On 17/02/2021 17:41, Kinsey Moore wrote:
I would remove the SUBALIGN() from the linker script. You can also add a
new test case for splinkersets01 similar to struct s from above. Then we
should check if the test fails on aarch64 and why it fails.
The example above actually shows the issue I'm having in _Linker_set_i_begin
and _Linker_set_i_end. The alignment expands for the larger struct, but does
not shrink for data types smaller than 8 bytes, leaving padding that the test
interprets as additional space in the linker set.
I still don't see why a larger alignment is an issue. The begin/end
objects have a size of zero. Where do you observe a padding?
Even when the begin/end objects have a size of 0, they're aligned out to 8
bytes forcing empty bytes if the content does not exactly fill a multiple of 8
bytes. Those non-content bytes are counted as part of the section.
Ok, it took a while, but I understand now the problem. I think this is a
GCC bug.
cat test-align.c
char i;
char j[1];
char z[0];
unsigned long ai = _Alignof(i);
unsigned long aj = _Alignof(j);
unsigned long az = _Alignof(z);
aarch64-rtems6-gcc -O2 -S -o - test-align.c -mabi=ilp32 -fdata-sections
.arch armv8-a
.file "test-align.c"
.text
.global az
.global aj
.global ai
.global z
.global j
.global i
.section .bss.i,"aw",@nobits
.type i, %object
.size i, 1
i:
.zero 1
.section .bss.j,"aw",@nobits
.align 3
.type j, %object
.size j, 1
j:
.zero 1
.section .bss.z,"aw",@nobits
.align 3
.type z, %object
.size z, 0
z:
.section .data.ai,"aw"
.align 2
.type ai, %object
.size ai, 4
ai:
.word 1
.section .data.aj,"aw"
.align 2
.type aj, %object
.size aj, 4
aj:
.word 1
.section .data.az,"aw"
.align 2
.type az, %object
.size az, 4
az:
.word 1
.ident "GCC: (GNU) 10.2.1 20210205 (RTEMS 6, RSB
61dcadee0825867ebe51f9f367430ef75b8fe9c0, Newlib d4a756f)"
clang --target=aarch64 -O2 -S -o - test-align.c -fdata-sections -fno-common
test-align.c:4:20: warning: '_Alignof' applied to an expression is a GNU
extension [-Wgnu-alignof-expression]
unsigned long ai = _Alignof(i);
^
test-align.c:5:20: warning: '_Alignof' applied to an expression is a GNU
extension [-Wgnu-alignof-expression]
unsigned long aj = _Alignof(j);
^
test-align.c:6:20: warning: '_Alignof' applied to an expression is a GNU
extension [-Wgnu-alignof-expression]
unsigned long az = _Alignof(z);
^
.text
.file "test-align.c"
.type ai,@object // @ai
.section .data.ai,"aw",@progbits
.globl ai
.p2align 3
ai:
.xword 1 // 0x1
.size ai, 8
.type aj,@object // @aj
.section .data.aj,"aw",@progbits
.globl aj
.p2align 3
aj:
.xword 1 // 0x1
.size aj, 8
.type az,@object // @az
.section .data.az,"aw",@progbits
.globl az
.p2align 3
az:
.xword 1 // 0x1
.size az, 8
.type i,@object // @i
.section .bss.i,"aw",@nobits
.globl i
i:
.byte 0 // 0x0
.size i, 1
.type j,@object // @j
.section .bss.j,"aw",@nobits
.globl j
j:
.zero 1
.size j, 1
.type z,@object // @z
.section .bss.z,"aw",@nobits
.globl z
z:
.size z, 0
.ident "clang version 9.0.1 "
.section ".note.GNU-stack","",@progbits
.addrsig
3 warnings generated.
I would file a GCC bug report.
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