[llvm-dev] [ELF] String literals don't obey -fdata-sections

[Gaël Jobin via llvm-dev]

Hi there, 

When I compile my code with -fdata-sections and -ffunction-sections, I
still see some unused string in my shared library (Android). Actually,
the strings appear together inside a .rodata.str1.1 section instead of
getting their own section. It seems that the C-string literal are
considered differently than other constant and the -fdata-sections is
not respected in
https://github.com/llvm/llvm-project/blob/master/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp#L799.
[1]  I came across the following GCC bug
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=192 where they have fixed
the issue back in 2015. Any reason not to do so in LLVM? 

My code example:
- static library 1 : expose functions api1() and api3()

> #include "lib1.h" 
> 
> static char *test = "Test";
> static char *unused = "Unused"; 
> 
> void api1(){
> printf(test);
> } 
> 
> void api3(){
> printf(unused);
> }

- shared library : use only function api1() from static library 1

> #include "lib1.h" 
> 
> void test(){
> api1();
> }

Both compiled with "-fdata-sections -ffunction-sections
-fvisibility=hidden" and linked with "--gc-sections". 

While the api3() function is correctly gone, the result for the C-string
is the following (in Hopper):

> ; Section .rodata.str1.1 
> 
> ; Range: [0x63; 0x6f[ (12 bytes) 
> 
> ; File offset : [151; 163[ (12 bytes) 
> 
> ; Flags: 0x32 
> 
> ;   SHT_PROGBITS 
> 
> ;   SHF_ALLOC 
> 
> .L.str: 
> 
> 00000063         db         "Test", 0 
> 
> .L.str.1: 
> 
> 00000068         db         "Unused", 0

 

Links:
------
[1]
https://github.com/llvm/llvm-project/blob/master/llvm/lib/CodeGen/TargetLoweringObjectFileImpl.cpp#L799
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