https://llvm.org/bugs/show_bug.cgi?id=30774
Bug ID: 30774
Summary: ThinLTO undefined ref due to imported DIGlobalVariable
reference at -O0
Product: new-bugs
Version: 3.9
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: unassignedb...@nondot.org
Reporter: tejohn...@google.com
CC: jbc.enge...@gmail.com, llvm-bugs@lists.llvm.org,
mehdi.am...@apple.com, pe...@pcc.me.uk,
tejohn...@google.com
Classification: Unclassified
Johan reported an issue hit using the 3.9 compiler and ThinLTO, when building
with -O0 -g. The issue occurs when importing from a module with a const global
variable - while we don't import the global variable definition, we do import
the debug metadata, and in the 3.9 timeframe the DIGlobalVariable metadata
still contained a reference to the associated GlobalVariable. At -O2 the GV
declaration is eliminated as is the reference from the DIGlobalVariable, but at
-O0 we get an undefined reference linker error.
This doesn't occur at head due to pcc's change to reverse the
GlobalVariable/DIGlobalVariable edge (https://reviews.llvm.org/rL281284).
In reality, we shouldn't bother doing any importing at -O0 - inlining is
disabled, so this is just wasted work. My plan is to disable importing and
other ThinLTO optimizations at -O0, both in trunk and in 3.9 (which will fix
this 3.9 issue).
To reproduce:
$ cat global_main.c
extern int foo();
int main() {
return foo();
}
$ cat global.c
const int foobar = 10;
int foo() {
return 1;
}
$ clang -c -O2 global.c global_main.c -flto=thin -g
$ clang global.o global_main.o -flto=thin -g -O0
/tmp/lto-llvm-14880f.o(.debug_info+0x6f): error: undefined reference to
'foobar'
clang-3.9: error: linker command failed with exit code 1 (use -v to see
invocation)
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