[Ada] add new warning on asm statements

[Arnaud Charlet <charlet at adacore dot com>]

Tested on i686-linux, committed on trunk.

This patch adds some new warnings to code statements. In particular
if Volatile is not specified then a warning is given if two or more
code statements appear in sequence, or if either Input or Output
operands are not present.

Here are examples (these are x86 specific) (compiled with -gnatlj60)
                                                                    
     1. with System.Machine_Code; use System.Machine_Code;          
     2. function Exp1 (X : Long_Long_Float) return Long_Long_Float is
     3.    Result : Long_Long_Float := 0.0;                          
     4.    NL     : constant Character := ASCII.LF;
     5. begin                                      
     6.    Asm (Template =>
           |
        >>> warning: code statement with no inputs should
            usually be Volatile
                               
     7.           "fldl2e               " & NL
     8.         & "fscale               ",    
     9.         Inputs   => No_Input_Operands,
    10.         Outputs  => No_Output_Operands);
    11.    return Result;                       
    12. end;             
            
     1. with System.Machine_Code; use System.Machine_Code;
     2. function Exp3 (X : Long_Long_Float) return Long_Long_Float is
     3.    Result : Long_Long_Float;                                 
     4.    NL     : constant Character := ASCII.LF;
     5. begin                                      
     6.    Asm (Template =>
     7.           "faddp   %st, %st(1)",
     8.         Outputs  => Long_Long_Float'Asm_Output ("=t", Result),
     9.         Inputs   => Long_Long_Float'Asm_Input  ("0", X));     
    10.                                                          
    11.    Asm (Template =>
           |
        >>> warning: code statements in sequence should
            usually be Volatile (suggest using template
            with multiple instructions)
                                       
    12.           "fmulp   %st, %st(1)",
    13.         Outputs  => Long_Long_Float'Asm_Output ("=t", Result),
    14.         Inputs   => Long_Long_Float'Asm_Input  ("0", X));     
    15.    return Result;                                        
    16. end Exp3;        

2006-10-31  Robert Dewar  <dewar@adacore.com>

	* s-osinte-tru64.adb: 
	Mark Asm statements Volatile to prevent warnings (seems a
	reasonable change anyway)
	Fixes new warnings

	* s-mastop-irix.adb: Add Volatile to Asm statements
	Suppresses warning, and seems appropriate in any case

	* s-osinte-vms.adb: Add Volatile to Asm statement

	* s-vaflop-vms-alpha.adb: Add Volatile to Asm statements

	* exp_code.ads, exp_code.adb (Asm_Input_Value): Note that Error can be
	returned.
	Add call to Check_Code_Statement

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