[llvm-commits] [llvm] r154124 - /llvm/trunk/tools/lto/LTOCodeGenerator.cpp

[Bill Wendling]

On Apr 6, 2012, at 12:34 AM, Duncan Sands <baldrick at free.fr> wrote:

> Hi Bill,
> 
>> The internalize pass can be dangerous for LTO.
> 
> I'm kind of confused by your examples.  In your first example
> 
>> Consider the following program:
>> 
>> $ cat main.c
>> void foo(void) { }
>> 
>> int main(int argc, char *argv[]) {
>>     foo();
>>     return 0;
>> }
>> $ cat bundle.c
>> extern void foo(void);
>> 
>> void bar(void) {
>>      foo();
>> }
>> $ clang -o main main.c
>> $ clang -o bundle.so bundle.c -bundle -bundle_loader ./main
>> $ nm -m bundle.so
>> 0000000000000f40 (__TEXT,__text) external _bar
>>                  (undefined) external _foo (from executable)
>>                  (undefined) external dyld_stub_binder (from libSystem)
>> $ clang -o main main.c -O4
> 
> (^ If this is running internalize then that's a bug.  But it doesn't, right?)
> 
No. It's only run during linking.

>> $ clang -o bundle.so bundle.c -bundle -bundle_loader ./main
>> Undefined symbols for architecture x86_64:
>>   "_foo", referenced from:
>>       _bar in bundle-elQN6d.o
>> ld: symbol(s) not found for architecture x86_64
>> clang: error: linker command failed with exit code 1 (use -v to see invocation)
> 
> This looks like a bug: the linker should observe that bundle uses the symbol foo
> and that main provides the symbol foo, and pass "foo" to the internalize pass in
> the list of external symbols.  I think this is how the gold plugin does it. It's
> not a reason to turn off internalize altogether.  Hopefully Rafael can comment.
> 
The darwin linker uses export lists for these purposes. If the gold plugin automagically detects it, all the better.

>> The linker was told that the 'foo' in 'main' was 'internal' and had no uses, so
>> it was dead stripped.
>> 
>> Another situation is something like:
>> 
>> define void @foo() {
>>   ret void
>> }
>> 
>> define void @bar() {
>>   call asm volatile "call _foo" ...
>>   ret void
>> }
>> 
>> The only use of 'foo' is inside of an inline ASM call. Since we don't look
>> inside those for uses of functions, we don't specify this as a "use."
> 
> This is user error: the user should add a "used" attribute to foo.  This isn't
> anything new: lots of people make this mistake (it's come up again and again on
> the mailing list and in bugzilla).  The user should be educated here.
> 
I've sent the user an email about this. :)

-bw