Extended Description

Building the linux kernel for i386, I get an assembler warning in one file:

/tmp/slab_common-bf04f6.s: Assembler messages:
/tmp/slab_common-bf04f6.s:38: Warning: ignoring changed section attributes for .data..ro_after_init

I reduced the test case to

int a attribute((section(".data..ro_after_init")));
static int b[4] attribute((section(".data..ro_after_init")));
int c;
int fn1(void);
void fn2() { c = b[fn1()]; }

clang-9 -fno-common -no-integrated-as -O2 -fno-strict-overflow -c test.c

Initializing 'a' to zero shuts up the warning, and causes this difference in the assembler output:

--- slab_common-broken.s 2019-07-10 22:36:57.262501626 +0200
+++ slab_common-good.s 2019-07-10 22:37:17.418912892 +0200
@@ -12,36 +12,35 @@
cltq
movl b(,%rax,4), %eax
movl %eax, c(%rip)
popq %rax
.cfi_def_cfa_offset 8
retq
.Lfunc_end0:
.size fn2, .Lfunc_end0-fn2
.cfi_endproc
# -- End function

• .type a,@object # @​a
• .section .data..ro_after_init,"aw",@progbits
• .globl a
• .p2align 2
  +a:
• .long 0 # 0x0
• .size a, 4
• .type b,@object # @​b

• .section .data..ro_after_init,"aM",@progbits,16
  .p2align 4
  b:
  .zero 16
  .size b, 16

  .type c,@object # @​c
  .bss
  .globl c
  .p2align 2
  c:
  .long 0 # 0x0
  .size c, 4

• .type a,@object # @​a

• .section .data..ro_after_init,"aM",@progbits,16

• .globl a

• .p2align 2
  -a:

• .long 0 # 0x0

• .size a, 4

.ident	"clang version 9.0.0-svn363902-1~exp1+0~20190620001509.2315~1.gbp76e756 (trunk)"
.section	".note.GNU-stack","",@progbits

See also https://godbolt.org/z/hBPgpb