svn commit: r234014 - head/lib/libc/arm/gen

[Andrew Turner]

On Sun, 8 Apr 2012 18:36:45 -0700
Juli Mallett <jmallett at FreeBSD.org> wrote:

> On Sat, Apr 7, 2012 at 21:45, Andrew Turner <andrew at fubar.geek.nz>
> wrote:
> > On Sun, 8 Apr 2012 04:36:27 +0000 (UTC)
> > Andrew Turner <andrew at FreeBSD.org> wrote:
> >
> >> Author: andrew
> >> Date: Sun Apr  8 04:36:27 2012
> >> New Revision: 234014
> >> URL: http://svn.freebsd.org/changeset/base/234014
> >>
> >> Log:
> >>   Unlike other functions __aeabi_read_tp function must preserve
> >> r1-r3. The currently generated code clobbers r3. Fix this by
> >> loading ARM_TP_ADDRESS using inline assembly.
> >>
> >>   Approved by:        imp (mentor)
> >
> > This fixes thread local storage on ARM in cases when the compiler
> > loads the offset of the variable in r3 before calling
> > __aeabi_read_tp as has been observed when the variable is in a
> > shared library.
> 
> I don't believe this is safe unless you specify __attribute__
> ((__naked__)) in the declaration — currently the compiler is free to
> clobber registers that the calling convention/ABI allows it to
> clobber.  I don't see the benefit of implementing this in C over using
> an assembly file, that said, although GCC does have this attribute
> precisely to allow one to write assembly in a C source file without
> having to worry about unexpected behavior.

We can implement it as a naked function but we will need to store all
registers other than r0 and pc which seems a waste.

The problem implementing it in an assembly file is we are unable to use
ARM_TP_ADDRESS is defined as:
#define ARM_TP_ADDRESS (ARM_VECTORS_HIGH + 0x1000)

Where ARM_VECTORS_HIGH is defined as:
#define ARM_VECTORS_HIGH 0xffff0000U

Binutils fails because of the U in ARM_VECTORS_HIGH eith the following:
Error: missing ')'
Error: junk at end of line, first unrecognized character is `U'

I could remove the U from ARM_VECTORS_HIGH however I'm not sure on the
implications of that change.


> How do you know GCC won't allocate r2 to be used as a temporary in
> between the assembly and the return?

The compiler is free to use r2 with the current code. I have attached a
patch that fools the compiler into thinking we are using r1-r3 by
placing them in the clobber list.

Andrew
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