FreeBSD/armv6z/clang on Raspberry Pi 512MB (with U-Boot + ubldr)

[Warner Losh]

On Dec 28, 2012, at 9:47 AM, Adrian Chadd wrote:

> .. the compiler should know the alignment for each of those types and
> pad the structure appropriately, right?

No, it doesn't. At least traditionally gcc and other compilers will assume you know what you are doing and use the natural accessors.

> david - what's the "right" behaviour here? Surely clang should be
> inserting 4 bytes of padding before that pointer?

For OABI, no padding is the correct answer. For EABI padding is, I believe, the right answer. If it matters, and if you are matching an external standard, be explicit in some way with packed attributes and/or explicit dummy padding members.

Warner

> 
> Adrian
> 
> 
> On 28 December 2012 08:41, Daisuke Aoyama <aoyama at peach.ne.jp> wrote:
>>>>> This is complete version of systimer patch.
>>>>> It should fix stopping interrupt and related things such as USB LAN is
>>>>> unstable,
>>>>> SSH is closed suddenly.
>>>>> (I've not yet finished all test at this time, but at least portsnap
>>>>> fetch is
>>>>> success.
>>>>> Now extracting the ports without any problems.)
>> 
>> 
>> Finished without any problems.
>> 
>> 
>> I'm checking the alignment of clang now. It seems no difference of location
>> of data structure.
>> However, access method is different.
>> 
>> I learned clang will combine two loads into one op.
>> This is a reason why the alignment seems difference between clang and gcc.
>> Also, a reason why clang binary is smaller than gcc code.
>> 
>> According to ARM manual, strd alignment is:
>> "The address must be a multiple of eight for doubleword transfers."
>> 
>> uboot/lib/api_public.h:
>> ----------------------------------------------------------------------
>> struct sys_info {
>>       unsigned long           clk_bus;
>>       unsigned long           clk_cpu;
>>       unsigned long           bar;
>>       struct mem_region       *mr;    /* << mr offset is 12!! (not DW
>> aligned) */
>>       int                     mr_no;  /* number of memory regions */
>> };
>> 
>> uboot/lib/glue.c:
>> ----------------------------------------------------------------------
>> struct sys_info *
>> ub_get_sys_info(void)
>> {
>>       int err = 0;
>> 
>>       memset(&si, 0, sizeof(struct sys_info));
>>       si.mr = mr;
>>       si.mr_no = UB_MAX_MR;
>>       memset(&mr, 0, sizeof(mr));
>> 
>>       if (!syscall(API_GET_SYS_INFO, &err, (u_int32_t)&si))
>>               return (NULL);
>> 
>>       return ((err) ? NULL : &si);
>> }
>> 
>> ----------------------------------------------------------------------
>> clang -O2 output (7 steps):
>>       bl      memset            ; ATPCS uses r0-r3 as parameters
>>       ldr     r0, .LCPI6_1      ; mr
>>       mov     r1, #5            ; UB_MAX_MR
>>       mov     r2, #60           ; sizeof(mr)
>>       strd    r0, r1, [r5, #12] ; r5 aligned but strd requires DW(8byte)
>> alignment (faulted here)
>>       mov     r1, #0
>>       bl      memset            ;  memset(&mr, 0, sizeof(mr));
>> 
>> clang final binary size(2.8% smaller):
>> -rwxr-xr-x  1 root  wheel  235984 Dec 28 23:49 ubldr
>> ----------------------------------------------------------------------
>> gcc 4.2 -O2 output (9 steps):
>>       bl      memset            ; ATPCS uses r0-r3 as parameters
>>       ldr     ip, .L162+4       ; mr
>>       mov     r3, #5            ; UB_MAX_MR
>>       mov     r1, r4            ; r4 is zero
>>       mov     r0, ip            ; << what??
>>       mov     r2, #60           ; sizeof(mr)
>>       str     r3, [r6, #16]     ; r6 aligned same as clang
>>       str     ip, [r6, #12]     ; r6 aligned same as clang
>>       bl      memset            ;  memset(&mr, 0, sizeof(mr));
>> 
>> gcc final binary size:
>> -rwxr-xr-x  1 root  wheel  242810 Dec 28 18:22 ubldr
>> ----------------------------------------------------------------------
>> I don't know gcc 4.3 or newer, but probably output is more smart.
>> It seems that there is no reason to use ip in this case.
>> 
>> Does any one know how to prevent above clang output?
>> (or how to solve this issue for all codes.)
>> 
>> Thanks
>> 
>> --
>> Daisuke Aoyama
>> 
>> 
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