[Bug middle-end/37285] [4.4 Regression] ICE while building binutils on ppc

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from rguenth at gcc dot gnu dot org  2008-08-31 11:59 -------
The code in purge_dead_edges looks broken.

  /* If we don't see a jump insn, we don't know exactly why the block would
     have been broken at this point.  Look for a simple, non-fallthru edge,
     as these are only created by conditional branches.  If we find such an
     edge we know that there used to be a jump here and can then safely
     remove all non-fallthru edges.  */

but for switches we never have an edge with fallthru set (at least not on
the tree level, even before VRP runs).

And the splitting code now splits

;; basic block 3, loop depth 0, count 0
;; prev block 12, next block 4
;; pred:       12 [100.0%]  (fallthru)
;; succ:       5 [61.0%]  4 [39.0%] 
(note 17 51 18 3 [bb 3] NOTE_INSN_BASIC_BLOCK)
(insn 18 17 19 3 t.i:5 (set (reg:DI 3 3)
        (reg/v:DI 120 [ l_symndx ])) -1 (nil))
(insn 19 18 20 3 t.i:5 (set (reg:DI 5 5)
        (const_int 1 [0x1])) -1 (nil))
(call_insn/u 20 19 21 3 t.i:5 (parallel [
            (set (reg:SI 3 3)
                (call (mem:SI (symbol_ref:SI ("__ucmpdi2") [flags 0x41]) [0 S4
A8])
                    (const_int 0 [0x0])))
            (use (const_int 16 [0x10]))
            (clobber (reg:SI 65 lr))
        ]) -1 (expr_list:REG_EH_REGION (const_int 0 [0x0])
        (nil))
    (expr_list:REG_DEP_TRUE (use (reg:DI 5 5))
        (expr_list:REG_DEP_TRUE (use (reg:DI 3 3))
            (nil))))
(insn 21 20 22 3 t.i:5 (set (reg:CCUNS 125)
        (compare:CCUNS (reg:SI 3 3)
            (const_int 1 [0x1]))) -1 (nil))
(jump_insn 22 21 23 3 t.i:5 (set (pc)
        (if_then_else (leu (reg:CCUNS 125)
                (const_int 0 [0x0]))
            (label_ref 36)
            (pc))) -1 (nil))
(insn 23 22 24 3 t.i:5 (set (reg:CC 126)
        (compare:CC (subreg:SI (reg/v:DI 120 [ l_symndx ]) 0)
            (const_int 0 [0x0]))) -1 (nil))
(jump_insn 24 23 25 3 t.i:5 (set (pc)
        (if_then_else (ne (reg:CC 126)
                (const_int 0 [0x0]))
            (label_ref 29)
            (pc))) -1 (nil))
(insn 25 24 26 3 t.i:5 (set (reg:CC 127)
        (compare:CC (subreg:SI (reg/v:DI 120 [ l_symndx ]) 4)
            (const_int 2 [0x2]))) -1 (nil))
(jump_insn 26 25 27 3 t.i:5 (set (pc)
        (if_then_else (ne (reg:CC 127)
                (const_int 0 [0x0]))
            (label_ref 29)
            (pc))) -1 (nil))
(jump_insn 27 26 28 3 t.i:5 (set (pc)
        (label_ref 30)) -1 (nil))
(barrier 28 27 29)
(code_label 29 28 30 3 5 "" [2 uses])

at the jump target (code_label 29 28 30 3 5 "" [2 uses]) so we enter
purge_dead_edges with

;; basic block 16, loop depth 0, count 0
;; prev block 15, next block 4
;; pred:      
;; succ:       5 [61.0%]  4 [39.0%] 
(code_label 29 28 55 16 5 "" [2 uses])
(note 55 29 30 16 [bb 16] NOTE_INSN_BASIC_BLOCK)

so in the end it looks like we can remove the assert which leaves us with
(IMHO, my ppc fu is not too great) correct assembly generated.

Proposed patch:

Index: gcc/cfgrtl.c
===================================================================
--- gcc/cfgrtl.c        (revision 139823)
+++ gcc/cfgrtl.c        (working copy)
@@ -2324,10 +2324,11 @@ purge_dead_edges (basic_block bb)
        ei_next (&ei);
     }

-  gcc_assert (single_succ_p (bb));
-
-  single_succ_edge (bb)->probability = REG_BR_PROB_BASE;
-  single_succ_edge (bb)->count = bb->count;
+  if (single_succ_p (bb))
+    {
+      single_succ_edge (bb)->probability = REG_BR_PROB_BASE;
+      single_succ_edge (bb)->count = bb->count;
+    }

   if (dump_file)
     fprintf (dump_file, "Purged non-fallthru edges from bb %i\n",


But this also shows a missed optimization as we now no longer merge
the case 0 ...1 range with the default case.  Before VRP we have

  switch (l_symndx_1(D)) <default: <L4>, case 0 ... 1: <L4>, case 2: <L3>>

so we could easily detect this.

Can you verify the generated assembly is correct with the proposed patch?

Thanks.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2008-08-31 11:59:27
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37285