Created attachment 50481 [details]
Demonstrator, with output assembler

In arm-eabi-gcc version 11.0.1 20210303 (experimental) (GCC),
assigning an aggregate to a memory-mapped register is performed by
assigning the components of the aggregate one-by-one to the register
rather than as a whole word.
The same bug is present in the x86_64-apple-darwin compiler.

In compiler releases up to & including 10, this was done using a
whole-word assignment.

   with ATSAM3X8E; use ATSAM3X8E;
   with ATSAM3X8E.PMC; use ATSAM3X8E.PMC;
   procedure Clock is
   begin
      PMC_Periph.CKGR_MOR :=
        (KEY      => 16#37#,
         MOSCXTEN => 1,     -- main crystal oscillator enable
         MOSCRCEN => 1,     -- main on-chip rc osc. enable
         MOSCXTST => 8,     -- startup time
         others   => <>);
   end Clock;

The register is a component of a record:

   type PMC_Peripheral is record
      ...
      CKGR_MOR   : aliased CKGR_MOR_Register;
      pragma Volatile_Full_Access (CKGR_MOR);
      ...
   end record
     with Volatile;

with object as so:

   PMC_Periph : aliased PMC_Peripheral
     with Import, Address => PMC_Base;

and the write respects the Volatile_Full_Access *as it writes each
component separately to the register*! This fails, in this case,
because the PMC hardware requires each write to this register to be
accompanied by a valid value of the KEY field.

The attached aggregate_assignment.zip contains Ada source, and the
10.1.0 and 11.0.1 versions of the arm-eabi cortex-m3 assembler output
by -S.

I've tried compiling the source in aggregate_assignment.zip with the
x86_64-apple-darwin compiler, and the same bug seems to be present.

NOTE, this bug is also present in GNAT CE 2020.