On Mon, 14 Sep 2015, Eric Botcazou wrote:
> > Ok, so it's folding
> >
> > x == 127 ? .gnat_rcheck_CE_Overflow_Check ("overflow_sum3.adb", 14);, 0 :
> > (short_short_integer) x + 1
> >
> > <= 127
> >
> > where op0 (the COND_EXPR) does not have TREE_SIDE_EFFECTS set but
> > its operand 1 has:
> >
> > (gdb) p debug_tree (op0)
> > <cond_expr 0x7ffff68cbf90
> > type <integer_type 0x7ffff6572dc8 short_short_integer sizes-gimplified
> > public visited QI
> > size <integer_cst 0x7ffff68ccca8 constant visited 8>
> > unit size <integer_cst 0x7ffff68cccc0 constant visited 1>
> > align 8 symtab 0 alias set -1 canonical type 0x7ffff6572dc8
> > precision 8 min <integer_cst 0x7ffff656a678 -128> max <integer_cst
> > 0x7ffff656a6c0 127> context <translation_unit_decl 0x7ffff7ff81e0 D.24> RM
> > size <integer_cst 0x7ffff68ccca8 8>
> > chain <type_decl 0x7ffff6900b48 short_short_integer>>
> >
> > arg 0 <eq_expr 0x7ffff6573938
> > ...
> > arg 1 <compound_expr 0x7ffff65739b0 type <integer_type 0x7ffff6572dc8
> > short_short_integer>
> > side-effects
> > ...
> > arg 2 <plus_expr 0x7ffff6573910 type <integer_type 0x7ffff6572dc8
> > short_short_integer>
> > ...
> >
> > that's unexpected to the code generated by genmatch and I don't see
> > how omit_one_operand would handle that either.
>
> The old code was propagating the comparison inside the arms of COND_EXPR
> (fold_binary_op_with_conditional_arg) before applying the transformation:
>
> if ((short_short_integer) x == 127 ? .gnat_rcheck_CE_Overflow_Check
> ("overflow_sum3.adb", 14);, 1 : 1)
>
> The new code does the reverse, but the old behavior can be easily restored:
>
> Index: fold-const.c
> ===================================================================
> --- fold-const.c (revision 227729)
> +++ fold-const.c (working copy)
> @@ -9025,10 +9025,6 @@ fold_binary_loc (location_t loc,
> && tree_swap_operands_p (arg0, arg1, true))
> return fold_build2_loc (loc, swap_tree_comparison (code), type, op1,
> op0);
>
> - tem = generic_simplify (loc, code, type, op0, op1);
> - if (tem)
> - return tem;
> -
> /* ARG0 is the first operand of EXPR, and ARG1 is the second operand.
>
> First check for cases where an arithmetic operation is applied to a
> @@ -9114,6 +9110,10 @@ fold_binary_loc (location_t loc,
> }
> }
>
> + tem = generic_simplify (loc, code, type, op0, op1);
> + if (tem)
> + return tem;
> +
> switch (code)
> {
> case MEM_REF:
>
> is sufficient to fix the regression.
The newly generated code is better though and I can't see how we
should allow fold_binary_op_with_conditional_arg to be required
for correctness. Iff then the "fix" would not be the above but
to move fold_binary_op_with_conditional_arg to match.pd itself.
> > The COND_EXPR is originally built with TREE_SIDE_EFFECTS set but:
> >
> > Hardware watchpoint 7: *$43
> >
> > Old value = 65595
> > New value = 59
> > emit_check (gnu_cond=<eq_expr 0x7ffff6573938>,
> > gnu_expr=<plus_expr 0x7ffff6573910>, reason=10, gnat_node=2320)
> > at /space/rguenther/src/svn/trunk/gcc/ada/gcc-interface/trans.c:8823
> > 8823 return gnu_result;
> > $45 = 0
> >
> > so the Ada frontend resets the flag (improperly?):
> >
> > emit_check (gnu_cond=<eq_expr 0x7ffff6573938>,
> > gnu_expr=<plus_expr 0x7ffff6573910>, reason=10, gnat_node=2320)
> > at /space/rguenther/src/svn/trunk/gcc/ada/gcc-interface/trans.c:8823
> > 8823 return gnu_result;
> > $45 = 0
> > (gdb) l
> > 8818
> > 8819 /* GNU_RESULT has side effects if and only if GNU_EXPR has:
> > 8820 we don't need to evaluate it just for the check. */
> > 8821 TREE_SIDE_EFFECTS (gnu_result) = TREE_SIDE_EFFECTS (gnu_expr);
> > 8822
> > 8823 return gnu_result;
> > 8824 }
>
> That's old code and the comment makes it quite clear why this is done though.
Yeah, but then here "we don't need to evaluate it just for the check"
applies - the check is dead code as the outer comparison is always
false. I think what the code in the Ada frontend tries to achieve
is not actually what it does. Or the testcase is invalid (or rather
dependent on optimization performed).
Richard.
--
Richard Biener <rguent...@suse.de>
SUSE LINUX GmbH, GF: Felix Imendoerffer, Jane Smithard, Graham Norton, HRB
21284 (AG Nuernberg)
