As noted by Matthias when bootstrapping with AMD GCN support [PR97827]: Assembler source code generated by GCC might no longer assembly with LLVM's 'mc' since LLVM 11.
The reason is that GCC generates on purpose first the section with the flags, e.g. (via mergeable_constant_section) .section .rodata.cst8,"aM",@progbits,8 and then for subsequent uses, it does not repeat the flags: .section .rodata.cst8 GNU assembler warns (and with as >=2.35 gives an error) if the flags do not match, but not if the attributes/flags are left in the other same-named sections (as above) – just if they are specified and different. LLVM since February (in git) and released with LLVM 11 (12 Oct 2020) does a similar check – but without the no-error-if-no-flag exception: strtod.s:4472:2: error: changed section flags for .rodata.cst8, expected: 0x12 strtod.s:4472:2: error: changed section entsize for .rodata.cst8, expected: 8 The solution done by the attached patch is to emit the full flags also for SECTION_MERGE. Side note: For AMD GCN, we rely on LLVM as "GNU as" does not handle this target, yet; still, also in general, it makes sense to be compatible with llvm-mc. OK? Tobias References: - GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97827 with some analysis and quotes from LLVM MC and GNU AS - Filled https://bugs.llvm.org/show_bug.cgi?id=48201 about this issue but no one commented on that issue so far - Added to LLVM with commit https://reviews.llvm.org/D73999 - Some alignment was tried with GNU as (see all three links above) but the only "if (attr != 0)" then do check of gas/config/obj-elf.c was missed when implementing the if (Section->getType() != Type) in LLVM MC. ----------------- Mentor Graphics (Deutschland) GmbH, Arnulfstraße 201, 80634 München / Germany Registergericht München HRB 106955, Geschäftsführer: Thomas Heurung, Alexander Walter
varasm.c: Always output flags in merged .section for LLVM assembler compatibility [PR97827] For compatibility with LLVM 11's 'mc' assembler, the flags have to be repeated every time. See also LLVM Bug 48201 for this issue and https://reviews.llvm.org/D73999 for the patch causing the issue. gcc/ PR target/97827 * varasm.c (default_elf_asm_named_section): Always output all flags if SECTION_MERGE, even if already declared before. diff --git a/gcc/varasm.c b/gcc/varasm.c index ada99940f65..51a507393a8 100644 --- a/gcc/varasm.c +++ b/gcc/varasm.c @@ -6738,9 +6738,11 @@ default_elf_asm_named_section (const char *name, unsigned int flags, /* If we have already declared this section, we can use an abbreviated form to switch back to it -- unless this section is part of a COMDAT groups, in which case GAS requires the full - declaration every time. */ + declaration every time. LLVM's MC linker requires that the + flags are identical, thus avoid the abbreviated form with MERGE. */ if (!(HAVE_COMDAT_GROUP && (flags & SECTION_LINKONCE)) - && (flags & SECTION_DECLARED)) + && (flags & SECTION_DECLARED) + && !(flags & SECTION_MERGE)) { fprintf (asm_out_file, "\t.section\t%s\n", name); return;
