[ELF] Improve --icf=safe diagnostic

[MaskRay]

Summary

The current diagnostic has confused users. The new wording is adapted from one suggested by Ian Lance Taylor.

[jhenderson]

Seems reasonable to me. I gues my only question is: will most users understand what the address-significance table is?

lld/ELF/InputFiles.cpp

596

This should probably use the action section name, rather than hard-coding it in, since there's no requirement that SHT_LLVM_ADDRSIG sections be a specific name, to my knowledge. The section index might also be good. See also the describe method used by llvm-readobj for how we report issues with sections there.

[MaskRay]

comment

[jhenderson]

Looks good to me.

lld/ELF/InputFiles.cpp

if ((sec.sh_flags & SHF_EXCLUDE) && !config->relocatable) {
       if (sec.sh_type == SHT_LLVM_ADDRSIG) {
         // We ignore the address-significance table if we know that the object
         // file was created by objcopy or ld -r. This is because these tools
         // will reorder the symbols in the symbol table, invalidating the data
         // in the address-significance table, which refers to symbols by index.
         if (sec.sh_link != 0)
           this->addrsigSec = &sec;
         else if (config->icf == ICFLevel::Safe)
-          warn(toString(this) + ": --icf=safe is incompatible with object "
-                                "files created using objcopy or ld -r");
+          warn(toString(this) +
+               ": --icf=safe conservatively ignores "
+               "SHT_LLVM_ADDRSIG [index " +
+               Twine(i) +
+               "] with sh_link=0 "
+               "(likely created using objcopy or ld -r)");
       }
       this->sections[i] = &InputSection::discarded;
       continue;
     }
 
     switch (sec.sh_type) {
     case SHT_GROUP: {
       // De-duplicate section groups by their signatures.

lld/test/ELF/icf-safe.s

 # ALL-EXPORT:   removing identical section {{.*}}:(.rodata.l4)
 # ALL-EXPORT-NOT: selected section
 # ALL-EXPORT: selected section {{.*}}:(.text.f1)
 # ALL-EXPORT:   removing identical section {{.*}}:(.text.f2)
 # ALL-EXPORT:   removing identical section {{.*}}:(.text.non_addrsig1)
 # ALL-EXPORT:   removing identical section {{.*}}:(.text.non_addrsig2)
 # ALL-EXPORT-NOT: selected section
 
-# OBJCOPY: --icf=safe is incompatible with object files created using objcopy or ld -r
+# OBJCOPY: --icf=safe conservatively ignores SHT_LLVM_ADDRSIG [index [[#]]] with sh_link=0 (likely created using objcopy or ld -r)
 
 .section .text.f1,"ax",@progbits
 .globl f1
 f1:
 ret
 
 .section .text.f2,"ax",@progbits
 .globl f2