[PATCH] MIPS/Gas: Support .L/$ as the mark of local symbol

[PATCH] MIPS/Gas: Support .L/$ as the mark of local symbol

[YunQiang Su]

In as.texi, there are lines:
  A local symbol is any symbol beginning with certain local
  label prefixes. By default, the local label prefix is
  @samp{.L} for ELF systems or @samp{L} for traditional a.out
  systems, but each target may have its own set of local
  label prefixes.

Let's support it for MIPS:
1) For OldABI, GCC uses "$" to mark local symbols, and
   for NewABI, ".L" is used. So let's support both for
   OldABI, and ".L" only for NewABI.
2) Emit an fatal error, if a local symbol is used, while
   not defined, just like LLVM does.
---
 gas/config/tc-mips.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/gas/config/tc-mips.c b/gas/config/tc-mips.c
index 43c12de2c8a..a84b13e1815 100644
--- a/gas/config/tc-mips.c
+++ b/gas/config/tc-mips.c
@@ -17725,6 +17725,16 @@ pic_need_relax (symbolS *sym)
   if (symbol_section_p (sym))
     return true;
 
+  if (strstr (S_GET_NAME (sym), ".L") == S_GET_NAME (sym)
+      || ((mips_abi == O64_ABI || mips_abi == O32_ABI)
+	   && strstr (S_GET_NAME (sym), "$") == S_GET_NAME (sym)))
+    {
+      if (!S_IS_DEFINED (sym))
+	as_fatal (_("undefined temporary symbol: %s"),
+		  S_GET_NAME (sym));
+      return true;
+    }
+
   symsec = S_GET_SEGMENT (sym);
 
   /* This must duplicate the test in adjust_reloc_syms.  */
-- 
2.39.2

Re: [PATCH] MIPS/Gas: Support .L/$ as the mark of local symbol

[YunQiang Su]

YunQiang Su <syq@gcc.gnu.org> 于2024年2月5日周一 18:14写道：
>
> In as.texi, there are lines:
>   A local symbol is any symbol beginning with certain local
>   label prefixes. By default, the local label prefix is
>   @samp{.L} for ELF systems or @samp{L} for traditional a.out
>   systems, but each target may have its own set of local
>   label prefixes.
>
> Let's support it for MIPS:
> 1) For OldABI, GCC uses "$" to mark local symbols, and
>    for NewABI, ".L" is used. So let's support both for
>    OldABI, and ".L" only for NewABI.
> 2) Emit an fatal error, if a local symbol is used, while
>    not defined, just like LLVM does.

@Maciej W. Rozycki ping.

> ---
>  gas/config/tc-mips.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
>
> diff --git a/gas/config/tc-mips.c b/gas/config/tc-mips.c
> index 43c12de2c8a..a84b13e1815 100644
> --- a/gas/config/tc-mips.c
> +++ b/gas/config/tc-mips.c
> @@ -17725,6 +17725,16 @@ pic_need_relax (symbolS *sym)
>    if (symbol_section_p (sym))
>      return true;
>
> +  if (strstr (S_GET_NAME (sym), ".L") == S_GET_NAME (sym)
> +      || ((mips_abi == O64_ABI || mips_abi == O32_ABI)
> +          && strstr (S_GET_NAME (sym), "$") == S_GET_NAME (sym)))
> +    {
> +      if (!S_IS_DEFINED (sym))
> +       as_fatal (_("undefined temporary symbol: %s"),
> +                 S_GET_NAME (sym));
> +      return true;
> +    }
> +
>    symsec = S_GET_SEGMENT (sym);
>
>    /* This must duplicate the test in adjust_reloc_syms.  */
> --
> 2.39.2
>

Re: [PATCH] MIPS/Gas: Support .L/$ as the mark of local symbol

[Maciej W. Rozycki]

On Mon, 5 Feb 2024, YunQiang Su wrote:

> In as.texi, there are lines:
>   A local symbol is any symbol beginning with certain local
>   label prefixes. By default, the local label prefix is
>   @samp{.L} for ELF systems or @samp{L} for traditional a.out
>   systems, but each target may have its own set of local
>   label prefixes.
> 
> Let's support it for MIPS:
> 1) For OldABI, GCC uses "$" to mark local symbols, and
>    for NewABI, ".L" is used. So let's support both for
>    OldABI, and ".L" only for NewABI.
> 2) Emit an fatal error, if a local symbol is used, while
>    not defined, just like LLVM does.

 I gather you want to avoid a case of an undefined reference making it to 
output for a symbol whose name suggests its a local label, right?

 But that seems like material for the generic part of GAS rather than the 
MIPS backend only (and then specifically SVR4 PIC only and not non-PIC or 
VxWorks, according to your code as submitted), so that all targets behave 
consistently, and I can't see e.g. i386 GAS doing it.

 So a question raises: what problem are you trying to solve?  Can you give 
us a piece of code that you think is handled incorrectly by GAS, and why?

  Maciej

Re: [PATCH] MIPS/Gas: Support .L/$ as the mark of local symbol

[YunQiang Su]

Maciej W. Rozycki <macro@orcam.me.uk> 于2024年2月21日周三 21:33写道：
>
> On Mon, 5 Feb 2024, YunQiang Su wrote:
>
> > In as.texi, there are lines:
> >   A local symbol is any symbol beginning with certain local
> >   label prefixes. By default, the local label prefix is
> >   @samp{.L} for ELF systems or @samp{L} for traditional a.out
> >   systems, but each target may have its own set of local
> >   label prefixes.
> >
> > Let's support it for MIPS:
> > 1) For OldABI, GCC uses "$" to mark local symbols, and
> >    for NewABI, ".L" is used. So let's support both for
> >    OldABI, and ".L" only for NewABI.
> > 2) Emit an fatal error, if a local symbol is used, while
> >    not defined, just like LLVM does.
>
>  I gather you want to avoid a case of an undefined reference making it to
> output for a symbol whose name suggests its a local label, right?
>
>  But that seems like material for the generic part of GAS rather than the
> MIPS backend only (and then specifically SVR4 PIC only and not non-PIC or
> VxWorks, according to your code as submitted), so that all targets behave
> consistently, and I can't see e.g. i386 GAS doing it.
>
>  So a question raises: what problem are you trying to solve?  Can you give
> us a piece of code that you think is handled incorrectly by GAS, and why?
>

main:
     la $4,$hello

$ mipsel-linux-gnu-as -KPIC tt.s && objdump -dr a.out

00000000 <main>:
   0: 8f840000 lw a0,0(gp)
0: R_MIPS_GOT16 $hello

In this case, GAS treat $hello as an external symbol, so only a single
R_MIPS_GOT16 is used.

>   Maciej

Re: [PATCH] MIPS/Gas: Support .L/$ as the mark of local symbol

[Maciej W. Rozycki]

On Wed, 21 Feb 2024, YunQiang Su wrote:

> > > In as.texi, there are lines:
> > >   A local symbol is any symbol beginning with certain local
> > >   label prefixes. By default, the local label prefix is
> > >   @samp{.L} for ELF systems or @samp{L} for traditional a.out
> > >   systems, but each target may have its own set of local
> > >   label prefixes.
> > >
> > > Let's support it for MIPS:
> > > 1) For OldABI, GCC uses "$" to mark local symbols, and
> > >    for NewABI, ".L" is used. So let's support both for
> > >    OldABI, and ".L" only for NewABI.
> > > 2) Emit an fatal error, if a local symbol is used, while
> > >    not defined, just like LLVM does.
> >
> >  I gather you want to avoid a case of an undefined reference making it to
> > output for a symbol whose name suggests its a local label, right?
> >
> >  But that seems like material for the generic part of GAS rather than the
> > MIPS backend only (and then specifically SVR4 PIC only and not non-PIC or
> > VxWorks, according to your code as submitted), so that all targets behave
> > consistently, and I can't see e.g. i386 GAS doing it.
> >
> >  So a question raises: what problem are you trying to solve?  Can you give
> > us a piece of code that you think is handled incorrectly by GAS, and why?
> >
> 
> main:
>      la $4,$hello
> 
> $ mipsel-linux-gnu-as -KPIC tt.s && objdump -dr a.out
> 
> 00000000 <main>:
>    0: 8f840000 lw a0,0(gp)
> 0: R_MIPS_GOT16 $hello
> 
> In this case, GAS treat $hello as an external symbol, so only a single
> R_MIPS_GOT16 is used.

 So that's indeed a waste of a GOT entry if `$hello' does end up local in 
the link, but I fail to see it being a problem itself.  It's no different 
from where you have forced an ordinary symbol local via a linker script 
really: it will have retained its individual GOT entry even though it 
cannot be preempted anymore and could well use a shared entry with an 
offset.

 If you're concerned as to the piece of GAS documentation you quoted, then 
refer to the paragraph that follows:

  "Local symbols are defined and used within the assembler, but they are 
normally not saved in object files.  Thus, they are not visible when 
debugging.  You may use the `-L' option (see Section 2.7 [Include Local 
Symbols], page 26) to retain the local symbols in the object files."

-- which clarifies that this is specifically about symbols automatically 
produced internally by GAS for its own use, e.g. referring to `.' will 
emit such a symbol.  In addition to the prefix described in the previous 
paragraph they use a non-printable character immediately following so as 
to make sure they don't clash with a symbol provided by assembly code 
supplied, cf. LOCAL_LABEL_CHAR.

 Then symbols supplied by assembly code are interpreted according to their 
attributes and an undefined symbol is necessarily global.  If the supplier 
of code produced non-compliant code, then the GIGO principle applies, GAS 
just does what it's been told to, just as will say a C compiler when you 
use a symbol from a reserved namespace in ordinary user code.  Tools must 
not stand in the way.

 If you think a warning would be useful here, then I guess that might be 
acceptable, but as I say that would have to go into a generic part of GAS.

  Maciej