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Re: RFA: MN10300: Add redundant comparison elimination pass

On 11/10/2010 09:34 AM, Nick Clifton wrote:
Hi Guys,

   As Richard pointed out my previous patch (for LIW/SETLB) included a
   separate feature which should have been submitted on its own.  So here
   it is - a machine reorg pass to eliminate redundant compares.

   The scan could be improved to look further back through the
   instruction stream for insns that set the EPSW register, but I am
   leaving that for a future patch.

Tested without regressions on an mn10300-elf toolchain.

OK to apply ?


2010-11-10  Nick Clifton<>

	* config/mn10300/mn10300.c (scan_for_redundant_compares): New
	(mn10300_reorg): New function.

Index: gcc/config/mn10300/mn10300.c =================================================================== --- gcc/config/mn10300/mn10300.c (revision 166474) +++ gcc/config/mn10300/mn10300.c (working copy) @@ -2403,6 +2403,128 @@ /* Extract the latency value from the timings attribute. */ return timings< 100 ? (timings % 10) : (timings % 100); } + +static void +scan_for_redundant_compares (void) +{ + rtx cur_insn; + + /* Look for this sequence: + + (set (reg X) (arith_op (...))) + (set (reg CC) (compare (reg X) (const_int 0))) + (set (pc) (if_then_else (EQ|NE (...)) (...) (...))) + + And remove the compare as the flags in the + EPSW register will already be correctly set. */

This pass is only necessary because you are not modeling MODE_CC modes correctly. You should use a specific mode in SELECT_CC_MODE for EQ/NE comparisons (e.g. CCZmode) and add patterns like

  (parallel [
     (set (reg X) (arith_op (...))
     (set (reg:CCZ CC) (compare:CCZ (arith_op (...)) (const_int 0)))])

Then combine will be able to change the mode of the COMPARE to CCZmode (also in the use), and merge the two instructions.

I guess it's okay to do it in mdreorg, but you should at least add a comment saying that the above would allow a more precise representation of the instruction stream, and also it would allow better register allocation in case X dies after the comparison.


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