* [PATCH] Fix folding of -(unsigned)(a * -b)
@ 2011-07-07 13:22 Richard Guenther
2011-07-07 15:48 ` Michael Matz
0 siblings, 1 reply; 2+ messages in thread
From: Richard Guenther @ 2011-07-07 13:22 UTC (permalink / raw)
To: gcc-patches
Folding of $subject is currently broken (noticed that when playing
with types in pointer_int_sum). We happily ignore the fact
that the negate operates on an unsigned type and change it to
operate on a signed one - which may cause new undefined overflow.
Seen with the testcase below which aborts with current trunk.
The fix is to not strip sign-changing conversions as already
done for ABS_EXPR.
Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
Richard.
2011-07-07 Richard Guenther <rguenther@suse.de>
* fold-const.c (fold_unary_loc): Do not strip sign-changes
for NEGATE_EXPR.
* gcc.dg/ftrapv-3.c: New testcase.
Index: gcc/fold-const.c
===================================================================
--- gcc/fold-const.c (revision 175962)
+++ gcc/fold-const.c (working copy)
@@ -7561,7 +7561,7 @@ fold_unary_loc (location_t loc, enum tre
if (arg0)
{
if (CONVERT_EXPR_CODE_P (code)
- || code == FLOAT_EXPR || code == ABS_EXPR)
+ || code == FLOAT_EXPR || code == ABS_EXPR || code == NEGATE_EXPR)
{
/* Don't use STRIP_NOPS, because signedness of argument type
matters. */
Index: gcc/testsuite/gcc.dg/ftrapv-3.c
===================================================================
--- gcc/testsuite/gcc.dg/ftrapv-3.c (revision 0)
+++ gcc/testsuite/gcc.dg/ftrapv-3.c (revision 0)
@@ -0,0 +1,16 @@
+/* { dg-do run } */
+/* { dg-options "-ftrapv" } */
+
+extern void abort (void);
+unsigned long
+foo (long i, long j)
+{
+ /* We may not fold this to (unsigned long)(i * j). */
+ return -(unsigned long)(i * -j);
+}
+int main()
+{
+ if (foo (-__LONG_MAX__ - 1, -1) != -(unsigned long)(-__LONG_MAX__ - 1))
+ abort ();
+ return 0;
+}
^ permalink raw reply [flat|nested] 2+ messages in thread
* Re: [PATCH] Fix folding of -(unsigned)(a * -b)
2011-07-07 13:22 [PATCH] Fix folding of -(unsigned)(a * -b) Richard Guenther
@ 2011-07-07 15:48 ` Michael Matz
0 siblings, 0 replies; 2+ messages in thread
From: Michael Matz @ 2011-07-07 15:48 UTC (permalink / raw)
To: Richard Guenther; +Cc: gcc-patches
Hi,
On Thu, 7 Jul 2011, Richard Guenther wrote:
> Index: gcc/fold-const.c
> ===================================================================
> --- gcc/fold-const.c (revision 175962)
> +++ gcc/fold-const.c (working copy)
> @@ -7561,7 +7561,7 @@ fold_unary_loc (location_t loc, enum tre
> if (arg0)
> {
> if (CONVERT_EXPR_CODE_P (code)
> - || code == FLOAT_EXPR || code == ABS_EXPR)
> + || code == FLOAT_EXPR || code == ABS_EXPR || code == NEGATE_EXPR)
> {
> /* Don't use STRIP_NOPS, because signedness of argument type
> matters. */
Um, so why would stripping a signchange ever be okay? There are many
other unary codes that behave similar enough to FLOAT_EXPR, or
CONVERT_EXPR that it's not obvious to me why those would allow sign
stripping but the above not. When the operand is float or fixed point
types then STRIP_SIGN_NOPS and STRIP_NOPS aren't different, and when the
operands are integer types I don't see how we can ignore sign-changing
nops. I'm thinking about:
VEC_UNPACK_HI_EXPR, VEC_UNPACK_LO_EXPR and PAREN_EXPR
Perhaps BIT_NOT_EXPR. Perhaps also NON_LVALUE_EXPR. All these can
conceivably have integer operands, where signedness seems to matter.
I think these are harmless: CONJ_EXPR, FIXED_CONVERT_EXPR, FIX_TRUNC_EXPR,
ADDR_SPACE_CONVERT_EXPR as their operands are either float/fixed-point
types or pointers, but as said in those cases STRIP_NOPS and
STRIP_SIGN_NOPS are equivalent.
So, why not simply always use STRIP_SIGN_NOPS?
Ciao,
Michael.
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