Re: Managing GCC Assembly Code Size (AArch64)

["Cohen, Eugene" <eugene@hp.com>]

Ard, as usual you rock...

> FYI there is a null token \() for GAS which you can use to concatenate
> a string with a macro argument, e.g.,
> 
>    .macro func, x
>    .globl \x
>    .type \x, %function
>    .section .text.\x
> \x\():
>   .endm
>

Using the GAS .macro syntax this all collapses nicely.  I tested it with one assembly function and all the right stuff happens.

So the request becomes: can we modify all of the assembly (at least Aarch64 please) to use this?  How would you like to phase this in?

> > I think this would be an improvement, so go for it. The only thing to
> > be wary of is routines that fall through into the subsequent one.
> > Those need to remain in the same section.

Yes, I've accidentally modified these with disastrous results.  I now know to stay away from them (ExceptionSupport.S in particular).  :)

Thanks,

Eugene

> -----Original Message-----
> From: Ard Biesheuvel [mailto:ard.biesheuvel@linaro.org]
> Sent: Thursday, August 04, 2016 1:47 PM
> To: Cohen, Eugene <eugene@hp.com>
> Cc: Leif Lindholm <leif.lindholm@linaro.org>; edk2-devel@lists.01.org
> Subject: Re: Managing GCC Assembly Code Size (AArch64)
> 
> On 4 August 2016 at 21:18, Ard Biesheuvel
> <ard.biesheuvel@linaro.org> wrote:
> > On 4 August 2016 at 20:08, Cohen, Eugene <eugene@hp.com>
> wrote:
> >> Ard and Leif,
> >>
> >> I've been too backlogged to provide a real patchset at this point but
> wanted to get your approval on this proposal...
> >>
> >>
> >> As you know we have some code size sensitive uncompressed XIP
> stuff going on.  For C code we get dead code stripping thanks to the "-
> ffunction-sections" switch which places each function in its own
> section so the linker can strip unreferenced sections.
> >>
> >> For assembly there is not a solution that's as easy.  For RVCT we
> handled this with an assembler macro that combined the procedure
> label definition, export of global symbols and placement of the
> procedure in its own section.  For GCC I haven't found a way to fully do
> this because we rely on the C preprocessor for assembly which means
> you cannot expand to multi-line macros.  (The label and assembler
> directives require their own lines but the preprocessor collapses stuff
> onto one line because in the C language newlines don't matter.)
> >>
> >> So the solution I've settled on is to do this:
> >>
> >> in MdePkg\Include\AArch64\ProcessorBind.h define:
> >>
> >>   /// Macro to place a function in its own section for dead code
> elimination
> >>   /// This must be placed directly before the corresponding code
> since the
> >>   /// .section directive applies to the code that follows it.
> >>   #define GCC_ASM_EXPORT_SECTION(func__)   \
> >>           .global  _CONCATENATE (__USER_LABEL_PREFIX__, func__)
> ;\
> >>           .section  .text._CONCATENATE (__USER_LABEL_PREFIX__,
> func__)    ;\
> >>           .type ASM_PFX(func__), %function; \
> >>
> >> This has the effect of placing the function in a section called
> .text.<func__> so the linker can do its dead code stripping stuff.  It also
> absorbs the making the symbol globally visible so the corresponding
> GCC_ASM_EXPORT statement can be removed.
> >>
> >> then for every single assembly procedure change from this:
> >>
> >> [top of file]
> >> GCC_ASM_EXPORT (ArmInvalidateDataCacheEntryByMVA)
> >>
> >> [lower down]
> >> ASM_PFX(ArmInvalidateDataCacheEntryByMVA):
> >>   dc      ivac, x0    // Invalidate single data cache line
> >>   ret
> >>
> >> to this:
> >>
> >>
> GCC_ASM_EXPORT_SECTION(ArmInvalidateDataCacheEntryByMVA)
> >> ASM_PFX(ArmInvalidateDataCacheEntryByMVA):
> >>   dc      ivac, x0    // Invalidate single data cache line
> >>   ret
> >>
> >> Because the assembly label must appear in column 1 I couldn't find
> a way to use the C preprocessor to absorb it so hence the two lines.  If
> you can find a way to improve on this it would be great.
> >>
> >
> > What about GAS macros (.macro / .endm). I prefer those over cpp
> macros
> > in assembler anyway.
> >
> 
> FYI there is a null token \() for GAS which you can use to concatenate
> a string with a macro argument, e.g.,
> 
>    .macro func, x
>    .globl \x
>    .type \x, %function
>    .section .text.\x
> \x\():
>   .endm
> 
> 
> >> I'm not sure what impacts this might have to other toolchains - can
> this be translated to CLANG and ARM Compiler?
> >>
> >
> > The asm dialect is 99% aligned between CLANG and GNU as, so this
> > shouldn't be a problem
> >
> >> I'd like to get your OK on this conceptually and then I could
> upstream some patches that modify the AArch64 *.S files to use this
> approach.  Unfortunately it won't be complete because I only updated
> the libraries that we use.  My hope is that long term all assembly (or at
> least assembly in libraries) adopt this approach so we are positioned
> for maximum dead code stripping.
> >>
> >
> > I think this would be an improvement, so go for it. The only thing to
> > be wary of is routines that fall through into the subsequent one.
> > Those need to remain in the same section.