Dear Wolfgang Denk,
On Tuesday, September 17, 2013 1:47:02 PM, Wolfgang Denk wrote:
Dear Benoît Thébaudeau,
In message 1135126743.1842859.1379415574013.JavaMail.zimbra@advansee.com you wrote:
So how can the above not properly align the buffer? Is that a compiler bug then?
btw. using ALLOC_CACHE_ALIGN_BUFFER will be safer were someone to change FEC_MAX_PKT_SIZE, the buffer would still be safe for cache flush/inval ops.
I have encountered the same kind of alignment issue recently on something else. It looks like GCC for ARM just silently ignores the aligned attribute for auto (stacked) variables.
I would really like to see the generated code from such a system, so we verify if this is indeed true, or if something else is causing such issues.
Even if the suggested patch fixes the current problem, it leaves a bad feeling as it's only based on speculation about the causes.
Here is a basic alignment test that I have run on ARM: --- #include <stdio.h>
void foo(void *var) { printf("var=0x%.8x\n", (int)var); }
int main(void) { unsigned char var[1536] __attribute__((__aligned__(64))); unsigned int i;
for (i = 0; i < 10; i++) foo(&var); return 0; } ---
I have built it using: $ cross-gcc align.c -o align
With GCC 4.5.4, the kind of output that I get is 'var=0x7ee1a6b8' (i.e. not aligned as requested).
The generated asm is: --- 0000849c <main>: 849c: e92d4800 push {fp, lr} 84a0: e28db004 add fp, sp, #4 84a4: e24ddc06 sub sp, sp, #1536 ; 0x600 84a8: e24dd008 sub sp, sp, #8 84ac: e3a03000 mov r3, #0 84b0: e50b3008 str r3, [fp, #-8] 84b4: ea000007 b 84d8 <main+0x3c> 84b8: e24b3c06 sub r3, fp, #1536 ; 0x600 84bc: e2433004 sub r3, r3, #4 84c0: e2433008 sub r3, r3, #8 84c4: e1a00003 mov r0, r3 84c8: ebffffe7 bl 846c <foo> 84cc: e51b3008 ldr r3, [fp, #-8] 84d0: e2833001 add r3, r3, #1 84d4: e50b3008 str r3, [fp, #-8] 84d8: e51b3008 ldr r3, [fp, #-8] 84dc: e3530009 cmp r3, #9 84e0: 9afffff4 bls 84b8 <main+0x1c> 84e4: e3a03000 mov r3, #0 84e8: e1a00003 mov r0, r3 84ec: e24bd004 sub sp, fp, #4 84f0: e8bd8800 pop {fp, pc} ---
With GCC 4.6.2, the kind of output that I get is 'var=0x7e808680' (i.e. aligned as requested).
The generated asm is: --- 000083a4 <main>: 83a4: e92d4810 push {r4, fp, lr} 83a8: e28db008 add fp, sp, #8 83ac: e24dd00c sub sp, sp, #12 83b0: e24ddd19 sub sp, sp, #1600 ; 0x640 83b4: e1a0300d mov r3, sp 83b8: e283303f add r3, r3, #63 ; 0x3f 83bc: e1a03323 lsr r3, r3, #6 83c0: e1a04303 lsl r4, r3, #6 83c4: e3a03000 mov r3, #0 83c8: e50b3010 str r3, [fp, #-16] 83cc: ea000004 b 83e4 <main+0x40> 83d0: e1a00004 mov r0, r4 83d4: ebffffe6 bl 8374 <foo> 83d8: e51b3010 ldr r3, [fp, #-16] 83dc: e2833001 add r3, r3, #1 83e0: e50b3010 str r3, [fp, #-16] 83e4: e51b3010 ldr r3, [fp, #-16] 83e8: e3530009 cmp r3, #9 83ec: 9afffff7 bls 83d0 <main+0x2c> 83f0: e3a03000 mov r3, #0 83f4: e1a00003 mov r0, r3 83f8: e24bd008 sub sp, fp, #8 83fc: e8bd8810 pop {r4, fp, pc} ---
I did not succeed to duplicate the issue with GCC 4.6.2, while GCC 4.5.4 almost always produces an unexpected alignment for auto variables.
Best regards, Benoît