SGI MIPSpro C Compiler 7.2/SGI MIPSpro C Compiler 7.2.iso/relnotes/c_fe/ch4.z/ch4

Text File | 1997-09-05 | 15KB | 528 lines

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                    7.2 ANSI C Front-End Release Notes




























































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                    DDDDooooccccuuuummmmeeeennnntttt NNNNuuuummmmbbbbeeeerrrr 000000007777----1111666655559999----000011110000


                    4.  _N_e_w__F_e_a_t_u_r_e_s__o_f__T_h_i_s__R_e_l_e_a_s_e

                    This chapter contains the differences between
                    this release and the 7.1 release of MIPSpro C
                    compiler.


                    4.1  _R_e_p_a_c_k_a_g_i_n_g__o_f__F_r_o_n_t_-_E_n_d__S_u_b_s_y_s_t_e_m_s


                    Under the 7.2 release, the C compiler front-end
                    is packaged in its own subsystem on a separate
                    CD. For proper installation, you must install
                    _c_o_m_p_i_l_e_r__d_e_v, _c_o_m_p_i_l_e_r__e_o_e and _c__d_e_v from the
                    IRIX Development Foundation CD as well as _c__f_e
                    from the MIPSpro C Compiler CD.



                    4.2  _N_e_w__O_p_t_i_o_n_s__a_n_d__D_e_f_a_u_l_t_s


                    A new -_D_E_B_U_G:_o_p_t_i_o_n control group has been
                    created to allow insertion of code to assist in
                    the debugging of programs.  For example,
                    -_D_E_B_U_G:_d_i_v__c_h_e_c_k=_N replaces -_T_E_N_V:_c_h_e_c_k__d_i_v=_N
                    and the 7.2 compiler, by default, inserts code
                    to check for divide by zero (N=1).

                    _N_O_T_E:  The default value for -_T_E_N_V:_c_h_e_c_k__d_i_v=_N
                    under MIPSpro 7.1 was N=0 (no checks).

                    For more information, please refer to the _c_c(1)
                    and _D_E_B_U_G__g_r_o_u_p(5) man pages.

                    The -_L_I_S_T: options control group has been
                    enhanced to the create a listing file (.l) that
                    contains the values of all flags  modified,
                    directly in the command line, or indirectly as a
                    side effect of other options. For example:

                    % cc -n32 -LIST:options=ON foo.c

                    will create foo.l which contains a listing that
                    contains the default values of certain options
                    from the -OPT, -LNO, -TARG and -TENV option
                    control groups.












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                    The following command:

                    % cc -n32 -LIST:all_options=ON foo.c

                    will create foo.l which contains a listing that
                    contains the default values of all options from
                    all of the option control groups.

                    For more information, please refer to the _c_c(1)
                    man page.


                    4.3  _O_b_s_o_l_e_t_e__O_p_t_i_o_n_s


                    Several compile-time flags have been obsoleted.
                    These include: -_T_E_N_V:_m_i_s_a_l_i_g_n_e_m_n_t=_N,
                    -_T_E_N_V:_a_l_i_g_n__e_x_t_e_r_n=_N and -_T_E_N_V:_a_l_i_g_n_e_d=_T_R_U_E.
                    Their use will generate a warning message in
                    both the compiler front-end and backend. For
                    example:

                    % cc -n32 -TENV:misalignment=3 reshape.c
                    Warning: Obsolete option "-TENV:misalignment=3" -- ignored
                    Warning: Obsolete option "-TENV:misalignment=3" -- ignored

                    The -_T_E_N_V:_v_a_r_a_r_g_s__p_r_o_t_o_t_y_p_e_s=_T_R_U_E flag has been
                    replaced by -_D_E_B_U_G:_v_a_r_a_r_g_s__p_r_o_t_o_t_y_p_e_s=_T_R_U_E.

                    For more information, please refer to the _c_c(1)
                    and _D_E_B_U_G__g_r_o_u_p(5) man pages.



                    4.4  _V_a_r_i_a_b_l_e__L_e_n_g_t_h__A_r_r_a_y_s


                    MIPSpro 7.2 now supports variable length arrays.
                    Array dummy arguments and local arrays can now
                    be declared as variable length arrays rather
                    than as pointers.  This allows the compiler to
                    make assumptions about aliasing and can allow
                    more optimizations to be applied to code
                    containing references to the array.  This is
                    particularly beneficial for parallelization.
                    The syntax for variable length arrays is:
















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                        int foo( int n, float a[n][n] ) {
                           float b[n]
                        }


                    In the example above, dummy argument a and local
                    variable b are both variable length arrays.


                    4.5  _F_r_e_q_u_e_n_c_y__h_i_n_t__p_r_a_g_m_a_s


                    MIPSpro 7.2 now supports a new pragma "#pragma
                    mips_freqency_hint" that is used to tell the
                    compiler that a branch or function is rarely or
                    frequently executed.  The compiler uses this
                    information to make better optimization
                    decisions. A branch can be marked as infreqently
                    taken in this fashion:


                        if ( a < b ) {
                        #pragma mips_frequency_hint NEVER
                           printf("error condition0);
                        }


                    While a function can be so marked this way:


                        void handle_error( int i );
                        #pragma mips_frequency_hint NEVER handle_error


                    To specify that a block of code gets frequently
                    executed, you can use:

                    #pragma mips_frequency_hint FREQUENT

                    Frequency information about a function gets
                    propogated to all instances where the function
                    is used. So a branch that contains a call to an
                    infrequent function is considered infrequently
                    executed itself.


















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                    4.6  _F_i_l_l__a_n_d__A_l_i_g_n__P_r_a_g_m_a__S_u_p_p_o_r_t


                    MIPSpro 7.2 C supports new types of pragmas to
                    facilitate padding and alignment of variables
                    within cachelines and pages of memory. They are
                    outlined below.


                    #pragma fill_symbol (s, L1cacheline)
                    #pragma fill_symbol (s, L2cacheline)
                    #pragma fill_symbol (s, page)
                    #pragma fill_symbol (s, <user-specified-power-of-two>)
                    #pragma align_symbol (s, L1cacheline)
                    #pragma align_symbol (s, L2cacheline)
                    #pragma align_symbol (s, page)
                    #pragma align_symbol (s, <user-specified-power-of-two>)


                    The fill_symbol and align_symbol pragmas take a
                    symbol (i.e. a variable that may be a C global,
                    an automatic variable, but not a formal and not
                    an element of a structured type like a struct or
                    an array).  The second argument in the pragma
                    may be one of the keywords L1cacheline (machine
                    specific first-level cache line size, typically
                    32 bytes), L2cacheline (machine specific
                    second-level cache line size, typically 128
                    bytes), page (machine specific page size,
                    typically 16 Kbytes), or a user-specified
                    power-of-two value.

                    The align_symbol pragma aligns the start of the
                    named symbol at the specified alignment, i.e.
                    the symbol "s" will start at the specified
                    alignment boundary.

                    The fill_symbol pragma pads the named symbol
                    with additional storage so that the symbol is
                    assured not to overlap with any other data item
                    within the storage of the specified size. The
                    additional padding required is heuristically
                    divided between each end of the specified
                    variable. For instance, a fill_symbol pragma for
                    the L1cacheline will guarantee that the
                    specified symbol will not suffer from false-
                    sharing for the L1 cache line.

                    For global variables these pragma must be
                    specified at the variable definition, and are
                    optional at the declarations of the variable.











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                    The align_symbol pragma is ineffective for local
                    variables of fixed-size symbols, such as simple
                    scalars or arrays of known size. The pragma
                    continues to be effective for stack-allocated
                    arrays of dynamically-determined size.

                    A variable cannot have both fill_symbol and
                    align_symbol pragma applied to it.


                    4.7  _E_n_h_a_n_c_e_m_e_n_t__f_o_r__M_u_l_t_i_p_r_o_c_e_s_s_i_n_g


                    Several new enhancements related to
                    multiprocessing have been made to MIPSpro C 7.2:


                    4.7.1  _D_a_t_a__D_i_s_t_r_i_b_u_t_i_o_n__P_r_a_g_m_a_s   MIPSpro 7.2 C
                    now supports pragmas for page-level data
                    distribution and dynamic redistribution. These
                    include _p_a_g_e__p_l_a_c_e, _d_i_s_t_r_i_b_u_t_e, _d_i_s_t_r_i_b_u_t_e
                    _r_e_s_h_a_p_e, _d_y_n_a_m_i_c, _r_e_d_i_s_t_r_i_b_u_t_e.



                    4.7.2  _O_t_h_e_r__N_e_w__M_P__P_r_a_g_m_a_s   The following list
                    of pragmas are new for 7.2 and related to
                    multiprocessing:  _c_o_n_c_u_r_r_e_n_t, _s_e_r_i_a_l, _c_o_n_c_u_r_r_e_n_t
                    _c_a_l_l, _c_o_n_c_u_r_e_n_t_i_z_e, _n_o_c_o_n_c_u_r_r_e_n_t_i_z_e, _p_r_e_f_e_r
                    _s_e_r_i_a_l, _p_r_e_f_e_r _c_o_n_c_u_r_r_e_n_t, _p_e_r_m_u_t_a_t_i_o_n.

                    The full set of pragmas supported by C are
                    described in the new _M_I_P_S_p_r_o _C _a_n_d _C++ _P_r_a_g_m_a_s
                    manual.



                    4.7.3  _M_P___S_L_A_V_E___S_T_A_C_K_S_I_Z_E  The stacksize of
                    slave processes can be controlled through the
                    environment variable MP_SLAVE_STACKSIZE, which
                    may be set to the desired stacksize in bytes.
                    The default value is 16 Mbytes (4 MB for greater
                    than 64 threads).



















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                    4.8  _S_t_r_i_c_t_e_r__E_r_r_o_r__M_e_s_s_a_g_e


                    The example below would incorretly compile with
                    MIPSpro 7.1 without any errors. The MIPSpro 7.2
                    compilers will emit an error:

                    Example:

                    %cat a.c
                    struct st1_t;
                    struct st2_t {
                        struct st1_t st1;
                    };
                    struct st2_t st2;
                    main()
                    {
                    }
                    %cc -n32 -c a.c
                    "a.c", line 3: error(1070): incomplete type is not allowed
                     struct st1_t st1;
                     ^

                    1 error detected in the compilation of "a.c".

                    The fix is to declare a complete type before its
                    use.



                    4.9  _S_t_r_i_c_t_e_r__W_a_r_n_i_n_g__M_e_s_s_a_g_e


                    The C compiler now checks the arguments with the
                    corresponding conversion specification in the
                    format strings of calls to printf(), sprintf(),
                    fprintf(), scanf(), fscanf, and sscanf(). This
                    can result in the following warning:

                    warning/error(1178): argument is incompatible with corresponding
                    format string conversion

                    This is normally a warning. However, it will be
                    reported as an error if the -_d_i_a_g__e_r_r_o_r option
                    is used. The fix is to use the correct
                    conversion character for the type of the
                    argument.















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                    4.10  _P_R_I_N_T_F_L_I_K_E___l_i_n_t___s_t_y_l_e___c_o_m_m_e_n_t


                    These are lint comments for checking validity of
                    arguments They applies lint style check to the
                    first (n-1) arguments as usual. The nth argument
                    is interpreted as a printf format string that is
                    used to check the remaining arguments.


                    4.11  _S_C_A_N_F_L_I_K_E___l_i_n_t___s_t_y_l_e___c_o_m_m_e_n_t


                    This comment applies lint style check to the
                    first (n-1) arguments as usual. The nth argument
                    is interpreted as a scanf format string that is
                    used to check the remaining arguments.