better redundant compare elimination (original) (raw)

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• To: egcs at cygnus dot com
• Subject: better redundant compare elimination
• From: John Carr
• Date: Thu, 03 Sep 1998 07:02:36 EDT
• cc: egcs-patches at cygnus dot com

This patch eliminates redundant compares when a label is only reached from a jump by setting CC status according to the instruction preceding the branch.

This has a very small effect on the code I tried it on. Is it worth installing?

Any other comments?

*** final.c 1998/08/26 08:11:20 1.55 --- final.c 1998/09/03 01:59:08 *************** final (first, file, optimize, prescan) *** 1970,1975 **** --- 1970,1990 ---- if (GET_CODE (insn) == NOTE && NOTE_LINE_NUMBER (insn) > max_line) max_line = NOTE_LINE_NUMBER (insn); }

• #ifdef HAVE_cc0

• /* If CC tracking across branches is enabled, record the insn which

•  jumps to each branch only reached from one place.  */

• if (flag_expensive_optimizations)

• for (insn = first; insn; insn = NEXT_INSN (insn))

•   {

• rtx lab;

• if (GET_CODE (insn) == JUMP_INSN

•     && (lab = JUMP_LABEL (insn))

•     && LABEL_NUSES (lab) == 1)

•   {

•     LABEL_REFS (lab) = insn;

•   }

•   }

• #endif

  line_note_exists = (char *) oballoc (max_line + 1); bzero (line_note_exists, max_line + 1);

*************** final_scan_insn (insn, file, optimize, p *** 2283,2289 **** --- 2298,2327 ---- ASM_OUTPUT_ALIGN (file, align); #endif }

• #ifdef HAVE_cc0 CC_STATUS_INIT;

•   /* If this label is reached from only one place, set the condition

•  codes from the instruction just before the branch.  */

•   if (flag_expensive_optimizations && LABEL_NUSES (insn) == 1)

• {

•   rtx jump = LABEL_REFS (insn);

•   rtx barrier = prev_nonnote_insn (insn);

•   rtx prev;

•   /* If the LABEL_REFS field of this label has been set to point

•      at a branch, the predecessor of the branch is a regular

•      insn, and that branch is the only way to reach this label,

•      set the condition codes based on the branch and its

•      predecessor.  */

•   if (barrier && GET_CODE (barrier) == BARRIER

•       && jump && GET_CODE (jump) == JUMP_INSN

•       && (prev = prev_nonnote_insn (jump))

•       && GET_CODE (prev) == INSN)

•     {

•       NOTICE_UPDATE_CC (PATTERN (prev), prev);

•       NOTICE_UPDATE_CC (PATTERN (jump), jump);

•     }

• }

• #endif if (prescan > 0) break; new_block = 1;

• Follow-Ups:
  • Re: better redundant compare elimination
    * From: Jeffrey A Law