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Re: arm-gcc, Cortex-M0+, uint64_t and alignment

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Il 20/01/2026 17:07, David Brown ha scritto:
> On 20/01/2026 13:26, pozz wrote:
>> I just discovered that my arm-gcc assigns an alignment of 8 to a 
>> struct with uint64_t member.
>>
>> First of all: I can't explain why. Cortex-M0+ shouldn't have any 
>> special load/store instructions for 64-bits data. I think the uint64_t 
>> variable is *always* accessed with two separate instructions.
>>
> 
> There are other Cortex-M devices that /can/ access 64 bit data with a 
> single instruction (though not always as an atomic function).
> 
> Compilers use family ABI's, not ABI's specifically tuned for exact 
> devices.  The EABI for 32-bit ARM says long long's are 8 byte aligned, 
> so that's what is used for all targets that use the EABI.  (There's a 
> lot to dislike about the EABI - this is not the worst thing.)

So the ABI used by arm gcc is EABI that is valid for a list of Cortex-M 
devices, a few of these that require 8-byte alignment of 64-bits integers.


>> Second thing. Is it safe to force the alignment of such structs to 4 
>> with __attribute__((aligned(4)))?
> 
> You can't reduce the alignment of a struct or its elements by adding an 
> __aligned_ attribute to the struct itself or any of its fields.  The 
> best you can do on the struct itself is __attribute__((packed)).  But 
> that can come with disadvantages, and inefficient use.

But this is the opposite of what you write below!


>> I have big arrays of structs that contains uint64_t members, so I'm 
>> thinking how to save some space.
> 
> The best way is to organise the fields so that they are naturally 
> aligned, and don't have padding for alignment.  I like "-Wpadded" to 
> tell me if there is unexpected padding.
> 
> 
> What you /can/ do, however, is define a type that is 64 bits, but 4 byte 
> alignment:
> 
> typedef uint64_t __attribute__((aligned(4)) uint64_a;
> 
> Now you can use "uint64_a" instead of "uint64_t", and it will have 4 
> byte alignment.

Before you wrote it's impossible to reduce the alignment from 8 to 4 
with __attribute__((aligned(4))), but now you write it is possible.

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Thread

arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-20 13:26 +0100
  Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 17:07 +0100
    Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-20 16:41 +0000
      Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-20 18:09 +0100
        Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 18:44 +0100
          Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-21 09:11 +0100
            Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-21 10:02 +0100
              Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-21 15:58 +0100
                Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-21 17:13 +0100
                Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-21 17:57 +0100
                Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-22 10:03 +0100
        Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-20 17:48 +0000
      Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 18:41 +0100
        Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-20 18:10 +0000
          Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 22:32 +0100
            Re: arm-gcc, Cortex-M0+, uint64_t and alignment Grant Edwards <invalid@invalid.invalid> - 2026-01-21 03:38 +0000
              Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-21 08:54 +0100
    Re: arm-gcc, Cortex-M0+, uint64_t and alignment pozz <pozzugno@gmail.com> - 2026-01-20 17:55 +0100
      Re: arm-gcc, Cortex-M0+, uint64_t and alignment David Brown <david.brown@hesbynett.no> - 2026-01-20 22:24 +0100

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