[cfe-dev] proposal - pragma section directive in clang

James Molloy via cfe-dev cfe-dev at lists.llvm.org
Fri Mar 10 01:42:27 PST 2017


Hi Reid, all,

+llvm-dev as this RFC involves changes in Clang and LLVM.

This RFC has stagnated and I think that's partially because the proposal
isn't particularly elegant and is light on details. We've been having a
rethink and have a slightly different implementation to propose that we (I)
hope will be nicer.

** Rationale (for llvm-dev) **

The goal of this proposed feature is to provide a migration path toward
Clang for developers in the automotive domain. As Javed has mentioned,
AUTOSAR, an automotive standard, mandates the use of a #pragma in header
files to determine in which sections initialized and uninitialized data get
put.

This feature is implemented in our legacy ARM Compiler 5 toolchain and
we're also aware of GCC forks used across the automotive space that have
this feature implemented compatible with the ARM Compiler 5 implementation.

The documentation is here:
http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0472m/chr1359124985290.html

** Proposed syntax and (vague) semantics **

As this is a new pragma for Clang and isn't ARM-specific, we've invented a
less ARM-specific syntax. Bikeshedding is expected and welcome.

  #pragma clang section bss(".mybss") rodata(".myrodata") data(".mydata")
text(".mytext")

The pragma applies to all global variable and function declarations from
the pragma to the end of the translation unit. The pragma should ideally be
pushable and poppable, but that is outside the scope of this RFC. The
pragma also applies to static local declarations within functions.

All global variables and functions affected by this pragma have their
default ELF section destinations changed. Globals with
__attribute__((section())) are not affected (the attribute trumps the
pragma).

This pragma is only defined to work sensibly for ELF targets.

** Proposed implementation **

There are, I believe, three possible implementation stategies:
  1) Clang internally sets the "section" on all globals it creates. No
changes in LLVM.
  2) Clang sets some module-level attribute describing the default section
names. The LLVM backend takes this into account when deciding the section
name for a global when it emits it (AsmPrinter).
  3) Clang sets the default section names as attributes on all globals. The
LLVM backend takes this into account when deciding the section name for a
global when it emits it (AsmPrinter).

(1) and (3) work with LTO. (2) interacts badly with LTO so is discounted.

(1) requires Clang to perform the decision into exactly what section a
global will go, which is the wrong place for several previously mentioned
reasons (midend optimizations could promote .data -> .bss, clang currently
doesn't have the mechanics to test if an initializer is zero or not, LLVM
does).

(2) and (3) have the advantage that the section type does not need to be
inferred by LLVM from its name. This means we don't need the horrible
string matching (m/^.bss./ -> BSS, m/^.data./ -> Data, etc) in the
AsmPrinter - users can specify whatever names they like for bss sections
without confusing the compiler.

Our previous proposal was (1). We are now proposing (3).

For (3), I think there are three distinct steps, none of which are
*particularly* invasive:

  a) Allow arbitrary attributes on GlobalVariables. Currently these are
restricted to Functions only; I believe mainly because noone had a usecase
for attributes on variables.
  b) Teach the clang frontend about the new pragma and CodeGen to add the
attributes to globals
  c) Teach AsmPrinter to inspect these attributes if they exist and take
them into account when choosing sections.

All comments more than welcome,

James

On Fri, 3 Mar 2017 at 09:09 James Molloy <james at jamesmolloy.co.uk> wrote:

> Hi Reid,
>
> Thanks for your comments. In truth, we don't have a particular requirement
> for behaviour under LTO or pushability/poppability, so we can define this
> in terms of what's best for Clang.
>
> As context is always useful, the goal of this proposed feature is to
> provide a migration path toward Clang for developers in the automotive
> domain. As Javed has mentioned, AUTOSAR which is an automotive standard,
> mandates the use of a #pragma in header files to determine in which
> sections initialized and uninitialized data get put.
>
> This feature is implemented in our legacy ARM Compiler 5 toolchain and
> we're also aware of GCC forks used across the automotive space that have
> this feature implemented compatible with the ARM Compiler 5 implementation.
>
> The documentation is here:
> http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0472m/chr1359124985290.html
>
> We do not aim for or anticipate syntax-level compatibility; merely the
> ability to do something similar in Clang. In particular Clang can't support
> the "section_type" specifier without significant rewrite, as LLVM decides
> the section type (NOBITS/PROGBITS) by textual matching on the section name
> (gross, but the fix is well beyond our scope).
>
> Given that documentation, you can also see why the Microsoft compatibility
> option *almost* met our requirements - all except the behaviour in this
> case:
>
> #pragma bss_seg('.bss.mybss')
> int i; // Microsoft extension will put i in .bss.mybss
> int j = 0; // Microsoft extension will put j in .data, whereas we really
> need it in .bss.mybss
>
> So to specifically answer your questions:
>
> In ARM Compiler 5, all pragmas are pushable and poppable. Clang doesn't
> have this feature generally yet, but when/if it does, I don't see why this
> pragma shouldn't be affected. So yes, we should consider it pushable and
> poppable.
>
> I think the only reasonable behaviour under LTO must be that the two TUs
> (may) have different bss and data sections. Anything else would be very
> strange behaviour from the user's perspective.
>
> Your example of a static global migrating from .data into .bss after
> optimization is interesting. With our proposal to explicitly name sections
> in IR, this optimization would be inhibited. I personally think that's fine
> :)
>
> Cheers,
>
> James
>
> On Thu, 2 Mar 2017 at 21:42 Reid Kleckner via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Would these pragmas be translation-unit global or could they be pushed and
> popped like the MSVC pragmas?
>
> If two TUs are combined through LTO, can the two TUs have different bss
> and data sections?
>
> If the answer to both is "no", then I think we should use a module flag
> instead of manually setting the section from the frontend.
>
> Here is a C++ example where, after optimization, a global may end up in
> .bss instead of .data:
> // c++
> static int f() { return 0; }
> static int x = f();
> int *g() { return &x; }
>
> After global opt we get this:
> @_ZL1x = internal global i32 0, align 4
>
> Normally LLVM will put this in .bss or use .lcomm for it. Your proposal
> will put it in the user's data section instead of their bss section. If
> that's important, we should use a module flag for this.
>
> On Thu, Mar 2, 2017 at 11:35 AM, Javed Absar via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> Hi all:
>
>
>
> We would like to propose a clang pragma directive to allow specialized
> section names.
> The semantics of it could be as follows. The pragma section name is
> declared in global
> scope. All global variables and functions get assigned to the
> corresponding specialized
> section name if one is present. With this feature, the following code:
>
> // foo.c
> #pragma bss_section(".bss.alpha")
> #pragma data_section(".data.beta")
> #pragma code_section(".code.gamma")
> #pragma const_section(".const.delta")
> int a;
> int b=2;
> const int d = 5;
> int c(){
>   return d;
> }
>
> ..will emit llvm-ir as:
>
> target triple = "armv7-arm-none-eabi"
> @a = global i32 0, section ".bss.alpha", align 4
> @b = global i32 2, section ".data.beta", align 4
> @d = constant i32 5, section ".const.delta", align 4
>
> ; Function Attrs: noinline nounwind
> define i32 @c() #0 section ".code.gamma" {
> entry:
>   ret i32 5
> }
>
> This pragma will be very useful for embedded code which
> need to control where the different sections are placed in memory.
>
> Microsoft -fms-extension provides similar feature, but our proposal is for
> a
> general use.  Attributes are an alternative that is also currently
> available,
> but attributes are applicable only to specific declarations and not entire
> file. Many real embedded users prefer the pragma option.
> This will be a welcome enabler for them. Also, AUTOSAR, which is an
>
> automotive standard mandates use of a #pragma solution over an attribute
> one
>
> Looking forward to comments and suggestions.
> Best Regards
>
> Javed
>
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