Groups | Search | Server Info | Login | Register

Groups > comp.arch.embedded > #32463

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

Show all headers | View raw

On 2026-01-20, David Brown <david.brown@hesbynett.no> wrote:

> 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.

Yep making a structure aligned is an excellent way to introduce subtle
bugs that happen when somebody, somewhere passes a pointer to one of
those structure fields to some library function. Somebody I used to
work with was very fond of making all of his structures aligned (for
no apparent reason). Then he would test his code on an X86 desktop
machine. It worked fine because the X86 support unaligned
accesses. Then he would move to an ARM target, and it would
fail. Inevitably the cry "The compiler's broken!" would be heard, and
I would have to explain to him for the Nth time about misaligned
accesses on different ARM targets.  Some of our targets generate a bus
fault, some just silently read/write only part of the data.

That same guy once insisted that with the 32-bit GCC compiler we were
using "unsigned long variables work, but unsigned variables don't". So
he was busily changing all of his "unsigned" variables to "unsigned
long".  I printed out the assembly generated for both cases showing
that it was identical. He then insisted that the linker must be doing
something to break unsigned integers.

And then there was the time he decided that cross compiling on a
single-core Linux host worked but compiling on a dual-core
didn't. [Both cases using a single-threaded "make".]

And the time he decided that he needed to upgrade a buch of the Ubuntu
X11 libraries on the X86 host machine to fix a problem in the ARM
target.

--
Grant

Back to comp.arch.embedded | Previous | Next — Previous in thread | Next in thread | Find similar

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

csiph-web