Re: [PATCH V2] place `const volatile' objects in read-only sections

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

ping

> [Changes from V1:
> - Added a test.]
>
> It is common for C BPF programs to use variables that are implicitly
> set by the BPF loader and run-time.  It is also necessary for these
> variables to be stored in read-only storage so the BPF verifier
> recognizes them as such.  This leads to declarations using both
> `const' and `volatile' qualifiers, like this:
>
>   const volatile unsigned char is_allow_list = 0;
>
> Where `volatile' is used to avoid the compiler to optimize out the
> variable, or turn it into a constant, and `const' to make sure it is
> placed in .rodata.
>
> Now, it happens that:
>
> - GCC places `const volatile' objects in the .data section, under the
>   assumption that `volatile' somehow voids the `const'.
>
> - LLVM places `const volatile' objects in .rodata, under the
>   assumption that `volatile' is orthogonal to `const'.
>
> So there is a divergence, that has practical consequences: it makes
> BPF programs compiled with GCC to not work properly.
>
> When looking into this, I found this bugzilla:
>
>   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=25521
>   "change semantics of const volatile variables"
>
> which was filed back in 2005, long ago.  This report was already
> asking to put `const volatile' objects in .rodata, questioning the
> current behavior.
>
> While discussing this in the #gcc IRC channel I was pointed out to the
> following excerpt from the C18 spec:
>
>    6.7.3 Type qualifiers / 5 The properties associated with qualified
>          types are meaningful only for expressions that are
>          lval-values [note 135]
>
>    135) The implementation may place a const object that is not
>         volatile in a read-only region of storage. Moreover, the
>         implementation need not allocate storage for such an object if
>         its $ address is never used.
>
> This footnote may be interpreted as if const objects that are volatile
> shouldn't be put in read-only storage.  Even if I personally was not
> very convinced of that interpretation (see my earlier comment in BZ
> 25521) I filed the following issue in the LLVM tracker in order to
> discuss the matter:
>
>   https://github.com/llvm/llvm-project/issues/56468
>
> As you can see, Aaron Ballman, one of the LLVM hackers, asked the WG14
> reflectors about this.  He reported that the reflectors don't think
> footnote 135 has any normative value.
>
> So, not having a normative mandate on either direction, there are two
> options:
>
> a) To change GCC to place `const volatile' objects in .rodata instead
>    of .data.
>
> b) To change LLVM to place `const volatile' objects in .data instead
>    of .rodata.
>
> Considering that:
>
> - One target (bpf-unknown-none) breaks with the current GCC behavior.
>
> - No target/platform relies on the GCC behavior, that we know.
>
> - Changing the LLVM behavior at this point would be very severely
>   traumatic for the BPF people and their users.
>
> I think the right thing to do at this point is a).
> Therefore this patch.
>
> Regtested in x86_64-linux-gnu and bpf-unknown-none.
> No regressions observed.
>
> gcc/ChangeLog:
>
> 	PR middle-end/25521
> 	* varasm.cc (categorize_decl_for_section): Place `const volatile'
> 	objects in read-only sections.
> 	(default_select_section): Likewise.
>
> gcc/testsuite/ChangeLog:
>
> 	PR middle-end/25521
> 	* lib/target-supports.exp (check_effective_target_elf): Define.
> 	* gcc.dg/pr25521.c: New test.
> ---
>  gcc/testsuite/gcc.dg/pr25521.c        | 10 ++++++++++
>  gcc/testsuite/lib/target-supports.exp | 10 ++++++++++
>  gcc/varasm.cc                         |  3 ---
>  3 files changed, 20 insertions(+), 3 deletions(-)
>  create mode 100644 gcc/testsuite/gcc.dg/pr25521.c
>
> diff --git a/gcc/testsuite/gcc.dg/pr25521.c b/gcc/testsuite/gcc.dg/pr25521.c
> new file mode 100644
> index 00000000000..74fe2ae6626
> --- /dev/null
> +++ b/gcc/testsuite/gcc.dg/pr25521.c
> @@ -0,0 +1,10 @@
> +/* PR middle-end/25521 - place `const volatile' objects in read-only
> +   sections.
> +
> +   { dg-require-effective-target elf }
> +   { dg-do compile } */
> +
> +const volatile int foo = 30;
> +
> +
> +/* { dg-final { scan-assembler "\\.rodata" } } */
> diff --git a/gcc/testsuite/lib/target-supports.exp b/gcc/testsuite/lib/target-supports.exp
> index 04a2a8e8659..c663d59264b 100644
> --- a/gcc/testsuite/lib/target-supports.exp
> +++ b/gcc/testsuite/lib/target-supports.exp
> @@ -483,6 +483,16 @@ proc check_effective_target_alias { } {
>      }
>  }
>  
> +# Returns 1 if the target uses the ELF object format, 0 otherwise.
> +
> +proc check_effective_target_elf { } {
> +    if { [gcc_target_object_format] == "elf" } {
> +	return 1;
> +    } else {
> +	return 0;
> +    }
> +}
> +
>  # Returns 1 if the target toolchain supports ifunc, 0 otherwise.
>  
>  proc check_ifunc_available { } {
> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
> index 4db8506b106..7864db11faf 100644
> --- a/gcc/varasm.cc
> +++ b/gcc/varasm.cc
> @@ -6971,7 +6971,6 @@ default_select_section (tree decl, int reloc,
>      {
>        if (! ((flag_pic && reloc)
>  	     || !TREE_READONLY (decl)
> -	     || TREE_SIDE_EFFECTS (decl)
>  	     || !TREE_CONSTANT (decl)))
>  	return readonly_data_section;
>      }
> @@ -7005,7 +7004,6 @@ categorize_decl_for_section (const_tree decl, int reloc)
>        if (bss_initializer_p (decl))
>  	ret = SECCAT_BSS;
>        else if (! TREE_READONLY (decl)
> -	       || TREE_SIDE_EFFECTS (decl)
>  	       || (DECL_INITIAL (decl)
>  		   && ! TREE_CONSTANT (DECL_INITIAL (decl))))
>  	{
> @@ -7046,7 +7044,6 @@ categorize_decl_for_section (const_tree decl, int reloc)
>    else if (TREE_CODE (decl) == CONSTRUCTOR)
>      {
>        if ((reloc & targetm.asm_out.reloc_rw_mask ())
> -	  || TREE_SIDE_EFFECTS (decl)
>  	  || ! TREE_CONSTANT (decl))
>  	ret = SECCAT_DATA;
>        else