[LLVMbugs] [Bug 12440] New: Runtime linker issue with X11R6 on i386 with -O3 optimization

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http://llvm.org/bugs/show_bug.cgi?id=12440

             Bug #: 12440
           Summary: Runtime linker issue with X11R6 on i386 with -O3
                    optimization
           Product: libraries
           Version: trunk
          Platform: PC
        OS/Version: OpenBSD
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: marco at peereboom.us
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Created attachment 8316
  --> http://llvm.org/bugs/attachment.cgi?id=8316
lib a .i file

I was told to get this issue in the bug tracker so here goes.

The executive summary is that with any optimization higher than -O0 clang
generates code that is incompatible with X11's assumptions.  When a TAILCALL is
generated it stuffs a function pointer into the GOT.  When a library is
dlopen'ed it fails because it can't resolve the GOT function name.

This looks like a bug to me because the assumption that a function call is
resolved late is widely used in code that does "modules" of sorts.

Code was compiled on OpenBSD with clang 3.0-release and clang 3.1 trunk. 

======================================================================== 
With -O0 which works as X expects: 
======================================================================== 
$ make clean 
rm -f a.o b.o liba.so libb.so app 
$ make CFLAGS=-O0 
clang -O0 -fpic -c a.c 
clang -shared -o liba.so a.o 
clang -O0 -fpic -c b.c 
clang -shared -o libb.so b.o 
clang -O0 -fpic app.c -o app 
$ ./app 
opening a 
opening b 
clang and X sitting in a tree :) 

The relevant bits of objdump -R 
$ objdump -R liba.so | grep ex_func 
2000210c R_386_JUMP_SLOT   ex_func 

Looking at the asm here shows us that ex_func is always looked up in the 
PLT. 
$ grep ex_func a.s 
        calll   ex_func at PLT 
        calll   ex_func at PLT 


======================================================================== 
With -O3 that doesn't work like X expects: 
======================================================================== 
$ make clean 
rm -f a.o b.o liba.so libb.so app 
$ make CFLAGS=-O3 
clang -O3 -fpic -c a.c 
clang -shared -o liba.so a.o 
clang -O3 -fpic -c b.c 
clang -shared -o libb.so b.o 
clang -O3 -fpic app.c -o app 
$ ./app 
opening a 
./app:./liba.so: undefined symbol 'ex_func' 
app: dlopen a Cannot load specified object 

The relevant bits of objdump -R 
$ objdump -R liba.so | grep ex_func 
2000211c R_386_GLOB_DAT    ex_func 
20002108 R_386_JUMP_SLOT   ex_func 

Now in the asm here we can see that in the TAILCALL case the ex_func is 
looked up in GOT. 
$ grep ex_func a.s         
        calll   ex_func at PLT 
        movl    ex_func at GOT(%esi), %eax 
... 
        jmpl    *%eax                   # TAILCALL 

See the attachments for the various temporary files and code.

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