在 2024/2/9 下午4:08, Xi Ruoyao 写道:
On Fri, 2024-02-09 at 00:02 +0800, chenglulu wrote:
在 2024/2/7 上午12:23, Xi Ruoyao 写道:
Hi Lulu,
I'm proposing to backport r14-4674 "LoongArch: Delete macro definition
ASM_OUTPUT_ALIGN_WITH_NOP." to releases/gcc-12 and releases/gcc-13. The
reasons:
1. Strictly speaking, the old ASM_OUTPUT_ALIGN_WITH_NOP macro may cause
a correctness issue. For example, a developer may use -falign-
functions=16 and then use the low 4 bits of a function pointer to encode
some metainfo. Then ASM_OUTPUT_ALIGN_WITH_NOP causes the functions not
really aligned to a 16 bytes boundary, causing some breakage.
2. With Binutils-2.42, ASM_OUTPUT_ALIGN_WITH_NOP can cause illegal
opcodes. For example:
.globl _start
_start:
.balign 32
nop
nop
nop
addi.d $a0, $r0, 1
.balign 16,54525952,4
addi.d $a0, $a0, 1
is assembled and linked to:
0000000000000220 <_start>:
220: 03400000 nop
224: 03400000 nop
228: 03400000 nop
22c: 02c00404 li.d $a0, 1
230: 00000000 .word 0x00000000 # <== OOPS!
234: 02c00484 addi.d $a0, $a0, 1
Arguably this is a bug in GAS (it should at least error out for the
unsupported case where .balign 16,54525952,4 appears with -mrelax; I'd
prefer it to support the 3-operand .align directive even -mrelax for
reasons I've given in [1]). But we can at least work it around by
removing ASM_OUTPUT_ALIGN_WITH_NOP to allow using GCC 13.3 with Binutils
2.42.
3. Without ASM_OUTPUT_ALIGN_WITH_NOP, GCC just outputs something like
".align 5" which works as expected since Binutils-2.38.
4. GCC < 14 does not have a default setting of -falign-*, so changing
this won't affect anyone who do not specify -falign-* explicitly.
[1]:https://github.com/loongson-community/discussions/issues/41#issuecomment-1925872603
Is it OK to backport r14-4674 into releases/gcc-12 and releases/gcc-13
then?
Ok, I agree with you.
Thanks!
Oops, with Binutils-2.41 GAS will fail to assemble some conditional
branches if we do this :(.
Not sure what to do (maybe backporting both this and a simplified
version of PR112330 fix?) Let's reconsider after the holiday...
To solve this problem,based on r14-4674, r14-5434 also needs to be
transplanted.
But I took a look and r14-5434 modified relatively many files.
So I think that without worrying about performance and ensuring that
there is no problem
with binutils, I think we can make the following modifications:
-/* "nop" instruction 54525952 (andi $r0,$r0,0) is
- used for padding. */
+/* ".align num,,4" will insert "nop"(andi $r0,$r0,0) into padding by
+ default. */
#define ASM_OUTPUT_ALIGN_WITH_NOP(STREAM, LOG) \
- fprintf (STREAM, "\t.align\t%d,54525952,4\n", (LOG))
+ fprintf (STREAM, "\t.align\t%d,,4\n", (LOG))
What do you think of it?
