[ELF] Allow placing non-string SHF_MERGE sections with different alignments into the same MergeSyntheticSection
The difference from D63432/r365015 is that this patch does not place
SHF_STRINGS sections with different alignments into the same
MergeSyntheticSection. Doing that would:
(1) create unnecessary padding and thus waste space.
Add a test tail-merge-string-align2.s to check no extra padding is created.
(2) make some input sections unaligned when tail merge (-O2) is enabled.
The alignment of MergeTailAlignment::Builder was out of sync in D63432. MOVAPS on such unaligned strings can raise SIGSEGV.
This should fix PR42289: the Linux kernel has a use case that input
files have .rodata.cst32 sections with different alignments. The
expectation (and what ld.bfd and gold do) is that in the -r link, there
is only one .rodata.cst32 (SHF_MERGE sections with different alignments
can be combined), but lld currently creates one for each different
The current merging strategy:
- Group SHF_MERGE sections by (name, sh_flags, sh_entsize and sh_addralign). Merging is performed among a group, even if -O0 is specified.
- Create one output section for each group. This is a special case in addInputSec().
This patch changes 1) to:
- Group SHF_MERGE sections by (name, sh_flags, sh_entsize). Merging is performed among a group, even if -O0 is specified.
We will thus create just one .rodata.cst32 . This also improves merging
efficiency when sections with the same name but different alignments are
Reviewed By: peter.smith
Differential Revision: https://reviews.llvm.org/D64200