https://gcc.gnu.org/g:405f45aae779fe7694e8660cfa828f1cef58b101
commit r16-3116-g405f45aae779fe7694e8660cfa828f1cef58b101
Author: Andrew Pinski <quic_apin...@quicinc.com>
Date: Thu Aug 7 11:44:21 2025 -0700
varasm: Ensure each variable in mergeable section is the entity size
[PR121394]
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.
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>
Diff:
---
gcc/varasm.cc | 27 +++++++++++++++++++++++----
1 file changed, 23 insertions(+), 4 deletions(-)
diff --git a/gcc/varasm.cc b/gcc/varasm.cc
index 000ad9e26f6f..ee32cf1d7f69 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:
