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Subject: Re: ARM: Packed structure
To: "Barbier, Renaud" <Renaud.Barbier@ametek.com>,
 "gcc-help@gcc.gnu.org" <gcc-help@gcc.gnu.org>
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From: David Brown <david.brown@hesbynett.no>
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On 02/03/2022 17:29, Barbier, Renaud via Gcc-help wrote:
> I am building a boot loader (barebox) with gcc 10.2.0 for a NXP LS1021A for which I intend to use SPD.
> 
> The SPD structure for DDR3 (struct ddr3_spd_eeprom )can be found here: https://git.pengutronix.de/cgit/barebox/tree/include/ddr_spd.h
> 
> The problem I have is that the field after the union are not packed:
> 
> 	....
> 	} registered;
> 		unsigned char uc[57]; /* 60-116 Module-Specific Section */ ==> uc[56] is at the right place offset 116
> 	} mod_section;
> 
> 	/* Unique Module ID: Bytes 117-125 */
> 	unsigned char mmid_lsb;        /* 117 Module MfgID Code LSB - JEP-106 */ ===> mmid_lsb end up at offset 120 instead of offset 117
> 
> I tried the __packed attribute without effect
> 
> This is the debug output showing the offset from the start of SPD structure:
> uc[56]@ = 0x1000e2cc, 116
> mmid_lsb@ = 0x1000e2d0, 120  ==> should be 117.
> 
> Is there a bug in 10.2.0 that I have not found or a parameter I have not used to build the boot loader that could fix this problem.
> 
> Cheers,
> R
> 

It seems fine to me when I tried it on godbolt :

<https://godbolt.org/z/r1joYd135>