Re: Forcing GNAT to use 32-bit load/store instructions on ARM?

[anon@att.net]

     ...  -- other Ada instructions

      --  tells assembler to use 32-bit coding
      --  Must be before insert 32-bit instructions or the 
      --  assembler will try to decode the 32-bit Ada in 64-bit mode

      Asm ( Template => ".code32", 
            Volatile => True ) ;

--- 32-bit assembly instructions.
--  go here

      --  just an example: A 32-bit 'nop'
      --  In this case, only a single instruction 
      --  but could be more than 1

      Asm ( Template => " nop",   
            Volatile => True ) ;

--  go here
--- 32-bit assembly instructions

      --  tells assembler to switch back to 64-bit coding
      --  Must be after insert 32-bit instructions or the 
      --  assembler will try to decode the 64-bit Ada in 32-bit mode
      Asm ( Template => ".code64",
            Volatile => True ) ;

     ...  -- other Ada instructions


In <0e0b9ac2-e793-4cc5-8d8d-d3441ca28a58@googlegroups.com>, daniel.dmk@googlemail.com writes:
>Hello,
>
>I've started using GNAT GPL for ARM (hosted on Windows) and I'm working on =
>some code in SPARK 2014 to interface to memory-mapped registers to control =
>registers on the STM32 F4 microcontroller, however I'm having trouble where=
> GNAT is using byte loads/stores to the registers, instead of word load/sto=
>res (32-bit).
>
>For a lot of registers on the STM32 it is necessary to always access the re=
>gisters using 32-bit load/store instructions. Using half-word (16-bit) or b=
>yte (8-bit) accesses generates a CPU fault with these registers.
>
>I'm representing the 32-bit register as a record as follows:
>
>   type Bits_1 is mod 2**1 with Size =3D> 1;
>
>   type CR_Register is
>      record
>         RNGEN : Bits_1;
>         IE    : Bits_1;
>      end record;
>   for CR_Register use
>      record
>         RNGEN at 0 range 2 .. 2; -- bits 0 .. 1 are reserved
>         IE    at 0 range 3 .. 3;
>         -- bits 4 .. 31 are reserved
>      end record;
>   for CR_Register'Size use 32;
>
>I then define the register of type CR_Register at the peripheral's base add=
>ress:
>
>   CR : CR_Register
>     with Size =3D> 32,
>       Volatile,
>       Async_Readers, Async_Writers,
>       Address =3D> System'To_Address(Base_Address);
>
>I'm using a record instead of a simple Unsigned_32 type so that:
>1) I can hide access to the "reserved" parts of the register,
>2) The code to modify the register is straightfoward.
>
>For example, enabling the peripheral is done by setting the RNGEN bit to 1:
>
>   CR.RNGEN :=3D 1;
>
>However, the GNAT compiler uses the ARM instruction ldrb to load the lower =
>8 bits of the register, modify the byte (or 16#40#), then use strb to write=
> the lower 8-bits back to memory. This causes a fault since the register mu=
>st be accessed using a 32-bit load/store instructions (the ldr and str inst=
>ructions).
>
>Does anyone know how I can force GNAT to generate the appropriate instructi=
>ons?
>
>Things I have tried:
>1) Using the Atomic aspect on the record type and CR object had no effect o=
>n the code generated.
>2) Using Pack.
>3) Declaring the bits inside the register as: Bits_32 range 0 .. 1;
>
>So far, I am able to get GNAT to use 32-bit load/stores only when I read th=
>e entire record into a temporary variable, as follows:
>
>   declare
>      Temp : CR_Register;
>   begin
>      Temp :=3D CR;
>      Temp.RNGEN :=3D 1;
>      CR :=3D Temp;
>   end;
>
>However, this makes it more "clunky" to use in my opinion. I would prefer t=
>o be able to modify the bits directly, and have the compiler always use 32-=
>bit load/store instructions.
>
>Does anyone else have any ideas that I could try?
>
>Thanks,
>Dan