[llvm-dev] [EXTERNAL] Re: RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler
Peter Smith via llvm-dev
llvm-dev at lists.llvm.org
Wed May 1 03:09:51 PDT 2019
On Tue, 30 Apr 2019 at 19:11, Snider, Todd <t-snider at ti.com> wrote:
>
> Hi Peter,
>
> Thanks for the response.
>
> If we set aside the discussion of the relationship between sections and the application of the "location" or "at" attribute for a moment, do you have any objections to the proposed method of encoding metadata information about symbols (whether they are associated with actual data objects, functions, or sections) in the ELF object file?
>
> There are other use cases that would benefit from this encoding method besides the location attribute. The used attribute is an example. There are likely to be others.
>
I think that some problems map naturally to symbol, section,
relocation or even a separate custom metadata section. As Peter
Collingbourne points out, there can be problems with tools like
objdump when they encounter conventions they don't understand. I think
that the scheme as outlined may not be the best fit.
> I would agree with you that applying a "location" or "at" attribute to a data object or function definition must require that the compiler generate the definition of the applicable data object or function into its own section, and that section may only contain the definition of that data object or function.
>
> One of the advantages of attaching the location attribute information to the symbol is that if the symbol is associated with a common data object, then the location attribute ends up being applied to the definition that the common symbol resolves to.
>
In that particular case I'd say is it worth supporting common symbols
with this attribute? For example if I write something like int foo
__attribute__((location(0x1000))) in one file, and int foo
__attribute__((location(0x2000))) in another file, I'd probably want
that to be a multiple definition error rather than silently choosing
one of them and resolving it to one of the two locations silently, or
I'd want the linker to tell me about clashing metadata? When int foo
__attribute__((section("name"))); is used the definition is not common
even if -fcommon is in use, perhaps that would be a better model for
__attribute__((location(<address)))?
A possible weakness of using absolute metadata symbols is with comdat
groups. Although I suspect the number of sensible use cases of
__attribute__((location(<address>))) and templates is low, it is
possible that something similar to the contrived example below could
occur:
template <class T>
struct Foo {
T bar;
T foo() __attribute__((location(0x1000))) { return bar; }
};
A linker would have to know which absolute metadata symbols to select
and which to ignore, a solveable problem but it would be good to have
a way to have metadata drop out naturally through the group selection
process. Without common symbols it could be possible to define the
metadata symbols relative to the section. It could also be possible to
have a metadata section included in the group.
Peter
> ~ Todd
>
> -----Original Message-----
> From: Peter Smith [mailto:peter.smith at linaro.org]
> Sent: Tuesday, April 30, 2019 10:51 AM
> To: Snider, Todd
> Cc: llvm-dev
> Subject: [EXTERNAL] Re: [llvm-dev] RFC - a proposal to support additional symbol metadata in ELF object files in the ARM compiler
>
> 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
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