The following fixes a bug in fold-const.c which happily drops
side-effects when simplifying X - X to 0. I ran into this because
on match-and-simplify I cannot simplify expressions with
TREE_SIDE_EFFECTS and it happens that TYPE_SIZE for VLAs has
side-effects because it usually contains a SAVE_EXPR which are
forced to have side-effects (even if its arg does not have
side-effects). This is appearantly because you can use SAVE_EXPRs
to force creation of initialized temporaries at defined place
in the instruction stream. For example fold-const.c currently
would miscompile
(save_expr<global> - save_expr<global>, (global = 1, save_expr<global>))
Of course all this makes my life hard on match-and-simplify
branch and I wonder why we need to use SAVE_EXPRs for these cases
where the initialization of a new temporary could be done
explicitely (using INIT_EXPR or TARGET_EXPR).
Anyway - fixing the fold-const.c bug requires fixing the bug
I run into on match-and-simplify branch - here by simply
"stripping" the side-effects to get to the actual difference
of the two DECL_FIELD_OFFSETs (I wonder if I missed an obvious
tool to do such stripping).
Bootstrap & regtest running on x86_64-unknown-linux-gnu.
Of course this doesn't help me on match-and-simplify branch
(example testcase that ICEs there with the assert in stor-layout.c
visible below is gcc.c-torture/execute/20040308-1.c). In
theory it should be ok to simplify these late in GIMPLE and
for the stor-layout.c case unset TREE_SIDE_EFFECTS temporarily
to make generic_simplify happy ... (ugh).
Thanks,
Richard.
2014-10-22 Richard Biener <rguent...@suse.de>
* fold-const.c (fold_binary_loc): Preserve side-effects of
X - X when simplifying to 0.
* stor-layout.c (finish_bitfield_representative): Strip
side-effects of evaluating the difference of two DECL_FIELD_OFFSET.
Index: gcc/fold-const.c
===================================================================
--- gcc/fold-const.c (revision 216543)
+++ gcc/fold-const.c (working copy)
@@ -10749,7 +10768,7 @@ fold_binary_loc (location_t loc,
if ((!FLOAT_TYPE_P (type) || !HONOR_NANS (TYPE_MODE (type)))
&& operand_equal_p (arg0, arg1, 0))
- return build_zero_cst (type);
+ return omit_one_operand_loc (loc, type, build_zero_cst (type), arg0);
/* A - B -> A + (-B) if B is easily negatable. */
if (negate_expr_p (arg1)
Index: gcc/stor-layout.c
===================================================================
--- gcc/stor-layout.c (revision 216543)
+++ gcc/stor-layout.c (working copy)
@@ -1860,6 +1860,8 @@ finish_bitfield_representative (tree rep
size = size_diffop (DECL_FIELD_OFFSET (field),
DECL_FIELD_OFFSET (repr));
+ while (TREE_CODE (size) == COMPOUND_EXPR)
+ size = TREE_OPERAND (size, 1);
gcc_assert (tree_fits_uhwi_p (size));
bitsize = (tree_to_uhwi (size) * BITS_PER_UNIT
+ tree_to_uhwi (DECL_FIELD_BIT_OFFSET (field))
