gcc-11-20210515 is now available

2021-05-15 Thread GCC Administrator via Gcc 
Snapshot gcc-11-20210515 is now available on
  https://gcc.gnu.org/pub/gcc/snapshots/11-20210515/
and on various mirrors, see http://gcc.gnu.org/mirrors.html for details.

This snapshot has been generated from the GCC 11 git branch
with the following options: git://gcc.gnu.org/git/gcc.git branch 
releases/gcc-11 revision 3f3bcc115c877500851d55bf81dcfa37f6aaf085

You'll find:

 gcc-11-20210515.tar.xz   Complete GCC

  SHA256=358bf9256f2509a78e88d596db959fbae1e860d23ce9e09db882688047d088dc
  SHA1=5c82153fd49a67c23558a38342dc7fcfef55

Diffs from 11-20210508 are available in the diffs/ subdirectory.

When a particular snapshot is ready for public consumption the LATEST-11
link is updated and a message is sent to the gcc list.  Please do not use
a snapshot before it has been announced that way.

ELF world with default STB_GLOBAL -Bsymbolic-functions

2021-05-15 Thread Fangrui Song 

Hi, elves (people who work on ELF)!

Motivated by a recent rant from Linus Torvalds on shared objects' performance
issues and a previous post about CPython 1.3x speedup with
-fno-semantic-interposition 1.3x, I have thought about an ELF world with
STB_GLOBAL variant of -Bsymbolic-functions by default recently and filed some
GCC/binutils feature requests.

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98112 
-fno-direct-access-access-external-data for -fno-pic
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100593 -fno-pic: Use GOT to take 
address of an external default visibility function
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100618 
-fsemantic-interposition=variable
https://sourceware.org/bugzilla/show_bug.cgi?id=27871 ld: Add 
-Bsymbolic-global-functions which only applies to STB_GLOBAL STT_FUNC
Pending patch: remove HAVE_LD_PIE_COPYRELOC: 
https://gcc.gnu.org/pipermail/gcc-patches/2021-May/570139.html

I have written down my thoughts and a plan. I'll hereby humbly refer you to
https://maskray.me/blog/2021-05-16-elf-interposition-and-bsymbolic#the-last-alliance-of-elf-and-men

For your convenience, I have attached the paragraph.


I think distributions should default to a function-only variant of
`-fno-semantic-interposition` and (in the long term) a `STB_GLOBAL` variant of
`-Wl,-Bsymbolic-functions`, bringing back the lost performance for decades.

I wish that distributions default to a function-only variant of
-fno-semantic-interposition and (in the long term) a STB_GLOBAL variant of
-Wl,-Bsymbolic-functions, bringing back the lost performance for decades. macOS
(Mach-O), Windows (PE-COFF), and Solaris (ELF) direct bindings have set up
precedent so there is a good chance that most pieces of portable software are
already in a good state.

However, there is still some amount of work needed to annotate software which
cannot be built with -fsemantic-interposition=variable or
-Wl,-Bsymbolic-global-functions. Distributions have to put into resources. In
return, I estimate that many pieces of software may be 5% to 20% faster
(CPython is 1.3x faster) and a few percentage smaller in size.

There is a trade-off and the downside is that LD_PRELOAD replacing a function
definition in a shared object will be a non-default choice. The users can build
the software by themselves. (Note: malloc replacement/fakeroot are still 
supported).

We need an option (say, -fsemantic-interposition=variable) to disable
interposition for functions but enable interposition for variables, because we
want to be compatible with copy relocations, which will require years to fix.
GCC feature request: PR100618.

We may need a configure-time option for default
-fsemantic-interposition=variable, like GCC's --enable-default-pie.

We need a -Bsymbolic-functions variant which only applies to STB_GLOBAL symbols
(i.e. STB_WEAK symbols are excluded). {-Bsymbolic-global-functions}

We need a linker option to cancel default -Bsymbolic-global-functions. I have
added -Bno-symbolic to GNU ld and gold (binutils 2.37; PR27834) and ld.lld 13.

(From Peter Smith) The linker can introduce a debugging option for executables
to catch accidental interposition, say, --warn-interposition: "Warning symbol S
of type STT_FUNC is defined in executable A and shared objects B and C, using
definition in A."

GCC -fno-pic should be fixed to use GOT to take the address of an external
default visibility function. PR100593. {-fno-pic_got}

TBAA

2021-05-15 Thread Uecker, Martin 


Hi Richard,

I noticed that GCC 11 has different behavior in the following
example relative to 10.2 with -O2. I wonder whether this
is an intentional change and if yes, what are the rules?

Thanks!
Martin

https://godbolt.org/z/57res7ax1

#include 
#include 


__attribute__((__noinline__, __weak__))
void f(long unk, void *pa, void *pa2, void *pb, long *x) {
  for (long i = 0; i < unk; ++i) {
int oldy = *(int *)pa;
*(float *)pb = 42;
*(int *)pa2 = oldy ^ x[i];
  }
}

int main(void) {
  void *p = malloc(sizeof(int));
  *(int *)p = 13;
  f(1, p, p, p, (long []){ 0 });
  printf("*pa(%d)\n", *(int *)p);
}