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Christof Douma via llvm-dev

2019-May-01 12:22 UTC

[llvm-dev] RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler

Hi Snider.

As you and Peter mentioned there are indeed toolchains that allow location
placement from within the C/C++ source code, using attributes or similar. I
always wonder if such extension is worth the effort. There are downsides like
the non-standard ways of communicating this information to the linker, different
places that control location of things (linker and compiler sources). I would
love to understand more of what is problematic in the more common approach for
placement that is already available.

The conceptual model I follow is that the C/C++ source describes the semantics
of the program, and the linker sources (LD scripts or similar, depending on the
toolchain in use) describe the placement of the program on the system/device.
This gives rise to two common ways for placement that are used a lot that work
without any non-standard extensions:

* Define a variable in C/C++ in a dedicated section that a linker can move
individually ('section' attribute in the compiler, and regular section
placement in the linker).
* Define a symbol in the linker at a certain place and used an extern
declaration in C/C++. At this point you can either take the address of it
(commonly used) or use it as a regular object (less common).

I am very interested to hear what the weakness in these methods are, to
understand the need of a 'location' attribute.

Thanks,
Christof

On 30/04/2019, 16:51, "llvm-dev on behalf of Peter Smith via
llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of llvm-dev at
lists.llvm.org> wrote:

    On Tue, 30 Apr 2019 at 16:17, Snider, Todd via llvm-dev
    <llvm-dev at lists.llvm.org> wrote:
    >
    >
    >
    > Hello All,
    >
    >
    >
    > In ARM embedded applications, there are some compilers that support
useful function and variable attributes that help the compiler communicate
information about symbols to downstream object consumers (i.e. linkers).
    >
    >
    >
    > One such attribute is the “location” attribute. This attribute can be
applied to a global or local static data object or a function to indicate to the
linker that the definition of the data object or function should be placed at a
specific address in memory.
    >
    >
    >
    > For example, in the following code:
    >
    >
    >
    > #include <stdio.h>
    >
    >
    >
    > extern int a;
    >
    > int a __attribute__((location(0x1000))) = 4;
    >
    >
    >
    > struct bstruct
    >
    > {
    >
    >     int f1;
    >
    >     int f2;
    >
    > };
    >
    >
    >
    > struct bstruct b __attribute__((location(0x1004))) = {10, 12};
    >
    > double c __attribute__((location(0x1010))) = 1.0;
    >
    > char d[] __attribute__((location(0x2000)))  = {1, 2, 3, 4};
    >
    > void foo(double x) __attribute((location(0x4000)));
    >
    >
    >
    > void foo(double x) { printf("%f\n", x); }
    >
    >
    >
    > A location attribute has been applied to several  data objects and the
function “foo.”  The compiler would then encode information into the compiled
object file that tells the downstream linker about these memory placement
constraints on the data objects and function.
    >
    >
    >
    > Without extending the ELF object format, how would this work?
    >
    >
    >
    > I propose to encode metadata information about a symbol in special
absolute symbols, “__sym_attr_metadata.<int>”, that the linker can
recognize when scanning the symbol table for an incoming object file. In an ELF
symbol table entry:
    >
    >
    >
    > typedef struct {
    >
    >        Elf32_Word     st_name;
    >
    >        Elf32_Addr     st_value;
    >
    >        Elf32_Word     st_size;
    >
    >        unsigned char  st_info;
    >
    >        unsigned char  st_other;
    >
    >        Elf32_Half     st_shndx;
    >
    > } Elf32_Sym;
    >
    >
    >
    > typedef struct {
    >
    >        Elf64_Word     st_name;
    >
    >        unsigned char  st_info;
    >
    >        unsigned char  st_other;
    >
    >        Elf64_Half     st_shndx;
    >
    >        Elf64_Addr     st_value;
    >
    >        Elf64_Xword    st_size;
    >
    > } Elf64_Sym;
    >
    >
    >
    > The st_size and st_value fields could be used to represent attribute
information about a given symbol:
    >
    >
    >
    > The st_size field can be split into an attribute ID and a symbol index
for the symbol that the attribute applies to
    >
    > attribute ID: bits 0..7
    > symbol index: bits 8..31
    >
    > The st_value field can contain the value associated with the attribute
(i.e. the address argument of a location attribute)
    >
    >
    >
    > If the compiler is generating assembly code, a new directive similar to
the .eabi_attribute can be used:
    >
    >
    >
    >         .symbol_attribute <symbol name>, <attribute kind>,
<attribute value>
    >
    >
    >
    > Where:
    >
    > symbol name - will unambiguously identify the symbol that the
attribute/value pair applies to
    > attribute kind - is an unsigned integer between 1 and 255 that
specifies the kind of attribute to be applied to the symbol
    >
    > I propose a starting base set of 2 attribute IDs: used (1), location
(2)
    > the compiler will emit the integer constant that identifies the
attribute kind
    >
    > attribute value - a value that is appropriate for the specified
attribute kind
    >
    >
    >
    > Thoughts? Comments? Concerns?
    >

    Hello Todd,

    Thanks for bringing this up, I've got a few comments for you based on
    the implementation of a similar attribute in another Embedded Compiler
   
(http://infocenter.arm.com/help/topic/com.arm.doc.dui0472m/chr1359124981140.html).
     In that case it was __attribute__((at(address))) but the name is not
    that important.

    The communication with the linker in that case was via section name
    and not symbol, from memory at(<address>) translated to a section name
    of .ARM.__at_<address>. For us this had some advantages:
    - We could use
__attribute__((section(".ARM.__at_<address>")))  when
    the compiler didn't support the attribute, it also needed no support
    in the assembler. This wasn't ideal as it is nice to be able to use
    expressions for the address, but it gets you most of the way there.
    - In practice you'd likely need a separate section for each variable
    to avoid problems at link time. For example if you had two variables
    with non-contiguous locations you'd most likely not want these in the
    same section so this mapped quite well to something similar to
    __attribute__((section(name))).
    - We did find some properties of __attribute__((section("name")))
    inconvenient, especially that variables would come out as SHT_PROGBITS
    when in many cases the user wanted SHT_NOBITS (memory mapped
    peripheral), we had our custom attribute fix that.

    If you used a section name rather than a symbol then you may not need
    any backend changes and it would generalise over all ELF targets.
    Linker support is another question entirely though.

    Peter

    >
    >
    > The anticipated next steps would be to add support for the location
attribute and update the ARM/ELF LLVM back-end to support encoding the used
attribute with the new mechanism.
    >
    >
    >
    > ~ Todd Snider
    >
    >
    >
    > Code Generation Tools Group
    >
    > Texas Instruments Incorporated
    >
    >
    >
    >
    >
    > _______________________________________________
    > LLVM Developers mailing list
    > llvm-dev at lists.llvm.org
    > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
    _______________________________________________
    LLVM Developers mailing list
    llvm-dev at lists.llvm.org
    https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev


IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium. Thank you.

Finkel, Hal J. via llvm-dev

2019-May-01 14:03 UTC

head link

[llvm-dev] RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler

On 5/1/19 7:22 AM, Christof Douma via llvm-dev wrote:> Hi Snider.
>
> As you and Peter mentioned there are indeed toolchains that allow location
placement from within the C/C++ source code, using attributes or similar. I
always wonder if such extension is worth the effort. There are downsides like
the non-standard ways of communicating this information to the linker, different
places that control location of things (linker and compiler sources). I would
love to understand more of what is problematic in the more common approach for
placement that is already available.
>
> The conceptual model I follow is that the C/C++ source describes the
semantics of the program, and the linker sources (LD scripts or similar,
depending on the toolchain in use) describe the placement of the program on the
system/device. This gives rise to two common ways for placement that are used a
lot that work without any non-standard extensions:
>
> * Define a variable in C/C++ in a dedicated section that a linker can move
individually ('section' attribute in the compiler, and regular section
placement in the linker).
> * Define a symbol in the linker at a certain place and used an extern
declaration in C/C++. At this point you can either take the address of it
(commonly used) or use it as a regular object (less common).
>
> I am very interested to hear what the weakness in these methods are, to
understand the need of a 'location' attribute.

I like the idea of these fixed-location variables being defined as
actual global variables. The optimizer can actually reason about them
that way. The common alternative that I've seen is that programmers
don't generate variables at all, but rather, do something like this:

  #define DEV_DATA (*((volatile unsigned long *)(0x2000A000)))

and the optimizer needs to make very pessimistic assumptions about the
aliasing, etc. in this case. However, in the end, do we actually want
symbols that the linker resolves? Or do we want the immediate address?
Would the latter be more efficient?

Having to define sections for each of these variables and then maintain
the location mappings in a linker script can be annoying -- on the other
hand, if you target multiple systems for which the addresses might be
different then having the locations in a separate file might be best anyway.

What I don't understand about this proposal is how general it is. How
much of what is specified in a linker script can be specified this way?
Do we really just want a way to embed linker-script fragments into an
object file?

 -Hal
>
> Thanks,
> Christof
>
> On 30/04/2019, 16:51, "llvm-dev on behalf of Peter Smith via
llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of llvm-dev at
lists.llvm.org> wrote:
>
>     On Tue, 30 Apr 2019 at 16:17, Snider, Todd via llvm-dev
>     <llvm-dev at lists.llvm.org> wrote:
>     >
>     >
>     >
>     > Hello All,
>     >
>     >
>     >
>     > In ARM embedded applications, there are some compilers that
support useful function and variable attributes that help the compiler
communicate information about symbols to downstream object consumers (i.e.
linkers).
>     >
>     >
>     >
>     > One such attribute is the “location” attribute. This attribute can
be applied to a global or local static data object or a function to indicate to
the linker that the definition of the data object or function should be placed
at a specific address in memory.
>     >
>     >
>     >
>     > For example, in the following code:
>     >
>     >
>     >
>     > #include <stdio.h>
>     >
>     >
>     >
>     > extern int a;
>     >
>     > int a __attribute__((location(0x1000))) = 4;
>     >
>     >
>     >
>     > struct bstruct
>     >
>     > {
>     >
>     >     int f1;
>     >
>     >     int f2;
>     >
>     > };
>     >
>     >
>     >
>     > struct bstruct b __attribute__((location(0x1004))) = {10, 12};
>     >
>     > double c __attribute__((location(0x1010))) = 1.0;
>     >
>     > char d[] __attribute__((location(0x2000)))  = {1, 2, 3, 4};
>     >
>     > void foo(double x) __attribute((location(0x4000)));
>     >
>     >
>     >
>     > void foo(double x) { printf("%f\n", x); }
>     >
>     >
>     >
>     > A location attribute has been applied to several  data objects and
the function “foo.”  The compiler would then encode information into the
compiled object file that tells the downstream linker about these memory
placement constraints on the data objects and function.
>     >
>     >
>     >
>     > Without extending the ELF object format, how would this work?
>     >
>     >
>     >
>     > I propose to encode metadata information about a symbol in special
absolute symbols, “__sym_attr_metadata.<int>”, that the linker can
recognize when scanning the symbol table for an incoming object file. In an ELF
symbol table entry:
>     >
>     >
>     >
>     > typedef struct {
>     >
>     >        Elf32_Word     st_name;
>     >
>     >        Elf32_Addr     st_value;
>     >
>     >        Elf32_Word     st_size;
>     >
>     >        unsigned char  st_info;
>     >
>     >        unsigned char  st_other;
>     >
>     >        Elf32_Half     st_shndx;
>     >
>     > } Elf32_Sym;
>     >
>     >
>     >
>     > typedef struct {
>     >
>     >        Elf64_Word     st_name;
>     >
>     >        unsigned char  st_info;
>     >
>     >        unsigned char  st_other;
>     >
>     >        Elf64_Half     st_shndx;
>     >
>     >        Elf64_Addr     st_value;
>     >
>     >        Elf64_Xword    st_size;
>     >
>     > } Elf64_Sym;
>     >
>     >
>     >
>     > The st_size and st_value fields could be used to represent
attribute information about a given symbol:
>     >
>     >
>     >
>     > The st_size field can be split into an attribute ID and a symbol
index for the symbol that the attribute applies to
>     >
>     > attribute ID: bits 0..7
>     > symbol index: bits 8..31
>     >
>     > The st_value field can contain the value associated with the
attribute (i.e. the address argument of a location attribute)
>     >
>     >
>     >
>     > If the compiler is generating assembly code, a new directive
similar to the .eabi_attribute can be used:
>     >
>     >
>     >
>     >         .symbol_attribute <symbol name>, <attribute
kind>, <attribute value>
>     >
>     >
>     >
>     > Where:
>     >
>     > symbol name - will unambiguously identify the symbol that the
attribute/value pair applies to
>     > attribute kind - is an unsigned integer between 1 and 255 that
specifies the kind of attribute to be applied to the symbol
>     >
>     > I propose a starting base set of 2 attribute IDs: used (1),
location (2)
>     > the compiler will emit the integer constant that identifies the
attribute kind
>     >
>     > attribute value - a value that is appropriate for the specified
attribute kind
>     >
>     >
>     >
>     > Thoughts? Comments? Concerns?
>     >
>
>     Hello Todd,
>
>     Thanks for bringing this up, I've got a few comments for you based
on
>     the implementation of a similar attribute in another Embedded Compiler
>    
(http://infocenter.arm.com/help/topic/com.arm.doc.dui0472m/chr1359124981140.html).
>      In that case it was __attribute__((at(address))) but the name is not
>     that important.
>
>     The communication with the linker in that case was via section name
>     and not symbol, from memory at(<address>) translated to a section
name
>     of .ARM.__at_<address>. For us this had some advantages:
>     - We could use
__attribute__((section(".ARM.__at_<address>")))  when
>     the compiler didn't support the attribute, it also needed no
support
>     in the assembler. This wasn't ideal as it is nice to be able to use
>     expressions for the address, but it gets you most of the way there.
>     - In practice you'd likely need a separate section for each
variable
>     to avoid problems at link time. For example if you had two variables
>     with non-contiguous locations you'd most likely not want these in
the
>     same section so this mapped quite well to something similar to
>     __attribute__((section(name))).
>     - We did find some properties of
__attribute__((section("name")))
>     inconvenient, especially that variables would come out as SHT_PROGBITS
>     when in many cases the user wanted SHT_NOBITS (memory mapped
>     peripheral), we had our custom attribute fix that.
>
>     If you used a section name rather than a symbol then you may not need
>     any backend changes and it would generalise over all ELF targets.
>     Linker support is another question entirely though.
>
>     Peter
>
>     >
>     >
>     > The anticipated next steps would be to add support for the
location attribute and update the ARM/ELF LLVM back-end to support encoding the
used attribute with the new mechanism.
>     >
>     >
>     >
>     > ~ Todd Snider
>     >
>     >
>     >
>     > Code Generation Tools Group
>     >
>     > Texas Instruments Incorporated
>     >
>     >
>     >
>     >
>     >
>     > _______________________________________________
>     > LLVM Developers mailing list
>     > llvm-dev at lists.llvm.org
>     > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>     _______________________________________________
>     LLVM Developers mailing list
>     llvm-dev at lists.llvm.org
>     https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
> IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
medium. Thank you.
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-- 
Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

Peter Smith via llvm-dev

2019-May-01 15:27 UTC

head link

[llvm-dev] RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler

On Wed, 1 May 2019 at 15:03, Finkel, Hal J. <hfinkel at anl.gov>
wrote:>
> On 5/1/19 7:22 AM, Christof Douma via llvm-dev wrote:
> > Hi Snider.
> >
> > As you and Peter mentioned there are indeed toolchains that allow
location placement from within the C/C++ source code, using attributes or
similar. I always wonder if such extension is worth the effort. There are
downsides like the non-standard ways of communicating this information to the
linker, different places that control location of things (linker and compiler
sources). I would love to understand more of what is problematic in the more
common approach for placement that is already available.
> >
> > The conceptual model I follow is that the C/C++ source describes the
semantics of the program, and the linker sources (LD scripts or similar,
depending on the toolchain in use) describe the placement of the program on the
system/device. This gives rise to two common ways for placement that are used a
lot that work without any non-standard extensions:
> >
> > * Define a variable in C/C++ in a dedicated section that a linker can
move individually ('section' attribute in the compiler, and regular
section placement in the linker).
> > * Define a symbol in the linker at a certain place and used an extern
declaration in C/C++. At this point you can either take the address of it
(commonly used) or use it as a regular object (less common).
> >
> > I am very interested to hear what the weakness in these methods are,
to understand the need of a 'location' attribute.
>
>
> I like the idea of these fixed-location variables being defined as
> actual global variables. The optimizer can actually reason about them
> that way. The common alternative that I've seen is that programmers
> don't generate variables at all, but rather, do something like this:
>
>   #define DEV_DATA (*((volatile unsigned long *)(0x2000A000)))
>
> and the optimizer needs to make very pessimistic assumptions about the
> aliasing, etc. in this case. However, in the end, do we actually want
> symbols that the linker resolves? Or do we want the immediate address?
> Would the latter be more efficient?
>
> Having to define sections for each of these variables and then maintain
> the location mappings in a linker script can be annoying -- on the other
> hand, if you target multiple systems for which the addresses might be
> different then having the locations in a separate file might be best
anyway.
>
> What I don't understand about this proposal is how general it is. How
> much of what is specified in a linker script can be specified this way?
> Do we really just want a way to embed linker-script fragments into an
> object file?
>
I suspect that clang/llvm will be agnostic with respect to what can be
done in the linker. In effect the linker is given the instruction to
place a section at a particular address and it is up to the linker to
work out how to do that or error if it can't.

The majority of the cases I've seen this used for are memory mapped
peripheral registers that typically live way outside the normal memory
map covered by the linker script. These cases are not too difficult to
handle as the linker can generate its own fragment of linker script
(or equivalent) from the Input Section. The more difficult case is
where the location is in the middle of an existing OutputSection and
this can involve changes to the linker's layout to flow non-location
sections around it, this is a fertile source of corner case bugs. How
much or little of this to support might be best left to the linker.

Embedding linker script fragments is an interesting idea, and could
mean that any linker that supports GNU linker scripts could use the
feature. I think that there would be a number of challenges:
- Precedence of section selectors, i.e. how to stop an earlier linker
script pattern from matching the location, I guess a tempname style
section name might help, although wildcards might pick it up.
- The linker script fragment would need to not clash with an existing
OutputSection. I think that this could work for memory mapped
peripherals but it wouldn't for some of the other use cases that a
linker might want to support.
- Embedded ELF linkers may not support GNU Linker Script syntax.
Although custom targets could change the linker script format as they
see fit.

Will be interesting to hear what use cases Todd had in mind.

Peter>  -Hal
>
> >
> > Thanks,
> > Christof
> >
> > On 30/04/2019, 16:51, "llvm-dev on behalf of Peter Smith via
llvm-dev" <llvm-dev-bounces at lists.llvm.org on behalf of llvm-dev at
lists.llvm.org> wrote:
> >
> >     On Tue, 30 Apr 2019 at 16:17, Snider, Todd via llvm-dev
> >     <llvm-dev at lists.llvm.org> wrote:
> >     >
> >     >
> >     >
> >     > Hello All,
> >     >
> >     >
> >     >
> >     > In ARM embedded applications, there are some compilers that
support useful function and variable attributes that help the compiler
communicate information about symbols to downstream object consumers (i.e.
linkers).
> >     >
> >     >
> >     >
> >     > One such attribute is the “location” attribute. This
attribute can be applied to a global or local static data object or a function
to indicate to the linker that the definition of the data object or function
should be placed at a specific address in memory.
> >     >
> >     >
> >     >
> >     > For example, in the following code:
> >     >
> >     >
> >     >
> >     > #include <stdio.h>
> >     >
> >     >
> >     >
> >     > extern int a;
> >     >
> >     > int a __attribute__((location(0x1000))) = 4;
> >     >
> >     >
> >     >
> >     > struct bstruct
> >     >
> >     > {
> >     >
> >     >     int f1;
> >     >
> >     >     int f2;
> >     >
> >     > };
> >     >
> >     >
> >     >
> >     > struct bstruct b __attribute__((location(0x1004))) = {10,
12};
> >     >
> >     > double c __attribute__((location(0x1010))) = 1.0;
> >     >
> >     > char d[] __attribute__((location(0x2000)))  = {1, 2, 3, 4};
> >     >
> >     > void foo(double x) __attribute((location(0x4000)));
> >     >
> >     >
> >     >
> >     > void foo(double x) { printf("%f\n", x); }
> >     >
> >     >
> >     >
> >     > A location attribute has been applied to several  data
objects and the function “foo.”  The compiler would then encode information into
the compiled object file that tells the downstream linker about these memory
placement constraints on the data objects and function.
> >     >
> >     >
> >     >
> >     > Without extending the ELF object format, how would this work?
> >     >
> >     >
> >     >
> >     > I propose to encode metadata information about a symbol in
special absolute symbols, “__sym_attr_metadata.<int>”, that the linker can
recognize when scanning the symbol table for an incoming object file. In an ELF
symbol table entry:
> >     >
> >     >
> >     >
> >     > typedef struct {
> >     >
> >     >        Elf32_Word     st_name;
> >     >
> >     >        Elf32_Addr     st_value;
> >     >
> >     >        Elf32_Word     st_size;
> >     >
> >     >        unsigned char  st_info;
> >     >
> >     >        unsigned char  st_other;
> >     >
> >     >        Elf32_Half     st_shndx;
> >     >
> >     > } Elf32_Sym;
> >     >
> >     >
> >     >
> >     > typedef struct {
> >     >
> >     >        Elf64_Word     st_name;
> >     >
> >     >        unsigned char  st_info;
> >     >
> >     >        unsigned char  st_other;
> >     >
> >     >        Elf64_Half     st_shndx;
> >     >
> >     >        Elf64_Addr     st_value;
> >     >
> >     >        Elf64_Xword    st_size;
> >     >
> >     > } Elf64_Sym;
> >     >
> >     >
> >     >
> >     > The st_size and st_value fields could be used to represent
attribute information about a given symbol:
> >     >
> >     >
> >     >
> >     > The st_size field can be split into an attribute ID and a
symbol index for the symbol that the attribute applies to
> >     >
> >     > attribute ID: bits 0..7
> >     > symbol index: bits 8..31
> >     >
> >     > The st_value field can contain the value associated with the
attribute (i.e. the address argument of a location attribute)
> >     >
> >     >
> >     >
> >     > If the compiler is generating assembly code, a new directive
similar to the .eabi_attribute can be used:
> >     >
> >     >
> >     >
> >     >         .symbol_attribute <symbol name>, <attribute
kind>, <attribute value>
> >     >
> >     >
> >     >
> >     > Where:
> >     >
> >     > symbol name - will unambiguously identify the symbol that the
attribute/value pair applies to
> >     > attribute kind - is an unsigned integer between 1 and 255
that specifies the kind of attribute to be applied to the symbol
> >     >
> >     > I propose a starting base set of 2 attribute IDs: used (1),
location (2)
> >     > the compiler will emit the integer constant that identifies
the attribute kind
> >     >
> >     > attribute value - a value that is appropriate for the
specified attribute kind
> >     >
> >     >
> >     >
> >     > Thoughts? Comments? Concerns?
> >     >
> >
> >     Hello Todd,
> >
> >     Thanks for bringing this up, I've got a few comments for you
based on
> >     the implementation of a similar attribute in another Embedded
Compiler
> >    
(http://infocenter.arm.com/help/topic/com.arm.doc.dui0472m/chr1359124981140.html).
> >      In that case it was __attribute__((at(address))) but the name is
not
> >     that important.
> >
> >     The communication with the linker in that case was via section
name
> >     and not symbol, from memory at(<address>) translated to a
section name
> >     of .ARM.__at_<address>. For us this had some advantages:
> >     - We could use
__attribute__((section(".ARM.__at_<address>")))  when
> >     the compiler didn't support the attribute, it also needed no
support
> >     in the assembler. This wasn't ideal as it is nice to be able
to use
> >     expressions for the address, but it gets you most of the way
there.
> >     - In practice you'd likely need a separate section for each
variable
> >     to avoid problems at link time. For example if you had two
variables
> >     with non-contiguous locations you'd most likely not want these
in the
> >     same section so this mapped quite well to something similar to
> >     __attribute__((section(name))).
> >     - We did find some properties of
__attribute__((section("name")))
> >     inconvenient, especially that variables would come out as
SHT_PROGBITS
> >     when in many cases the user wanted SHT_NOBITS (memory mapped
> >     peripheral), we had our custom attribute fix that.
> >
> >     If you used a section name rather than a symbol then you may not
need
> >     any backend changes and it would generalise over all ELF targets.
> >     Linker support is another question entirely though.
> >
> >     Peter
> >
> >     >
> >     >
> >     > The anticipated next steps would be to add support for the
location attribute and update the ARM/ELF LLVM back-end to support encoding the
used attribute with the new mechanism.
> >     >
> >     >
> >     >
> >     > ~ Todd Snider
> >     >
> >     >
> >     >
> >     > Code Generation Tools Group
> >     >
> >     > Texas Instruments Incorporated
> >     >
> >     >
> >     >
> >     >
> >     >
> >     > _______________________________________________
> >     > LLVM Developers mailing list
> >     > llvm-dev at lists.llvm.org
> >     > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >     _______________________________________________
> >     LLVM Developers mailing list
> >     llvm-dev at lists.llvm.org
> >     https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
> >
> > IMPORTANT NOTICE: The contents of this email and any attachments are
confidential and may also be privileged. If you are not the intended recipient,
please notify the sender immediately and do not disclose the contents to any
other person, use it for any purpose, or store or copy the information in any
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> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory
>

llvm dev - May 2019 - RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler

[llvm-dev] RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler

[llvm-dev] RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler

[llvm-dev] RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler