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24.3 Output Variables in Inline Assembler

The examples in this section, showing how to access the processor flags, illustrate how to specify the destination operands for assembly language statements.

 
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Get_Flags is
   Flags : Unsigned_32;
   use ASCII;
begin
   Asm ("pushfl"          & LF & HT & -- push flags on stack
        "popl %%eax"      & LF & HT & -- load eax with flags
        "movl %%eax, %0",             -- store flags in variable
        Outputs => Unsigned_32'Asm_Output ("=g", Flags));
   Put_Line ("Flags register:" & Flags'Img);
end Get_Flags;

In order to have a nicely aligned assembly listing, we have separated multiple assembler statements in the Asm template string with linefeed (ASCII.LF) and horizontal tab (ASCII.HT) characters. The resulting section of the assembly output file is:

 
#APP
   pushfl
   popl %eax
   movl %eax, -40(%ebp)
#NO_APP

It would have been legal to write the Asm invocation as:

 
Asm ("pushfl popl %%eax movl %%eax, %0")

but in the generated assembler file, this would come out as:

 
#APP
   pushfl popl %eax movl %eax, -40(%ebp)
#NO_APP

which is not so convenient for the human reader.

We use Ada comments at the end of each line to explain what the assembler instructions actually do. This is a useful convention.

When writing Inline Assembler instructions, you need to precede each register and variable name with a percent sign. Since the assembler already requires a percent sign at the beginning of a register name, you need two consecutive percent signs for such names in the Asm template string, thus %%eax. In the generated assembly code, one of the percent signs will be stripped off.

Names such as %0, %1, %2, etc., denote input or output variables: operands you later define using Input or Output parameters to Asm. An output variable is illustrated in the third statement in the Asm template string:
 
movl %%eax, %0
The intent is to store the contents of the eax register in a variable that can be accessed in Ada. Simply writing movl %%eax, Flags would not necessarily work, since the compiler might optimize by using a register to hold Flags, and the expansion of the movl instruction would not be aware of this optimization. The solution is not to store the result directly but rather to advise the compiler to choose the correct operand form; that is the purpose of the %0 output variable.

Information about the output variable is supplied in the Outputs parameter to Asm:
 
Outputs => Unsigned_32'Asm_Output ("=g", Flags));

The output is defined by the Asm_Output attribute of the target type; the general format is
 
Type'Asm_Output (constraint_string, variable_name)

The constraint string directs the compiler how to store/access the associated variable. In the example
 
Unsigned_32'Asm_Output ("=m", Flags);
the "m" (memory) constraint tells the compiler that the variable Flags should be stored in a memory variable, thus preventing the optimizer from keeping it in a register. In contrast,
 
Unsigned_32'Asm_Output ("=r", Flags);
uses the "r" (register) constraint, telling the compiler to store the variable in a register.

If the constraint is preceded by the equal character (=), it tells the compiler that the variable will be used to store data into it.

In the Get_Flags example, we used the "g" (global) constraint, allowing the optimizer to choose whatever it deems best.

There are a fairly large number of constraints, but the ones that are most useful (for the Intel x86 processor) are the following:

=
output constraint
g
global (i.e. can be stored anywhere)
m
in memory
I
a constant
a
use eax
b
use ebx
c
use ecx
d
use edx
S
use esi
D
use edi
r
use one of eax, ebx, ecx or edx
q
use one of eax, ebx, ecx, edx, esi or edi

The full set of constraints is described in the gcc and as documentation; note that it is possible to combine certain constraints in one constraint string.

You specify the association of an output variable with an assembler operand through the %n notation, where n is a non-negative integer. Thus in
 
Asm ("pushfl"          & LF & HT & -- push flags on stack
     "popl %%eax"      & LF & HT & -- load eax with flags
     "movl %%eax, %0",             -- store flags in variable
     Outputs => Unsigned_32'Asm_Output ("=g", Flags));
%0 will be replaced in the expanded code by the appropriate operand, whatever the compiler decided for the Flags variable.

In general, you may have any number of output variables:

For example:
 
Asm ("movl %%eax, %0" & LF & HT &
     "movl %%ebx, %1" & LF & HT &
     "movl %%ecx, %2",
     Outputs => (Unsigned_32'Asm_Output ("=g", Var_A),   --  %0 = Var_A
                 Unsigned_32'Asm_Output ("=g", Var_B),   --  %1 = Var_B
                 Unsigned_32'Asm_Output ("=g", Var_C))); --  %2 = Var_C
where Var_A, Var_B, and Var_C are variables in the Ada program.

As a variation on the Get_Flags example, we can use the constraints string to direct the compiler to store the eax register into the Flags variable, instead of including the store instruction explicitly in the Asm template string:

 
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Get_Flags_2 is
   Flags : Unsigned_32;
   use ASCII;
begin
   Asm ("pushfl"      & LF & HT & -- push flags on stack
        "popl %%eax",             -- save flags in eax
        Outputs => Unsigned_32'Asm_Output ("=a", Flags));
   Put_Line ("Flags register:" & Flags'Img);
end Get_Flags_2;

The "a" constraint tells the compiler that the Flags variable will come from the eax register. Here is the resulting code:

 
#APP
   pushfl
   popl %eax
#NO_APP
   movl %eax,-40(%ebp)

The compiler generated the store of eax into Flags after expanding the assembler code.

Actually, there was no need to pop the flags into the eax register; more simply, we could just pop the flags directly into the program variable:

 
with Interfaces; use Interfaces;
with Ada.Text_IO; use Ada.Text_IO;
with System.Machine_Code; use System.Machine_Code;
procedure Get_Flags_3 is
   Flags : Unsigned_32;
   use ASCII;
begin
   Asm ("pushfl"  & LF & HT & -- push flags on stack
        "pop %0",             -- save flags in Flags
        Outputs => Unsigned_32'Asm_Output ("=g", Flags));
   Put_Line ("Flags register:" & Flags'Img);
end Get_Flags_3;


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