I saw a "Relocation to discarded section" warning in lld from .gcc_except_table to a COMDAT basic block section and this patch makes lld ignore this.
With --emit-relocs, lld safely ignores such relocations: https://github.com/llvm/llvm-project/blob/master/lld/ELF/InputSection.cpp#L453 whereas it doesn't do so in the normal case of scan relocations : https://github.com/llvm/llvm-project/blob/master/lld/ELF/Relocations.cpp#L966
I ran into this problem with basic block sections. Here is a repro for the problem and I have attached a very condensed test that pretty much does the same thing.
foo.cc
#include <iostream>
using namespace std;
inline int foo () {
try {
foo();
} catch (const char* msg) {
}
return 0;}
int bar();
int main() {
cerr << &foo; return 0;
}
bar.cc
#include <iostream>
using namespace std;
inline int foo() {
return 0;
}
int bar() {
cerr << &foo; return 0;
}
Now,
$ clang++ -fbasic-block-sections=all -funique-basic-block-section-names -c foo,cc
$ clang++ -fbasic-block-sections=all -funique-basic-block-section-names -c bar.cc
Now link with bar.o before foo.o
$ clang++ -fuse-ld=lld bar.o foo.o
error: relocation refers to a discarded section: .text._Z3foov._Z3foov.2
foo.o(gcc_except_table)
Reason is that foo.o uses exceptions and .gcc_except_table has a relocation pointing to a bb section there whereas the COMDAT copy in bar.o does not. Since that is kept, we see the error. gcc_except_table cannot be placed in the COMDAT group so this can be safely ignored.
Infact, the code in copy relocations shows that it can be ignored safely.