On Sat, Jan 9, 2021, at 5:09 AM, Christophe Lyon wrote:
> On Sat, 9 Jan 2021 at 13:27, Daniel Engel <lib...@danielengel.com> wrote:
> >
> > On Thu, Jan 7, 2021, at 4:56 AM, Richard Earnshaw wrote:
> > > On 07/01/2021 00:59, Daniel Engel wrote:
> > > > --snip--
> > > >
> > > > On Wed, Jan 6, 2021, at 9:05 AM, Richard Earnshaw wrote:
> > > >
> > > >> -- snip --
> > > >>
> > > >> - finally, your popcount implementations have data in the code segment.
> > > >> That's going to cause problems when we have compilation options such
> > > >> as
> > > >> -mpure-code.
> > > >
> > > > I am just following the precedent of existing lib1funcs (e.g. __clz2si).
> > > > If this matters, you'll need to point in the right direction for the
> > > > fix. I'm not sure it does matter, since these functions are PIC anyway.
> > >
> > > That might be a bug in the clz implementations - Christophe: Any thoughts?
> >
> > __clzsi2() has test coverage in "gcc.c-torture/execute/builtin-bitops-1.c"
> Thanks, I'll have a closer look at why I didn't see problems.
>
> > The 'clzs' and 'ctz' functions should never have problems. -mpure-code
> > appears to be valid only when the 'movt' instruction is available, which
> > means that the 'clz' instruction will also be available, so no array loads.
> No, -mpure-code is also supported with v6m.
>
> > Is the -mpure-code state detectable as a preprocessor flag? While
> No.
>
> > 'movw'/'movt' appears to be the canonical solution, I'm not sure it
> > should be the default just because a processor supports Thumb-2.
> >
> > Do users wanting to use -mpure-code recompile the toolchain to avoid
> > constant data in compiled C functions? I don't think this is the
> > default for the typical toolchain scripts.
> No, users of -mpure-code do not recompile the toolchain.
I won't claim that my use of inline constants is correct. It was not
hard to find references to high security model processors that block
reading from executable sections.
However, if all of the above is true, I think libgcc as a whole
will have much bigger problems. I count over 500 other instances
in the disassembled v6m *.a file where functions load pc-relative
data from '.text'.
For example:
* C version of popcount
* __powidf2 (0x3FF00000)
* __mulsc3 (0x7F7FFFFF)
* Most soft-float functions.
Still not seeing a clear resolution here. Is it acceptable to use the
"ldr rD, =const"
pattern?
Thanks,
Daniel
