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

[Friedman, Eli via cfe-dev]

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?

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.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project