🚀 go-pugleaf

        *m68k.h: no force mem for mc680[346]0
                             Dmitry K. Butskoy   (buc@stu.spb.su)

GCC:    2.7.0
CPU:    m68k

(Our configuration:
        CPU:        mc68030/mc68882
        Computer:   BESTA   (similar to `mot3300 delta')
        System:     SVR3.1  (similar to sysV68)
        asm:        SGS  System V/68 R2V1  syntax  )

    We had some experiments to see the influence of different
`-f' options on the resulting assembler code and found that
`-fforce-mem' option doesn`t have a positive effect if the processor
has on-chip data cashe. ( mc680[012]0 have no such cashe, but
mc680[346]0 have ).


test    ( sysV library variant of strchr()  )

    ****file foo.c****

    char *strchr( char *a , char b ) {
            do if(*a == b) goto ok;
            while(*a++) ;
            a = 0;
    ok:     return a;
    }

    ******************

`gcc -S -O2 -fforce-mem foo.c'

result #1

            file  "foo.c"
    text
            even
    global strchr
    strchr:
            mov.l 4(%sp),%a0
            mov.b 11(%sp),%d1
    L%L2:
            mov.b (%a0),%d0
            cmp.b %d1,%d0
            beq.w L%L6
            add.w &1,%a0
            tst.b %d0
            bne.w L%L2

            sub.l %a0,%a0
    L%L6:
            rts
            nop

`gcc -S -O2 -fno-force-mem foo.c'

result #2

          file  "foo.c"
    text
            even
    global strchr
    strchr:
            mov.l 4(%sp),%a0
            mov.b 11(%sp),%d0
    L%L2:
            cmp.b %d0,(%a0)
            beq.w L%L6
            tst.b (%a0)+
            bne.w L%L2

            sub.l %a0,%a0
    L%L6:
            rts
            nop


    Within the loop ` L%l2: . . . . bne.w L%L2 ' the result #1
has 2 insns more than the result #2 . This is caused by the desire
to exclude extra memory references to one and the same address
(`force-mem'). This may be efficient for chips having no data cashe.
But in case there is such data cashe, the result #2 shows more
efficient, because there will not be any actual references to external
memory. Under these circumstances:
    -the code length appears to be less,
    -the code requires a bit less times for its execution,
    -no extra regs required.

    To make a more efficient code one should consider these processor
features somehow. The most evident way to do it is:  to give all possible
m68k CPU types in TARGET_SWITCHES macro definition (m68k.h) and
to use macro CC1_SPEC to set `-fforce-mem' on or off.  (`-fforce-mem'
normally is a part of `-O2' option).
    Hear are the patches needed:

`diff -p -c4'  for  config/m68k/m68k.h

*** m68k.h      Thu Jun 15 22:13:04 1995
--- m68kNEW.h   Fri Dec  1 19:23:26 1995
*************** extern int target_flags;
*** 103,136 ****
    { { "68020", -01400},                               \
      { "c68020", -01400},                      \
      { "68020", 5},                            \
      { "c68020", 5},                           \
!     { "68881", 2},                            \
!     { "bitfield", 4},                         \
      { "68000", -01405},                               \
      { "c68000", -01405},                      \
      { "soft-float", -01102},                  \
      { "nobitfield", -4},                      \
      { "rtd", 8},                              \
      { "nortd", -8},                           \
      { "short", 040},                          \
      { "noshort", -040},                               \
      { "fpa", 0100},                           \
      { "nofpa", -0100},                                \
      { "sky", 0200},                           \
      { "nosky", -0200},                                \
-     { "68020-40", 0407},                      \
-     { "68030", -01400},                               \
-     { "68030", 5},                            \
-     { "68040", 01007},                                \
      { "68851", 0},    /* Affects *_SPEC and/or GAS.  */       \
      { "no-68851", 0}, /* Affects *_SPEC and/or GAS.  */       \
      { "68302", 0},    /* Affects *_SPEC and/or GAS.  */       \
      { "no-68302", 0}, /* Affects *_SPEC and/or GAS.  */       \
      { "68332", 0},    /* Affects *_SPEC and/or GAS.  */       \
      { "no-68332", 0}, /* Affects *_SPEC and/or GAS.  */       \
      SUBTARGET_SWITCHES                                \
      { "", TARGET_DEFAULT}}
  /* TARGET_DEFAULT is defined in sun*.h and isi.h, etc.  */

  /* This is meant to be redefined in the host dependent files */
  #define SUBTARGET_SWITCHES
--- 103,147 ----
    { { "68020", -01400},                         \
      { "c68020", -01400},                        \
      { "68020", 5},                              \
      { "c68020", 5},                             \
!     { "68030", -01400},                         \
!     { "c68030", -01400},                        \
!     { "68030", 5},                              \
!     { "c68030", 5},                             \
!     { "68040", -0400},                          \
!     { "c68040", -0400},                         \
!     { "68040", 01007},                          \
!     { "c68040", 01007},                         \
!     { "68020-40", -01000},                      \
!     { "68020-40", 0407},                        \
      { "68000", -01405},                         \
      { "c68000", -01405},                        \
+     { "68010", -01405},                         \
+     { "c68010", -01405},                        \
+     { "68881", 2},                              \
      { "soft-float", -01102},                    \
+     { "bitfield", 4},                           \
      { "nobitfield", -4},                        \
      { "rtd", 8},                                \
      { "nortd", -8},                             \
+     { "regparm", 020},                          \
+     { "noregparm", -020},                       \
      { "short", 040},                            \
      { "noshort", -040},                         \
      { "fpa", 0100},                             \
      { "nofpa", -0100},                          \
      { "sky", 0200},                             \
      { "nosky", -0200},                          \
      { "68851", 0},      /* Affects *_SPEC and/or GAS.  */       \
      { "no-68851", 0},   /* Affects *_SPEC and/or GAS.  */       \
      { "68302", 0},      /* Affects *_SPEC and/or GAS.  */       \
      { "no-68302", 0},   /* Affects *_SPEC and/or GAS.  */       \
      { "68332", 0},      /* Affects *_SPEC and/or GAS.  */       \
      { "no-68332", 0},   /* Affects *_SPEC and/or GAS.  */       \
      SUBTARGET_SWITCHES                          \
      { "", TARGET_DEFAULT}}
+
  /* TARGET_DEFAULT is defined in sun*.h and isi.h, etc.  */

  /* This is meant to be redefined in the host dependent files */
  #define SUBTARGET_SWITCHES
*************** extern int target_flags;
*** 156,163 ****
--- 167,179 ----

  /* This is meant to be redefined in the host dependent files */
  #define SUBTARGET_OVERRIDE_OPTIONS

+ #ifndef CC1_SPEC
+ #define CC1_SPEC "%{O2:%{!m68000:%{!m68010:%{!m68020:-fno-force-mem}}}}"
+ #endif
+
+
  /* target machine storage layout */

  /* Define for XFmode extended real floating point support.
     This will automatically cause REAL_ARITHMETIC to be defined.  */



    *Notes*

    Macro CC1_SPEC is currently defined in  config/m68k/crds.h,
config/m68k-none.h, config/m68k/sun2.h  and  config/m68k/sun3.h .
To install this patch above definitions should be followed with
changes mentioned. I can send necessary patches later.

    By default (no `-m' switches) I define `-fno-force-mem' ,
considering that mc680[346]0 (have data cashe) are now more frequent.

                        Dmitry K. Butskoy,
                        Saint-Petersburg State Technical University,
                        St. Petersburg, Russia.

                        <buc@stu.spb.su>