(just a log of my thoughts while reading the patch)
On Thu, 4 Jan 2018, Wilco Dijkstra wrote:
Richard Biener wrote:
On Tue, Oct 17, 2017 at 6:28 PM, Wilco Dijkstra <wilco.dijks...@arm.com> wrote:
+(if (flag_unsafe_math_optimizations)
+ /* Simplify (C / x op 0.0) to x op 0.0 for C > 0. */
+ (for op (lt le gt ge)
+ neg_op (gt ge lt le)
+ (simplify
+ (op (rdiv REAL_CST@0 @1) real_zerop@2)
+ (switch
+ (if (real_less (&dconst0, TREE_REAL_CST_PTR (@0)))
Note that real_less (0., +Inf) so I think you either need to check C is 'normal'
or ! HONOR_INFINITIES.
Yes, it was missing an explicit check for infinity, now added.
I don't understand how the check you added helps.
There's also the underflow issue I guess this is what
-funsafe-math-optimizations
is for. I think ignoring underflows is dangerous though.
We could change C / x > 0 to x >= 0 so the underflow case is included.
However that still means x == 0.0 would behave differently - so the question is
what exactly does -funsafe-math-optimization allow?
That is indeed unclear. HONOR_SIGNED_ZEROS, HONOR_NANS, HONOR_INFINITIES
are better defined and could also see some use.
1/X>=0: if X can be zero (and the inverse can be infinite), the result
depends on the sign of that zero. If !HONOR_SIGNED_ZEROS ||
!HONOR_INFINITIES, it looks like we can simplify to either X>0 or X>=0,
doesn't matter which.
1/X>=0 is true iff X is not NaN and the sign bit is not set, so with
!HONOR_NANS it could also be turned into a bit test.
It works the same for C/X>=0 with 0<C<infinity. For C=infinity, the main
difference is that X=infinity now gives false (if HONOR_NANS).
C/X>0 is more tricky because small/big may round to zero. For C large
enough (assuming denormals, 1 is large enough for instance), the only new
issue is X=infinity. For smaller C, we start getting wrong results for
finite values of X, and presumably that's where
flag_unsafe_math_optimizations comes into play.
If we consider flag_unsafe_math_optimizations as a kitchen sink that
implies !HONOR_SIGNED_ZEROS && !HONOR_INFINITIES, then you don't even need
your REAL_VALUE_ISINF test.
--
Marc Glisse
