Should gcc builtins always be resolved during the compilation step, or the linker step? - gcc

Question

In running gcc 3.4.3 on a Solaris 5.11 box I see that builtin functions are left undefined during compilation, but are resolved against libgcc.a when the Solaris linker links against libgcc.a.
On gcc 4.5.2 on another Solaris box, the same builtin functions are resolved during compilation of the .o, and the linker isn't concerned about them.
A boiled down example file compiled on gcc 3.4.3:
# cat ctzll.c
int func(unsigned long long x)
{
return __builtin_ctzll(x);
}
# cat main.c
int main(void)
{
return func(0xc0ffee);
}
First compile ctzll.c, and check the symbols:
# nm ctzll.o
U __ctzdi2
0000000000000000 T func
Now compile main.c, and link the objects:
# gcc -m64 main.c -c
# gcc -m64 ctzll.o main.o -v
Reading specs from /usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/specs
Configured with: /build/i386/components/gcc3/gcc-3.4.3/configure --prefix=/usr/sfw --mandir=/usr/sfw/share/man --infodir=/usr/sfw/share/info --without-gnu-ld --with-ld=/usr/bin/ld --enable-languages=c,c++,f77,objc --enable-shared --with-gnu-as --with-as=/usr/gnu/bin/as
Thread model: posix
gcc version 3.4.3 (csl-sol210-3_4-20050802)
/usr/sfw/libexec/gcc/i386-pc-solaris2.11/3.4.3/collect2 -V -Y P,/lib/64:/usr/lib/64:/usr/sfw/lib/64 -R /lib/64:/usr/lib/64:/usr/sfw/lib/64 -Qy /usr/lib/amd64/crt1.o /usr/lib/amd64/crti.o /usr/lib/amd64/values-Xa.o /usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/amd64/crtbegin.o -L/usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/amd64 -L/usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/../../../amd64 -L/lib/amd64 -L/usr/lib/amd64 ctzll.o main.o -lgcc -lgcc_eh -lc -lgcc -lgcc_eh /usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/amd64/crtend.o /usr/lib/amd64/crtn.o
ld: Software Generation Utilities - Solaris Link Editors: 5.11-1.2276
# nm a.out | grep ctz
0000000000400fd0 T __ctzdi2
So, if I understand correctly, the Solaris linker resolved __ctzdi2 (the internal representation of __builtin_ctzll).
Now, compile with gcc 4.5.2 on the other Solaris machine:
#gcc -m64 ctzll.c -c
# nm ctzll.o
0000000000000000 T func
The symbol has been resolved fine in the object file, and it has been inlined into the .o assembly as:
8: 48 0f bc 45 f8 bsf -0x8(%rbp),%rax
Is the 3.4.3 compiler behaving correctly? I'd have expected the actual compilation to handle the builtins, like 4.5.2, by referencing the 64 bit version of libgcc.a. The lack of uniformity across the compilers is causing upstream problems in my project as the builtin remains undefined and the linker isn't resolving the symbols as I'm not linking against the OS specific 64 bit libraries (libgcc.a). I'm not sure if the 3.4.3 compiler is misconfigured causing the .o files to have undefined builtin functions caught by linking, or if the newer compilers are just smarter and I need to add the 64 bit libraries to the linker to handler the older compiler.
The 3.4.3 seems to show a valid libgcc.a which contains the definition of _ctzdi2:
# gcc -m64 -print-libgcc-file-name
/usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/amd64/libgcc.a
# nm /usr/sfw/lib/gcc/i386-pc-solaris2.11/3.4.3/amd64/libgcc.a | grep ctzdi2
0000000000000000 T __ctzdi2
_ctzdi2.o:

Answer 1

The whole point of libgcc.a is to implement builtin operations that gcc can't emit code for (which includes not only __builtin_ functions, but also things like 'long long' math operations on some 32-bit platforms).
Obviously the newer gcc got smarter about emitting code.