[LLVMdev] Create "appending" section that can be partially dead stripped

[Jonas Maebe]

Hi,

Is there a way in llvm IR to emit multiple data elements within a  
single compilation unit that
a) are guaranteed to appear sequentially in the final binary (in the  
order they appear in the llvm IR), and
b) will be removed on an individual basis by the optimizers and/or  
linker in case they are not referenced from anywhere
?

Defining them as "appending" puts them all into a single section  
definition, even when compiling with -fdata-sections, so as soon as  
one of the symbols in that section is live, all of them will remain  
live.

Example:

***
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() {
   %1 = getelementptr [2 x i8]* @arr1, i64 0, i32 0
   %2 = load i8* %1
   %3 = zext i8 %2 to i32
   ret i32 %3
}

@arr1 = appending global [2 x i8] [
    i8 1,
    i8 2
], section "mytest"

@arr2 = appending global [2 x i8] [
    i8 3,
    i8 4
], section "mytest"
***

Since @arr2 is not referenced, I would like it to not appear in the  
final binary, but "clang -fdata-sections" generates:

***
         .type   arr1, at object            # @arr1
         .section        mytest,"aw", at progbits
         .globl  arr1
arr1:
         .ascii  "\001\002"
         .size   arr1, 2

         .type   arr2, at object            # @arr2
         .globl  arr2
arr2:
         .ascii  "\003\004"
         .size   arr2, 2
***

Alternatively, in case the "appending" linkage type implies "this data  
is also accessible via a getelementptr based on other data in that  
same section, and therefore must remain live", is there another way to  
achieve this?

Thanks,


Jonas