This is the mail archive of the
gcc-patches@gcc.gnu.org
mailing list for the GCC project.
Re: Patch gcc-4.0 should not apply mathematical associative rules for
gdr@cs.tamu.edu (Gabriel Dos Reis) wrote on 07.10.04 in <m3y8iiteaa.fsf@merlin.cs.tamu.edu>:
> Richard Henderson <rth@redhat.com> writes:
>
> | On Thu, Oct 07, 2004 at 03:49:45PM -0500, Gabriel Dos Reis wrote:
> | > Richard Henderson <rth@redhat.com> writes:
> | >
> | > | On Wed, Oct 06, 2004 at 05:05:08PM -0700, Fariborz Jahanian wrote:
> | > | > * tree-ssa-dom.c (unsafe_associative_fp_binop): New
> | > | > function. (simplify_rhs_and_lookup_avail_expr): Disallow
> | > | > associativity and constant folding of floating point
> | > | > MULT_EXPR/PLUS_EXPR expressions.
> | > |
> | > | If you apply this, I'll immediately file a PR for a regression
> | > | and assign it to you.
> | > |
> | > | You should allow
> | > |
> | > | X op C1 op C2
> | > | to combine to
> | > | X op C3
> | > |
> | > | if C1 op C2 is exactly representable.
> | >
> | > Only if C3 does not overflow, which is the root of the issue why this
> | > transformation is unsafe in general.
> |
> | Well, overflow is a subset of "not exactly representable", yes.
>
> Yes, but that was not the point I was making.
>
> | But more than that, (X * 0.3 * 3) would not be folded because bits
> | would be lost off the least-significant end. Which is surely the
> | more serious folding error.
>
> My point was that it does not suffice that C1 and C2 be exactly
> representable, we also need C3 not to overflow.
Look above. The original statement was NOT "C1 and C2 are exactly
representable". It was "C1 op C2 is exactly representable". Which, as
Richard points out, includes that it does not overflow.
MfG Kai