Re: Using pointers with inline assembly in Ada

[NiGHTS <nights@unku.us>]

On Saturday, June 11, 2022 at 8:28:28 AM UTC-4, Simon Wright wrote:
> NiGHTS <nig...@unku.us> writes: 
> 
> > This was my first experiment which produces a memory access error. In 
> > this early version I'm just trying to write to the first element. 
> > 
> > declare 
> > type ff is array (0 .. 10) of Unsigned_32; 
> > pragma Pack(ff); 
> > Flags : aliased ff := (others => 0); 
> > Flag_Address : System.Address := Flags'Address; 
> > begin 
> > Asm ( "movl %0, %%eax" & 
> > "movl $1, (%%eax)" , 
> > Inputs => System.Address'Asm_Input ("g", Flag_Address), 
> > Clobber => "eax", 
> > Volatile => true 
> > ); 
> > Put_Line ("Output:" & Flags(0)'Img); 
> > end;
> I got an access error as well (macOS, GCC 12.1.0, 64 bits). 
> 
> Eventually, it turned out that the problem was that eax is a 32-bit 
> register. (the compiler used rdx, and the assembler told me that 
> movl %rdx, %eax wasn't allowed). 
> 
> This is my working code; Flags is volatile, because otherwise the 
> compiler doesn't realise (at -O2) that Flags (0) has been touched. 
> 
> ALso, note the Inputs line! 
> 
> ==== 
> with System.Machine_Code; use System.Machine_Code; 
> with Interfaces; use Interfaces; 
> with Ada.Text_IO; use Ada.Text_IO; 
> procedure Nights is 
> type Ff is array (0 .. 10) of Unsigned_32; 
> Flags : aliased Ff := (others => 0) with Volatile; 
> begin 
> Asm ( 
> "movq %0, %%rax" & ASCII.LF & ASCII.HT 
> & "movl $1, (%%rax)", 
> Inputs => System.Address'Asm_Input ("g", Flags (Flags'First)'Address), 
> Clobber => "rax", 
> Volatile => True
> ); 
> Put_Line ("Output:" & Flags(0)'Img);
> end Nights;
I haven't tested this yet but the solution makes sense to me. Thank you for your help!