[gPXE-devel] Weak pxe_menu_boot()

[Stefan Hajnoczi]

Here is the objdump -dr bin/autoboot.o output we want (using the glibc
weak alias technique):

20e:   e8 fc ff ff ff          call   20f <netboot+0xe2>
                        20f: R_386_PC32 printf
 213:   89 f0                   mov    %esi,%eax
 215:   e8 fc ff ff ff          call   216 <netboot+0xe9>
                        216: R_386_PC32 pxe_menu_boot

Notice that pxe_menu_boot is a relocation because at this point the
toolchain doesn't know whether or not to use the weak symbol yet.

Here is the broken objdump -dr bin/autoboot.o output we currently have
in gpxe.git:

00000000 <pxe_menu_boot>:
   0:   b8 fd 1f fc c3          mov    $0xc3fc1ffd,%eax
   5:   c3                      ret
[...]
20d:   e8 fc ff ff ff          call   20e <netboot+0xe1>
                        20e: R_386_PC32 printf
 212:   bf fd 1f fc c3          mov    $0xc3fc1ffd,%edi

The compiler has inlined pxe_menu_boot, there is no relocation!

Here is what readelf -a bin/autoboot.o says about the broken autoboot.o:
   Num:    Value  Size Type    Bind   Vis      Ndx Name
    23: 00000000     6 FUNC    WEAK   HIDDEN    1 pxe_menu_boot

I don't see anything particularly interesting in this readelf output
but I put it here for completeness.

So it seems that gcc inlined the function when it shouldn't have.
Here is what happens when CFLAGS += -Os is changed to CFLAGS += -O in
Makefile.housekeeping:

2ad:   e8 fc ff ff ff          call   2ae <netboot+0xf0>
                        2ae: R_386_PC32 printf
 2b2:   89 f0                   mov    %esi,%eax
 2b4:   e8 fc ff ff ff          call   2b5 <netboot+0xf7>
                        2b5: R_386_PC32 pxe_menu_boot

Suddenly there is a relocation for pxe_menu_boot again!

Compiler bug or do we not understand __attribute__ ((weak)) properly?
The plot thickens...

Stefan