On Thu, Aug 7, 2025 at 7:58 PM Andrew Pinski <quic_apin...@quicinc.com> wrote:
>
> Now there are mergeable sections which have an entity size, we can place
> decls (constants) that are smaller in size in these sections.
> An example is an `long double` which has a size of 12 bytes on i686 and is
> placed in the 16 bytes shareable section.
>
> For an example with the following C++ code:
> ```
> std::initializer_list<long double> a = {0.3l};
> ```
>
> We place the constant array in the .rodata.cst16 section but we don't add a
> padding to 16 bytes.
> ```
> .section .rodata.cst16,"aM",@progbits,16
> .align 16
> .type _ZGR1a_, @object
> .size _ZGR1a_, 12
> _ZGR1a_:
> .long -1717986918
> .long -1717986919
> .long 16381
> .text
> ```
> GAS has a workaround added to do the padding but other assemblers don't.
> The gas workaround was added with
> https://sourceware.org/legacy-ml/binutils/2002-11/msg00615.html .
> Now for the constant pool, GCC does emit a `.align` to padd out the size
> correctly.
> This was done in r0-46282-gf41115930523b3.
> The same padding should be done when emitting a variable contents when in a
> mergeable section.
>
> This patch implements the padding and we now get an addition `.zero 4` which
> pads out the section.
Note I separated this out from the other mergeability patch so it can
be backported to both GCC 15 and 14.
Thanks,
Andrew
>
> Bootstrapped and tested on x86_64-linux-gnu.
>
> PR middle-end/121394
> gcc/ChangeLog:
>
> * varasm.cc (assemble_variable_contents): Pad out
> mergeable sections if needed.
> (output_constant_pool_1): Change the padding to be explicit
> zeroing for mergeable sections.
>
> Signed-off-by: Andrew Pinski <quic_apin...@quicinc.com>
> ---
> gcc/varasm.cc | 27 +++++++++++++++++++++++----
> 1 file changed, 23 insertions(+), 4 deletions(-)
>
> diff --git a/gcc/varasm.cc b/gcc/varasm.cc
> index 000ad9e26f6..ee32cf1d7f6 100644
> --- a/gcc/varasm.cc
> +++ b/gcc/varasm.cc
> @@ -2475,6 +2475,19 @@ assemble_variable_contents (tree decl, const char
> *name,
> else
> /* Leave space for it. */
> assemble_zeros (tree_to_uhwi (DECL_SIZE_UNIT (decl)));
> + /* For mergeable section, make sure the section is zero filled up to
> + the entity size of the section. */
> + if (in_section
> + && (in_section->common.flags & SECTION_MERGE)
> + && tree_fits_uhwi_p (DECL_SIZE_UNIT (decl))
> + && ((in_section->common.flags & SECTION_ENTSIZE)
> + > tree_to_uhwi (DECL_SIZE_UNIT (decl))))
> + {
> + unsigned HOST_WIDE_INT entsize, declsize;
> + entsize = (in_section->common.flags & SECTION_ENTSIZE);
> + declsize = tree_to_uhwi (DECL_SIZE_UNIT (decl));
> + assemble_zeros (entsize - declsize);
> + }
> targetm.asm_out.decl_end ();
> }
> }
> @@ -4435,10 +4448,16 @@ output_constant_pool_1 (class constant_descriptor_rtx
> *desc,
>
> /* Make sure all constants in SECTION_MERGE and not SECTION_STRINGS
> sections have proper size. */
> - if (align > GET_MODE_BITSIZE (desc->mode)
> - && in_section
> - && (in_section->common.flags & SECTION_MERGE))
> - assemble_align (align);
> + if (in_section
> + && (in_section->common.flags & SECTION_MERGE)
> + && ((in_section->common.flags & SECTION_ENTSIZE)
> + > GET_MODE_SIZE (desc->mode)))
> + {
> + unsigned HOST_WIDE_INT entsize, constsize;
> + entsize = (in_section->common.flags & SECTION_ENTSIZE);
> + constsize = GET_MODE_SIZE (desc->mode);
> + assemble_zeros (entsize - constsize);
> + }
>
> #ifdef ASM_OUTPUT_SPECIAL_POOL_ENTRY
> done:
> --
> 2.43.0
>
