On Wed, Jul 30, 2025 at 5:33 PM H.J. Lu <hjl.to...@gmail.com> wrote: > > According to the TLS spec, linker can optimize the TLS access by > overriding the TLS access model. Do the same in compiler by using > the default TLS mode if it is more optimized than the TLS model > attribute.
So reading the old bug reports specifically the comment here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78294#c18 . This is by design of the attribute. And should not be changed. And the problem is mismatch of the decls with and without the tls_model attribute and merging them. Jakub specifically brings up should we warn if they are mismatched? RIght now your patch does causes what the user asked for to be overridden. And there is no way to force the model and such. Also from the source of the glibc issue, it is a linker issue in a similar way of the gold issue was with https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78294 . Thanks, Andrew Pinski > > gcc/ > > PR c/121318 > * doc/extend.texi: Update the tls_model attribute. > > gcc/c-family/ > > PR c/121318 > * c-attribs.cc (handle_tls_model_attribute): Use the default TLS > mode if it is more optimized. > > gcc/testsuite/ > > PR c/121318 > * gcc.dg/tls/pr121318.c: New test. > > Signed-off-by: H.J. Lu <hjl.to...@gmail.com> > --- > gcc/c-family/c-attribs.cc | 4 ++++ > gcc/doc/extend.texi | 8 +++++++- > gcc/testsuite/gcc.dg/tls/pr121318.c | 16 ++++++++++++++++ > 3 files changed, 27 insertions(+), 1 deletion(-) > create mode 100644 gcc/testsuite/gcc.dg/tls/pr121318.c > > diff --git a/gcc/c-family/c-attribs.cc b/gcc/c-family/c-attribs.cc > index 1f4a0df1205..d49d00c7471 100644 > --- a/gcc/c-family/c-attribs.cc > +++ b/gcc/c-family/c-attribs.cc > @@ -3664,6 +3664,10 @@ handle_tls_model_attribute (tree *node, tree name, > tree args, > name, > "local-exec", "initial-exec", "local-dynamic", "global-dynamic"); > > + /* Use the default TLS mode if it is more optimized. */ > + tls_model default_kind = decl_default_tls_model (decl); > + if (default_kind > kind) > + kind = default_kind; > set_decl_tls_model (decl, kind); > return NULL_TREE; > } > diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi > index 224d6197d63..b204d1ce02c 100644 > --- a/gcc/doc/extend.texi > +++ b/gcc/doc/extend.texi > @@ -7528,7 +7528,13 @@ The @code{tls_model} attribute sets thread-local > storage model > overriding @option{-ftls-model=} command-line switch on a per-variable > basis. > The @var{tls_model} argument should be one of @code{global-dynamic}, > -@code{local-dynamic}, @code{initial-exec} or @code{local-exec}. > +@code{local-dynamic}, @code{initial-exec} or @code{local-exec}. The > +@code{tls_model} attribute specifies the least optimized @acronym{TLS} > +access model. GCC may choose a more optimized @acronym{TLS} access > +model, according to > +@uref{https://www.akkadia.org/drepper/tls.pdf, > +ELF Handling For Thread-Local Storage}, ignoring the @code{tls_model} > +attribute. > > Not all targets support this attribute. > > diff --git a/gcc/testsuite/gcc.dg/tls/pr121318.c > b/gcc/testsuite/gcc.dg/tls/pr121318.c > new file mode 100644 > index 00000000000..68378635f1d > --- /dev/null > +++ b/gcc/testsuite/gcc.dg/tls/pr121318.c > @@ -0,0 +1,16 @@ > +/* { dg-do compile { target { tls && pie } } } */ > +/* { dg-options "-O2 -fPIE" } */ > + > +extern const int afoo[3]; > + > +__thread const int *pfoo __attribute__ ((tls_model ("initial-exec"))) = afoo; > + > +const int ** > +ppfoo (void) > +{ > + return &pfoo; > +} > + > +/* { dg-final { scan-assembler "addq\[ \t\]\\\$pfoo@tpoff, %rax" { target { > { i?86-*-linux* x86_64-*-linux* } && { lp64 } } } } } */ > +/* { dg-final { scan-assembler "leal\[ \t\]pfoo@tpoff(%rax), %eax" { target > { { i?86-*-linux* x86_64-*-linux* } && { x32 } } } } } */ > +/* { dg-final { scan-assembler "leal\[ \t\]pfoo@ntpoff, %eax" { target { { > i?86-*-linux* x86_64-*-linux* } && { ia32 } } } } } */ > -- > 2.50.1 >
