Even when compiling with -O0, GCC uses non-standard parameter passing conventions for static inline functions, which makes debugging difficult.
In this case, the function of interest converts a tagged value to a pointer; it's very helpful to be able to use this in GDB 'print' commands. In the assembly code shown, notice that the function is expecting to find its argument in %eax, even though the standard IA-32 calling conventions pass the first argument on the stack. The GDB session also demonstrates this. $ (cd ~/gcc/gcc; svn info) Path: . URL: svn://gcc.gnu.org/svn/gcc/trunk Repository Root: svn://gcc.gnu.org/svn/gcc Repository UUID: 138bc75d-0d04-0410-961f-82ee72b054a4 Revision: 144927 Node Kind: directory Schedule: normal Last Changed Author: gccadmin Last Changed Rev: 144927 Last Changed Date: 2009-03-17 17:16:40 -0700 (Tue, 17 Mar 2009) $ cat jsobj.i extern "C" { static inline struct JSObject *JSVAL_TO_OBJECT (int v) { return (struct JSObject *) (v & ~3); } } static int obj_toSource (int *vp) { JSVAL_TO_OBJECT (vp[0]); } int main(int argc, char **argv) { } $ c++ -v -O0 -g jsobj.i -o jsobj -save-temps Using built-in specs. Target: i686-pc-linux-gnu Configured with: ../gcc/configure --prefix=/home/jimb/gcc --enable-languages=c,c++ Thread model: posix gcc version 4.4.0 20090318 (experimental) (GCC) COLLECT_GCC_OPTIONS='-v' '-O0' '-g' '-o' 'jsobj' '-save-temps' '-shared-libgcc' '-mtune=generic' /home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/cc1plus -fpreprocessed jsobj.i -quiet -dumpbase jsobj.i -mtune=generic -auxbase jsobj -g -O0 -version -o jsobj.s GNU C++ (GCC) version 4.4.0 20090318 (experimental) (i686-pc-linux-gnu) compiled by GNU C version 4.4.0 20090318 (experimental), GMP version 4.2.2, MPFR version 2.3.2. GGC heuristics: --param ggc-min-expand=30 --param ggc-min-heapsize=4096 Compiler executable checksum: 00b593c3c99bbacc875575ff2b6655cc COLLECT_GCC_OPTIONS='-v' '-O0' '-g' '-o' 'jsobj' '-save-temps' '-shared-libgcc' '-mtune=generic' as -V -Qy -o jsobj.o jsobj.s GNU assembler version 2.18.93 (i486-linux-gnu) using BFD version (GNU Binutils for Ubuntu) 2.18.93.20081009 COMPILER_PATH=/home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/:/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/ LIBRARY_PATH=/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/:/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-O0' '-g' '-o' 'jsobj' '-save-temps' '-shared-libgcc' '-mtune=generic' /home/jimb/gcc/libexec/gcc/i686-pc-linux-gnu/4.4.0/collect2 --eh-frame-hdr -m elf_i386 -dynamic-linker /lib/ld-linux.so.2 -o jsobj /usr/lib/crt1.o /usr/lib/crti.o /home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/crtbegin.o -L/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0 -L/home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/../../.. jsobj.o -lstdc++ -lm -lgcc_s -lgcc -lc -lgcc_s -lgcc /home/jimb/gcc/lib/gcc/i686-pc-linux-gnu/4.4.0/crtend.o /usr/lib/crtn.o $ cat jsobj.s ... JSVAL_TO_OBJECT: .LFB0: .file 1 "jsobj.i" .loc 1 4 0 .cfi_startproc .cfi_personality 0x0,__gxx_personality_v0 pushl %ebp .cfi_def_cfa_offset 8 movl %esp, %ebp .cfi_offset 5, -8 .cfi_def_cfa_register 5 subl $4, %esp movl %eax, -4(%ebp) .loc 1 5 0 movl -4(%ebp), %eax andl $-4, %eax .loc 1 6 0 leave ret ... $ gdb jsobj GNU gdb (GDB) 6.8.50.20090106-cvs Copyright (C) 2009 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "i686-pc-linux-gnu". For bug reporting instructions, please see: <http://www.gnu.org/software/gdb/bugs/>... (gdb) start Temporary breakpoint 1 at 0x804844c: file jsobj.i, line 16. Starting program: /home/jimb/mc/b/static-call/jsobj Temporary breakpoint 1, main (argc=1, argv=0xbffff8a4) at jsobj.i:16 16 } Current language: auto; currently c++ (gdb) set $eax = 0x42424242 (gdb) print JSVAL_TO_OBJECT(0x53535353) $1 = (struct JSObject *) 0x42424240 (gdb) quit The program is running. Quit anyway (and kill it)? (y or n) y $ -- Summary: GCC uses non-standard calling conventions, even with -O0 Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: jimb at red-bean dot com GCC build triplet: i686-pc-linux-gnu GCC host triplet: i686-pc-linux-gnu GCC target triplet: i686-pc-linux-gnu http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39496