On 2015-08-12 08:40:47 -0400, Tim Prince wrote:
> On 8/12/2015 7:03 AM, Marc Glisse wrote:
> > On Wed, 12 Aug 2015, Marcin Krotkiewski wrote:
> >
> >> Hello, all,
> >>
> >> I have doubts about asm generated for the following code that
> >> performs orientation test for a point and a segment:
> >>
> >> double orient_test_2d(const double m[2], const double a[2], const
> >> double b[2])
> >> {
> >> double am1 = a[0]-m[0];
> >> double bm1 = b[1]-m[1];
> >> double am2 = a[1]-m[1];
> >> double bm2 = b[0]-m[0];
> >>
> >> return ((am1)*(bm1)) - ((am2)*(bm2));
> >> }
> >>
> >> In the return statement the operands are all in parentheses. gcc
> >> optimizes
> >
> > Parentheses don't have the meaning you believe they have in C. All
> > those in your return statement are useless.
> No, this is a -std=gnu99 peculiarity which resembles K&R treatment. I
> know some people do mean K&R when they say C.
ISO C. The parentheses are useless here. I mean, according to the
ISO C99 standard, they don't change the meaning of the expression.
AFAIK, the C99 standard just says:
A parenthesized expression is a primary expression. Its type and
value are identical to those of the unparenthesized expression.
Ditto for the current ISO C11 standard.
> >> the statement and introduces a FMA instruction. I think this is wrong
> >> because FMA causes the subtraction and multiplication to be
> >> effectively executed at the same time, while the source specifies
> >> that the multiplications should be performed before the subtraction.
> >
> > -ffp-contract=off
> >
> Why not gcc -std=c99 .... ? It appears to answer the original question.
Probably because with recent GCC versions, -ffp-contract=on is now
an alias for -ffp-contract=off in ISO C mode, due to the fact that
the FP_CONTRACT pragma is not implemented by GCC. This has been
changed (fixed) by:
2013-11-06 16:52:47 (rev 204460, jsm28)
c-family:
* c-opts.c (c_common_post_options): Set -ffp-contract=off in C
standards modes.
* c-cppbuiltin.c (cpp_iec_559_value): Consider -ffp-contract=fast
to mean lack of IEEE 754 support.
testsuite:
* gcc.dg/torture/c99-contract-1.c: New test.
gcc/c-family/c-opts.c now contains:
/* ISO C restricts floating-point expression contraction to within
source-language expressions (-ffp-contract=on, currently an alias
^^^^^^^^^
for -ffp-contract=off). */
if (flag_iso
&& !c_dialect_cxx ()
&& (global_options_set.x_flag_fp_contract_mode
== (enum fp_contract_mode) 0)
&& flag_unsafe_math_optimizations == 0)
flag_fp_contract_mode = FP_CONTRACT_OFF;
See: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=37845
But this may change once the FP_CONTRACT pragma is implemented in GCC.
So, for future GCC versions and other compilers, one *also* needs:
#pragma STDC FP_CONTRACT OFF
An advantage of the pragma is that one may forbid FP contraction
only on some parts of the program.
--
Vincent Lefèvre <vincent@xxxxxxxxxx> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
