llvm dev - Aug 2014 - [LLVMdev] Create "appending" section that can be partially dead stripped

On 04 Aug 2014, at 09:27, Reid Kleckner wrote:
> On Sat, Aug 2, 2014 at 7:51 AM, Jonas Maebe  
> <jonas.maebe at elis.ugent.be>
> wrote:
>
>> On 01/08/14 19:37, Reid Kleckner wrote:
>>
>>> What happens if you drop appending linkage?  I think it will just  
>>> work,
>>> since you are already using a custom section, which will ensure  
>>> that all
>>> the data appears contiguously in memory.
>>
>> Thanks for the suggestion, but it still puts everything in a single
>> .section statement.
>
> Try giving the globals linkonce_odr linkage instead of external  
> linkage
> manually?  This is essentially the effect of -fdata-sections, except  
> it
> happens later during codegen.
That one indeed works, thanks! (provided I process the .ll file with  
optimisations enabled).

Regarding the question whether the optimisers and analyses will deal  
correctly with me iterating over all of the elements in such a  
section, at least at first sight it seems that the test below looks  
fine when processed with full optimisations. That's of course no proof  
that there won't be any problems in any case.

However, I'm not sure how to add a symbol without data at the start  
and end of the section. Adding a [i32 x 0]-typed symbol still inserts  
a byte (see the @arrstart/@arrstop). I can't work with alias  
declarations to the first and last element, since then the first and  
last element will always be considered live.


Jonas

target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32- 
i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64- 
f80:128:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"

define i32 @main() {
Entry:
  %sumvar = alloca i32

; make the second and third element live by directly referring them
  %eleptr1 = getelementptr [2 x i32]* @arr2, i64 0, i32 0
  %eleptr2 = getelementptr [2 x i32]* @arr3, i64 0, i32 0
  %ele1 = load i32* %eleptr1
  %ele2 = load i32* %eleptr2
  %sum1 = add i32 %ele1, %ele2
  store i32 %sum1, i32* %sumvar

; now loop over the entire array by using @arrstart and @arrstop
  %loopstop = ptrtoint [0 x i32]* @arrstop to i64
  %loopstart = ptrtoint [0 x i32]* @arrstart to i64
  %loopcount = sub i64 %loopstop, %loopstart
  %looparrinit = bitcast [0 x i32]* @arrstart to i32*
  br label %LoopStart

; sum all elements in the array
LoopStart:
  %looparr = phi i32* [%looparrinit, %Entry], [%looparrnext, %LoopBody]
  %loopcond = icmp eq i64 %loopcount, 0
  br i1 %loopcond, label %LoopEnd, label %LoopBody
LoopBody:
  %val = load i32* %looparr
  %sum2 = load i32* %sumvar
  %sum3 = add i32 %val, %sum2
  store i32 %sum3, i32* %sumvar
  %looparrnext = getelementptr i32* %looparr, i64 1
br label %LoopStart

; return the sum
LoopEnd:
  %retval = load i32* %sumvar
  ret i32 %retval
}

; this declaration inserts a 0 byte
@arrstart = global [0 x i32] [ ], section "mytest"

@arr1 = linkonce_odr global [2 x i32] [
   i32 1,
   i32 2
], section "mytest"

@arr2 = linkonce_odr global [2 x i32] [
   i32 3,
   i32 4
], section "mytest"

@arr3 = linkonce_odr global [2 x i32] [
   i32 5,
   i32 6
], section "mytest"

; this declaration inserts a 0 byte
@arrstop = global [0 x i32] [ ], section "mytest"

On Tue, Aug 5, 2014 at 7:34 AM, Jonas Maebe <jonas.maebe at elis.ugent.be>
wrote:
>
> However, I'm not sure how to add a symbol without data at the start and
> end of the section. Adding a [i32 x 0]-typed symbol still inserts a byte
> (see the @arrstart/@arrstop). I can't work with alias declarations to
the
> first and last element, since then the first and last element will always
> be considered live.
>
I think you want to declare those [0 x i32] globals instead of defining
them by adding extern and dropping the empty brackets.  Typically they are
defined by the linker, and not every TU.

A good starting point for this kind of stuff is to look at what Clang does
on C code like this:

$ cat t.cpp
extern "C" int printf(const char *, ...);
extern int __start_my_section[];
extern int __stop_my_section[];
int __attribute__((section("my_section"))) a = 1;
int __attribute__((section("my_section"))) b = 2;
int main() {
  for (int *i = &__start_my_section[0], *e = &__stop_my_section[0]; i !=
e;
       ++i) {
    printf("%d ", *i);
  }
  printf("\n");
}

$ clang -cc1 t.cpp -emit-llvm -o -
... // omitted

Speaking of which, I believe both ld.bfd and ld.gold define
__start_my_section for you, which should make your life easier.
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On 05/08/14 19:31, Reid Kleckner wrote:

[section start/stop symbols]
> I think you want to declare those [0 x i32] globals instead of defining
> them by adding extern and dropping the empty brackets.  Typically they
> are defined by the linker, and not every TU.
In my case, the sections will be unique per compilation unit 
(translation unit?). I'm trying to add the start and stop symbols myself 
because at least on OS X, the linker doesn't appear to define any such 
symbols. Its man page mentions an option called "-sectobjectsymbols", 
but it's listed under the obsolete options (on OS X 10.9) and has as 
explanation "Adding a local label at a section start is no longer 
supported.  This option is obsolete."

[C example]
> Speaking of which, I believe both ld.bfd and ld.gold define
> __start_my_section for you, which should make your life easier.
For platforms using the GNU binutils, indeed. My original example was 
for Linux/x86-64, but if possible I'd like a solution that does not 
depend on the platform or used linker (other than the way how sections 
are defined, since e.g. on OS X you also have to specify a segment 
name). Maybe the easiest is to consciously just add dummy sentinels. Or 
maybe I can add the start and stop labels via module-level inline 
assembly...

Thanks,


Jonas