This bug-report is in fact a wishlist for an optimization described in GCC
manual, yet not implemented unfortunately (at least not for x86). About the
"internal" visibility of a symbol, the manual states: "By indicating that a
symbol cannot be called from outside the module, GCC may for instance omit the
load of a PIC register since it is known that the calling function loaded the
correct value."
This is a great idea, since loading the GOT register on x86 is a costly
operation. Even with "-march=pentium3" (it prevents the return address predictor
of the processor from going into lalaland because of the load), the PIC version
of the testcase still runs twice as slow. Although g() has already loaded the
GOT address in the %ebx callee-save register, f() will load it once again in
%ecx.
The optimization described for the "internal" visibility would prevent such a
reload, since GCC would have complete control over the callers. I did not see
anything in the psABI that would disallow such an optimization. Hence this
wishlist. This was tested with GCC 4.0.2 and compiled by "gcc -O -fPIC" (or
-fpic).
Testcase:
extern int a;
__attribute__((visibility("internal")))
void f(void) { ++a; }
void g(void) { a = 0; f(); }
Excerpt from the generated assembly code:
080483c9 <g>:
80483c9: 55 push %ebp
80483ca: 89 e5 mov %esp,%ebp
80483cc: 53 push %ebx
80483cd: e8 00 00 00 00 call 80483d2 <g+0x9> \
80483d2: 5b pop %ebx | first load
80483d3: 81 c3 2e 12 00 00 add $0x122e,%ebx /
80483d9: 8b 83 f8 ff ff ff mov 0xfffffff8(%ebx),%eax
80483df: c7 00 00 00 00 00 movl $0x0,(%eax)
80483e5: e8 c6 ff ff ff call 80483b0 <f>
...
080483b0 <f>:
80483b0: 55 push %ebp
80483b1: 89 e5 mov %esp,%ebp
80483b3: e8 00 00 00 00 call 80483b8 <f+0x8> \
80483b8: 59 pop %ecx | second load
80483b9: 81 c1 48 12 00 00 add $0x1248,%ecx /
80483bf: 8b 81 f8 ff ff ff mov 0xfffffff8(%ecx),%eax
80483c5: ff 00 incl (%eax)
80483c7: 5d pop %ebp
80483c8: c3 ret
Note: it is impossible to specify both the "internal" visibility and the
"static" qualifier (GCC complains). And using only "static" does not help here
either.
$ gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,java,f95,objc,ada,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib --enable-nls
--without-included-gettext --enable-threads=posix --program-suffix=-4.0
--enable-__cxa_atexit --enable-libstdcxx-allocator=mt --enable-clocale=gnu
--enable-libstdcxx-debug --enable-java-gc=boehm --enable-java-awt=gtk
--enable-gtk-cairo --with-java-home=/usr/lib/jvm/java-1.4.2-gcj-4.0-1.4.2.0/jre
--enable-mpfr --disable-werror --enable-checking=release i486-linux-gnu
Thread model: posix
gcc version 4.0.2 20050821 (prerelease) (Debian 4.0.1-6)
--
Summary: Missed optimization for PIC code with internal
visibility
Product: gcc
Version: 4.0.2
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: guillaume dot melquiond at ens-lyon dot fr
CC: gcc-bugs at gcc dot gnu dot org
GCC target triplet: i486-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23756
