobber list is ignored in this case, so if we're overwriting it in our asm
> code, we have problems.
> >
> > As the clobber list is a GCC extension, I looked at the GCC behaviour,
> and GCC is doing roughly the same thing. There is a GCC bug out on this (
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=11807), and the opinions are
> somewhat divided on how to proceed.
> >
> > There's a few ways in which we think we'd like to resolve this:
> > 1 - leave as is, and be in line with GCC
> > 2 - error when we see the SP and PC on the clobber list, otherwise store
> and load around the assembly code
> > 3 - error on any reserved register on the clobber list
> >
> > Where option 1 doesn't sound like a very nice option to me as it's
> counter-intuitive, and seems quite shoot-in-the-foot-y.
> >
> > Any opinions on this?
>

Would it be good enough if we changed LLVM to eliminate the need for the
third reserved register, the so-called "base pointer" that Eli mentioned?
This is usually what trips people up on X86 (RBX vs. cpuid:
https://llvm.org/pr16830). Most users know they aren't supposed to muck
with SP or FP registers. You can imagine making the base pointer into a
virtual register that could be spilled, reloaded, or rematerialized as
needed.


> Regardless of how much of a footgun this is, we'll get a ton of
> complaints if we print an error in any case where gcc doesn't, so any
> new diagnostics would need to be a warning, at least at first.
>
> "otherwise store and load around the assembly code" makes sense for
> cases where the compiler is arbitrarily reserving registers, like
> fp/base ptr in a function that contains a VLA.  In particular, the
> "base" register isn't really specified anywhere, so we can't expect
> users to consistently avoid it.
>
> Printing a diagnostic for code that tries to clobber "sp" or "pc" makes
> sense; that can't do anything useful in any context.  Probably also
> makes sense to print a diagnostic for other fundamentally reserved
> registers, like the frame pointer with -fno-omit-frame-pointer, or x18
> on AArch64 targets where it's reserved, rather than try to do something
> which might have surprising results.
>

At some point when we were working on MS inline asm, we decided that inline
asm clobbering SP should be treated as a dynamic alloca, i.e. a frame
pointer should be used to address local variables.
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