[cfe-dev] [LLVMdev] Named register variables GNU-style

[Richard Smith]

On Thu, Mar 27, 2014 at 9:30 AM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> > That's my idea, yes. I'm not sure how Clang would transform the named
> > registers into the intrinsic, but something along the lines of:
> >
> > i8* @SP = "SP";
> >
> > define void @step() nounwind {
> > entry:
> >   %0 = call i32 @llvm.read_register(i8* @SP)
> >   %1 = add i32 %0, i32 4
> >   call void @llvm.write_register(i8* @SP, %1)
> > }
> >
> > declare void @llvm.write_register(i8*, i32) nounwind readnone
> > declare i32 @llvm.read_register(i8*) nounwind readnone
>
> I would not produce any llvm global for it. So some insanity like
>
> register long a asm("rsp");
> long f(long x) {
>   long ret = a;
>   a = x;
>   return ret;
> }
>
> would compile to
>
> define i64 @f(i64 %x) {
>   %ret = call i64 @llvm.read_register("rsp");
>   call void @llvm.write_register("rsp", i64 %x)
>   ret %ret
> }
> declare void @llvm.write_register(i8*, i64)
> declare i64 @llvm.read_register(i8*)


I don't think that works. Per the GCC documentation, a global register
variable reserves the register entirely for use with that name in a
translation unit. We don't seem to want exactly that model, but the
approach you're suggesting doesn't seem to capture the semantics. For
instance:

register long a asm("r12");
void f(long n) {
  n *= 3;
  a += n;
}

... could do the wrong thing if the multiplication happens to use r12.

>> This is not exactly the semantics gcc uses since the register would
> >> still be allocatable, but should cover 99% of the uses, including
> >> reading the stack pointer in the kernel.
> >
> > http://gcc.gnu.org/onlinedocs/gcc/Global-Reg-Vars.html
> >
> > It seems that the semantics is to avoid PCS registers, or they will be
> > clobbered...
>
> Yes, it is really odd. It says "Global register variables reserve
> registers throughout the program.", which is obviously not the case
> since not all compile units might see it.
>
> >
> >> For example, is it legal to move the read of rsp out of  a
> >> loop?
> >
> > No. It should be a volatile read/write.
>
> Agreed. With the intrinsic the semantics are easy to represent.
>
> Cheers,
> Rafael
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