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Subject: [Bug middle-end/61893] Constant folding inhibits trapping with -ftrapv
Date: Fri, 25 Jul 2014 11:47:00 -0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61893

--- Comment #2 from Richard Biener <rguenth at gcc dot gnu.org> ---
Fixing bit-ccp with

Index: tree-ssa-ccp.c
===================================================================
--- tree-ssa-ccp.c      (revision 213040)
+++ tree-ssa-ccp.c      (working copy)
@@ -1456,8 +1456,19 @@ bit_value_unop (enum tree_code code, tre
   widest_int value, mask;
   prop_value_t val;

-  if (rval.lattice_val == UNDEFINED)
-    return rval;
+  if (rval.lattice_val == UNDEFINED
+      /* If the value is fully known constants assume that
+        earlier simplification failed for a reason, for example
+        due to -ftrapv.  */
+      || (rval.lattice_val == CONSTANT
+         && TREE_CODE (rval.value) == INTEGER_CST
+         && rval.mask == -1))
+    {
+      val.lattice_val = VARYING;
+      val.value = NULL_TREE;
+      val.mask = -1;
+      return val;
+    }

   gcc_assert ((rval.lattice_val == CONSTANT
               && TREE_CODE (rval.value) == INTEGER_CST)
@@ -1492,7 +1503,16 @@ bit_value_binop (enum tree_code code, tr
   prop_value_t val;

   if (r1val.lattice_val == UNDEFINED
-      || r2val.lattice_val == UNDEFINED)
+      || r2val.lattice_val == UNDEFINED
+      /* If both values are fully known constants assume that
+        earlier simplification failed for a reason, for example
+        due to -ftrapv.  */
+      || (r1val.lattice_val == CONSTANT
+         && TREE_CODE (r1val.value) == INTEGER_CST
+         && r1val.mask == 0
+         && r2val.lattice_val == CONSTANT
+         && TREE_CODE (r2val.value) == INTEGER_CST
+         && r2val.mask == 0))
     {
       val.lattice_val = VARYING;
       val.value = NULL_TREE;

reveals that expansion produces

(insn 5 4 6 (set (reg:SI 4 si)
        (const_int 1 [0x1])) t.c:7 -1
     (nil))

(insn 6 5 7 (set (reg:SI 5 di)
        (const_int 2147483647 [0x7fffffff])) t.c:7 -1
     (nil))

(call_insn/u 7 6 8 (set (reg:SI 0 ax)
        (call (mem:QI (symbol_ref:DI ("__addvsi3") [flags 0x41]) [0  S1 A8])
            (const_int 0 [0]))) t.c:7 -1
     (expr_list:REG_EH_REGION (const_int -2147483648 [0xffffffff80000000])
        (nil))
    (expr_list (use (reg:SI 4 si))
        (expr_list (use (reg:SI 5 di))
            (nil))))

(insn 8 7 9 (set (reg:SI 86 [ D.1753 ])
        (reg:SI 0 ax)) t.c:7 -1
     (expr_list:REG_EQUAL (plus:SI (const_int 2147483647 [0x7fffffff])
            (const_int 1 [0x1]))
        (nil)))

note the REG_EQUAL note which CSE happily simplifies and substitutes...

Index: optabs.c
===================================================================
--- optabs.c    (revision 213040)
+++ optabs.c    (working copy)
@@ -3985,7 +3985,8 @@ emit_libcall_block_1 (rtx insns, rtx tar
     }

   last = emit_move_insn (target, result);
-  set_dst_reg_note (last, REG_EQUAL, copy_rtx (equiv), target);
+  if (!equiv_may_trap)
+    set_dst_reg_note (last, REG_EQUAL, copy_rtx (equiv), target);

   if (final_dest != target)
     emit_move_insn (final_dest, target);


with that we finally trap with -O2...