[cfe-dev] A question on register allocation

[Goncalo Carvalho via cfe-dev]

Clang doesn't support global register variables like GCC afaik so it needs
to be emulated if really needed by forcing the register to the intended
value.

In general it can be clobbered by whatever else but such is the way without
backend support.

But someone more knowledgeable about Clang's backend may provide a better
insight.





On 4 October 2015 at 12:31, Viswesh Narayanan <visweshn92 at gmail.com> wrote:

> I think you have got my question wrong (Or perhaps my writing was bad).
>
> I want that register (say xmm15) to hold value 100 throughout the
> execution of program.
> And LLVM should lock down xmm15 for me and should always allocate some
> other register for other functions.
>
> The solution provided applies only to context of main() and is not
> generic if my source code grows bigger.
>
>
>
> On Sun, Oct 4, 2015 at 7:24 PM, Goncalo Carvalho <glslang at gmail.com>
> wrote:
> > I don't think clang supports global register variables in general
> although
> > 'sp' appears to work from clang-3.7 onwards. However it doesn't appear to
> > understand 'r13' on arm which is equivalent.
> >
> > You can always do something like
> >
> > asm ("movq %0, %%xmm15"::"x"(100):"xmm15");
> >
> > although it's not strictly equivalent.
> >
> > int main() {
> >
> >     asm ("movq %0, %%xmm15"::"x"(100):"xmm15");
> >
> >     return 0;
> >
> > }
> >
> > will generate
> >
> >        movl    $0, -4(%rbp)
> >
> >         movss   .LCPI0_0(%rip), %xmm0
> >
> >         #APP
> >
> >         movq    %xmm0, %xmm15
> >
> >         #NO_APP
> >
> >         xorl    %eax, %eax
> >
> >         popq    %rbp
> >
> >         retq
> >
> >
> > So you could just have that as a replacement everytime 'A' is written
> but is
> > not the same I appreciate that. The above also lacks context of the
> actual
> > application of what is that you're trying to do so it's possible better
> (and
> > faster) solutions exist!
> >
> >
> > On 4 October 2015 at 07:44, Viswesh Narayanan via cfe-dev
> > <cfe-dev at lists.llvm.org> wrote:
> >>
> >> My use case is to store a global value permanently in a register (say
> >> xmm15) using LLVM
> >>
> >> The following piece of code works as expected in GCC while it is not
> >> supported in LLVM. It complains with "fatal error: error in backend:
> >> Invalid register name global variable".
> >>
> >> Can any expert here provide me a solution to this in Clang/LLVM?
> >>
> >> Code:
> >> #include <stdio.h>
> >> volatile register int A asm ("xmm15");
> >> int main() {
> >>     A = 100;
> >>     return A;
> >> }
> >> _______________________________________________
> >> cfe-dev mailing list
> >> cfe-dev at lists.llvm.org
> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
> >
> >
> >
> >
> >
>



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