*[PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.@ 2022-09-17 8:24 Aldy Hernandez2022-09-19 7:37 ` Richard Biener 0 siblings, 1 reply; 8+ messages in thread From: Aldy Hernandez @ 2022-09-17 8:24 UTC (permalink / raw) To: Jakub Jelinek, Richard Henderson Cc: GCC patches, Andrew MacLeod, Aldy Hernandez Jakub has mentioned that for -funsafe-math-optimizations we may flush denormals to zero, in which case we need to be careful to extend the ranges to the appropriate zero. This patch does exactly that. For a range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] we flush to [+0.0, x]. It is unclear whether we should do this for Alpha, since I believe flushing to zero is the default, and the port requires -mieee for IEEE sanity. If so, perhaps we should add a target hook so backends are free to request flushing to zero. Thoughts? gcc/ChangeLog: * value-range.cc (frange::flush_denormals_to_zero): New. (frange::set): Call flush_denormals_to_zero. * value-range.h (class frange): Add flush_denormals_to_zero. --- gcc/value-range.cc | 24 ++++++++++++++++++++++++ gcc/value-range.h | 1 + 2 files changed, 25 insertions(+) diff --git a/gcc/value-range.cc b/gcc/value-range.cc index 67d5d7fa90f..f285734f0e0 100644 --- a/gcc/value-range.cc +++ b/gcc/value-range.cc @@ -267,6 +267,26 @@ tree_compare (tree_code code, tree op1, tree op2) return !integer_zerop (fold_build2 (code, integer_type_node, op1, op2)); } +// Flush denormal endpoints to the appropriate 0.0. + +void +frange::flush_denormals_to_zero () +{ + if (undefined_p () || known_isnan ()) + return; + + // Flush [x, -DENORMAL] to [x, -0.0]. + if (real_isdenormal (&m_max) && real_isneg (&m_max)) + { + m_max = dconst0; + if (HONOR_SIGNED_ZEROS (m_type)) + m_max.sign = 1; + } + // Flush [+DENORMAL, x] to [+0.0, x]. + if (real_isdenormal (&m_min) && !real_isneg (&m_min)) + m_min = dconst0; +} + // Setter for franges. void @@ -317,6 +337,10 @@ frange::set (tree min, tree max, value_range_kind kind) gcc_checking_assert (tree_compare (LE_EXPR, min, max)); normalize_kind (); + + if (flag_unsafe_math_optimizations) + flush_denormals_to_zero (); + if (flag_checking) verify_range (); } diff --git a/gcc/value-range.h b/gcc/value-range.h index 3a401f3e4e2..795b1f00fdc 100644 --- a/gcc/value-range.h +++ b/gcc/value-range.h @@ -327,6 +327,7 @@ private: bool union_nans (const frange &); bool intersect_nans (const frange &); bool combine_zeros (const frange &, bool union_p); + void flush_denormals_to_zero (); tree m_type; REAL_VALUE_TYPE m_min; -- 2.37.1 ^ permalink raw reply [flat|nested] 8+ messages in thread

*Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.2022-09-17 8:24 [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations Aldy Hernandez@ 2022-09-19 7:37 ` Richard Biener2022-09-19 8:25 ` Jakub Jelinek 2022-09-19 13:04 ` Aldy Hernandez 0 siblings, 2 replies; 8+ messages in thread From: Richard Biener @ 2022-09-19 7:37 UTC (permalink / raw) To: Aldy Hernandez;+Cc:Jakub Jelinek, Richard Henderson, GCC patches On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > Jakub has mentioned that for -funsafe-math-optimizations we may flush > denormals to zero, in which case we need to be careful to extend the > ranges to the appropriate zero. This patch does exactly that. For a > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] > we flush to [+0.0, x]. > > It is unclear whether we should do this for Alpha, since I believe > flushing to zero is the default, and the port requires -mieee for IEEE > sanity. If so, perhaps we should add a target hook so backends are > free to request flushing to zero. > > Thoughts? I'm not sure what the intention of this is - it effectively results in more conservative ranges for -funsafe-math-optimizations. That is, if -funsafe-math-optimizations says that denormals don't exist and are all zero then doesn't that mean we can instead use the smalles non-denormal number less than (or greater than) zero here? That said, the flushing you do is also "valid" for -fno-unsafe-math-optimizations in case we don't want to deal with denormals in range boundaries. It might also be a correctness requirement in case we don't know how targets handle denormals (IIRC even OS defaults might matter here) so we conservatively have to treat them as being flushed to zero. So ... if we want to be on the "safe" side then please always do this. At the same point you could do if (!HONOR_SIGNED_ZEROS ()) if (real_iszero (&m_max)) { if (real_iszero (&m_min)) m.min.sign = 1; m_max.sign = 1; } else if (real_iszero (&m_min)) m_min.sign = 0; so that we canonicalize a zero bound so that the sign is known for a range. Richard. > gcc/ChangeLog: > > * value-range.cc (frange::flush_denormals_to_zero): New. > (frange::set): Call flush_denormals_to_zero. > * value-range.h (class frange): Add flush_denormals_to_zero. > --- > gcc/value-range.cc | 24 ++++++++++++++++++++++++ > gcc/value-range.h | 1 + > 2 files changed, 25 insertions(+) > > diff --git a/gcc/value-range.cc b/gcc/value-range.cc > index 67d5d7fa90f..f285734f0e0 100644 > --- a/gcc/value-range.cc > +++ b/gcc/value-range.cc > @@ -267,6 +267,26 @@ tree_compare (tree_code code, tree op1, tree op2) > return !integer_zerop (fold_build2 (code, integer_type_node, op1, op2)); > } > > +// Flush denormal endpoints to the appropriate 0.0. > + > +void > +frange::flush_denormals_to_zero () > +{ > + if (undefined_p () || known_isnan ()) > + return; > + > + // Flush [x, -DENORMAL] to [x, -0.0]. > + if (real_isdenormal (&m_max) && real_isneg (&m_max)) > + { > + m_max = dconst0; > + if (HONOR_SIGNED_ZEROS (m_type)) > + m_max.sign = 1; > + } > + // Flush [+DENORMAL, x] to [+0.0, x]. > + if (real_isdenormal (&m_min) && !real_isneg (&m_min)) > + m_min = dconst0; > +} > + > // Setter for franges. > > void > @@ -317,6 +337,10 @@ frange::set (tree min, tree max, value_range_kind kind) > gcc_checking_assert (tree_compare (LE_EXPR, min, max)); > > normalize_kind (); > + > + if (flag_unsafe_math_optimizations) > + flush_denormals_to_zero (); > + > if (flag_checking) > verify_range (); > } > diff --git a/gcc/value-range.h b/gcc/value-range.h > index 3a401f3e4e2..795b1f00fdc 100644 > --- a/gcc/value-range.h > +++ b/gcc/value-range.h > @@ -327,6 +327,7 @@ private: > bool union_nans (const frange &); > bool intersect_nans (const frange &); > bool combine_zeros (const frange &, bool union_p); > + void flush_denormals_to_zero (); > > tree m_type; > REAL_VALUE_TYPE m_min; > -- > 2.37.1 > ^ permalink raw reply [flat|nested] 8+ messages in thread

*Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.2022-09-19 7:37 ` Richard Biener@ 2022-09-19 8:25 ` Jakub Jelinek2022-09-19 13:04 ` Aldy Hernandez 1 sibling, 0 replies; 8+ messages in thread From: Jakub Jelinek @ 2022-09-19 8:25 UTC (permalink / raw) To: Richard Biener;+Cc:Aldy Hernandez, Richard Henderson, GCC patches On Mon, Sep 19, 2022 at 09:37:22AM +0200, Richard Biener wrote: > On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches > <gcc-patches@gcc.gnu.org> wrote: > > > > Jakub has mentioned that for -funsafe-math-optimizations we may flush > > denormals to zero, in which case we need to be careful to extend the > > ranges to the appropriate zero. This patch does exactly that. For a > > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] > > we flush to [+0.0, x]. > > > > It is unclear whether we should do this for Alpha, since I believe > > flushing to zero is the default, and the port requires -mieee for IEEE > > sanity. If so, perhaps we should add a target hook so backends are > > free to request flushing to zero. > > > > Thoughts? > > I'm not sure what the intention of this is - it effectively results in > more conservative ranges for -funsafe-math-optimizations. That is, > if -funsafe-math-optimizations says that denormals don't exist and > are all zero then doesn't that mean we can instead use the smalles > non-denormal number less than (or greater than) zero here? Yes, with -funsafe-math-optimizations we will sometimes have slightly larger ranges, while when we know or assume we know that denormals will be honored we can be more precise. It is similar to the -fno-signed-zeros case, when we are more relaxed and say we don't care about sign of zeros and optimizations can be performed to leave the sign of zero in whatever state they like, we can't have [+0.0, xx] or [xx, -0.0] ranges and need to extend them to [-0.0, xx] or [xx, +0.0]. The honor denormals case is similar, if there is the possibility of having denormals flushed to zero, [+denormal, xx] needs to be extended to [+0.0, xx] and [xx, -denormal] to [xx, -0.0]. Now, I think we just should add a separate option whether denormals are honored or not, because always extending the ranges with denormals on the boundaries might prevent some useful optimizations. If we add such an option, it could default to not honoring them for -funsafe-math-optimizations, and say on alpha it should default to not honoring them by default always unless -mieee option is used, but all of this if the user didn't use the option explicitly, in that case follow what the user said. That way if users know they set DTZ flag themselves or use libraries that do so, they can tell the compiler about it. > That said, the flushing you do is also "valid" for > -fno-unsafe-math-optimizations > in case we don't want to deal with denormals in range boundaries. It is always valid to extend the range more, but could be harmful to optimizations. Say if we have then x > 0.0 test... Of course, without -ffast-math we won't DCE various floating point operations because they can raise exceptions, but at least we can DCE the basic block after it that depends on x > 0.0. Jakub ^ permalink raw reply [flat|nested] 8+ messages in thread

*Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.2022-09-19 7:37 ` Richard Biener 2022-09-19 8:25 ` Jakub Jelinek@ 2022-09-19 13:04 ` Aldy Hernandez2022-09-19 13:06 ` Aldy Hernandez 2022-09-19 13:44 ` Richard Biener 1 sibling, 2 replies; 8+ messages in thread From: Aldy Hernandez @ 2022-09-19 13:04 UTC (permalink / raw) To: Richard Biener;+Cc:Jakub Jelinek, Richard Henderson, GCC patches On Mon, Sep 19, 2022 at 9:37 AM Richard Biener <richard.guenther@gmail.com> wrote: > > On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches > <gcc-patches@gcc.gnu.org> wrote: > > > > Jakub has mentioned that for -funsafe-math-optimizations we may flush > > denormals to zero, in which case we need to be careful to extend the > > ranges to the appropriate zero. This patch does exactly that. For a > > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] > > we flush to [+0.0, x]. > > > > It is unclear whether we should do this for Alpha, since I believe > > flushing to zero is the default, and the port requires -mieee for IEEE > > sanity. If so, perhaps we should add a target hook so backends are > > free to request flushing to zero. > > > > Thoughts? > > I'm not sure what the intention of this is - it effectively results in > more conservative ranges for -funsafe-math-optimizations. That is, > if -funsafe-math-optimizations says that denormals don't exist and > are all zero then doesn't that mean we can instead use the smalles > non-denormal number less than (or greater than) zero here? > > That said, the flushing you do is also "valid" for > -fno-unsafe-math-optimizations > in case we don't want to deal with denormals in range boundaries. > > It might also be a correctness requirement in case we don't know how > targets handle denormals (IIRC even OS defaults might matter here) so > we conservatively have to treat them as being flushed to zero. Actually, rth suggested we always flush to zero because we don't know what the target would do. Again, I'm happy to do whatever you agree on. I have no opinion. My main goal here is correctness. > > So ... if we want to be on the "safe" side then please always do this. > > At the same point you could do > > if (!HONOR_SIGNED_ZEROS ()) > if (real_iszero (&m_max)) > { > if (real_iszero (&m_min)) > m.min.sign = 1; > m_max.sign = 1; > } But wouldn't that set [-0.0, -0.0] when encountering [+0, +0] ?? > else if (real_iszero (&m_min)) > m_min.sign = 0; Jakub actually suggested something completely different...just set +0 always for !HONOR_SIGNED_ZEROS. Aldy > > so that we canonicalize a zero bound so that the sign is known for a range. > > Richard. > > > gcc/ChangeLog: > > > > * value-range.cc (frange::flush_denormals_to_zero): New. > > (frange::set): Call flush_denormals_to_zero. > > * value-range.h (class frange): Add flush_denormals_to_zero. > > --- > > gcc/value-range.cc | 24 ++++++++++++++++++++++++ > > gcc/value-range.h | 1 + > > 2 files changed, 25 insertions(+) > > > > diff --git a/gcc/value-range.cc b/gcc/value-range.cc > > index 67d5d7fa90f..f285734f0e0 100644 > > --- a/gcc/value-range.cc > > +++ b/gcc/value-range.cc > > @@ -267,6 +267,26 @@ tree_compare (tree_code code, tree op1, tree op2) > > return !integer_zerop (fold_build2 (code, integer_type_node, op1, op2)); > > } > > > > +// Flush denormal endpoints to the appropriate 0.0. > > + > > +void > > +frange::flush_denormals_to_zero () > > +{ > > + if (undefined_p () || known_isnan ()) > > + return; > > + > > + // Flush [x, -DENORMAL] to [x, -0.0]. > > + if (real_isdenormal (&m_max) && real_isneg (&m_max)) > > + { > > + m_max = dconst0; > > + if (HONOR_SIGNED_ZEROS (m_type)) > > + m_max.sign = 1; > > + } > > + // Flush [+DENORMAL, x] to [+0.0, x]. > > + if (real_isdenormal (&m_min) && !real_isneg (&m_min)) > > + m_min = dconst0; > > +} > > + > > // Setter for franges. > > > > void > > @@ -317,6 +337,10 @@ frange::set (tree min, tree max, value_range_kind kind) > > gcc_checking_assert (tree_compare (LE_EXPR, min, max)); > > > > normalize_kind (); > > + > > + if (flag_unsafe_math_optimizations) > > + flush_denormals_to_zero (); > > + > > if (flag_checking) > > verify_range (); > > } > > diff --git a/gcc/value-range.h b/gcc/value-range.h > > index 3a401f3e4e2..795b1f00fdc 100644 > > --- a/gcc/value-range.h > > +++ b/gcc/value-range.h > > @@ -327,6 +327,7 @@ private: > > bool union_nans (const frange &); > > bool intersect_nans (const frange &); > > bool combine_zeros (const frange &, bool union_p); > > + void flush_denormals_to_zero (); > > > > tree m_type; > > REAL_VALUE_TYPE m_min; > > -- > > 2.37.1 > > > ^ permalink raw reply [flat|nested] 8+ messages in thread

*Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.2022-09-19 13:04 ` Aldy Hernandez@ 2022-09-19 13:06 ` Aldy Hernandez2022-09-19 13:44 ` Richard Biener 1 sibling, 0 replies; 8+ messages in thread From: Aldy Hernandez @ 2022-09-19 13:06 UTC (permalink / raw) To: Richard Biener;+Cc:Jakub Jelinek, Richard Henderson, GCC patches On Mon, Sep 19, 2022 at 3:04 PM Aldy Hernandez <aldyh@redhat.com> wrote: > > On Mon, Sep 19, 2022 at 9:37 AM Richard Biener > <richard.guenther@gmail.com> wrote: > > > > On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches > > <gcc-patches@gcc.gnu.org> wrote: > > > > > > Jakub has mentioned that for -funsafe-math-optimizations we may flush > > > denormals to zero, in which case we need to be careful to extend the > > > ranges to the appropriate zero. This patch does exactly that. For a > > > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] > > > we flush to [+0.0, x]. > > > > > > It is unclear whether we should do this for Alpha, since I believe > > > flushing to zero is the default, and the port requires -mieee for IEEE > > > sanity. If so, perhaps we should add a target hook so backends are > > > free to request flushing to zero. > > > > > > Thoughts? > > > > I'm not sure what the intention of this is - it effectively results in > > more conservative ranges for -funsafe-math-optimizations. That is, > > if -funsafe-math-optimizations says that denormals don't exist and > > are all zero then doesn't that mean we can instead use the smalles > > non-denormal number less than (or greater than) zero here? > > > > That said, the flushing you do is also "valid" for > > -fno-unsafe-math-optimizations > > in case we don't want to deal with denormals in range boundaries. > > > > It might also be a correctness requirement in case we don't know how > > targets handle denormals (IIRC even OS defaults might matter here) so > > we conservatively have to treat them as being flushed to zero. > > Actually, rth suggested we always flush to zero because we don't know > what the target would do. Again, I'm happy to do whatever you agree More specifically, we don't know what the OS will do, so we either should have a flag of some kind set to TRUE by default when unsafe math optimizations, or always assume denormal flushing could happen. Again, no opinion. Up to y'all. Aldy > on. I have no opinion. My main goal here is correctness. > > > > > So ... if we want to be on the "safe" side then please always do this. > > > > At the same point you could do > > > > if (!HONOR_SIGNED_ZEROS ()) > > if (real_iszero (&m_max)) > > { > > if (real_iszero (&m_min)) > > m.min.sign = 1; > > m_max.sign = 1; > > } > > But wouldn't that set [-0.0, -0.0] when encountering [+0, +0] ?? > > > else if (real_iszero (&m_min)) > > m_min.sign = 0; > > Jakub actually suggested something completely different...just set +0 > always for !HONOR_SIGNED_ZEROS. > > Aldy > > > > > so that we canonicalize a zero bound so that the sign is known for a range. > > > > Richard. > > > > > gcc/ChangeLog: > > > > > > * value-range.cc (frange::flush_denormals_to_zero): New. > > > (frange::set): Call flush_denormals_to_zero. > > > * value-range.h (class frange): Add flush_denormals_to_zero. > > > --- > > > gcc/value-range.cc | 24 ++++++++++++++++++++++++ > > > gcc/value-range.h | 1 + > > > 2 files changed, 25 insertions(+) > > > > > > diff --git a/gcc/value-range.cc b/gcc/value-range.cc > > > index 67d5d7fa90f..f285734f0e0 100644 > > > --- a/gcc/value-range.cc > > > +++ b/gcc/value-range.cc > > > @@ -267,6 +267,26 @@ tree_compare (tree_code code, tree op1, tree op2) > > > return !integer_zerop (fold_build2 (code, integer_type_node, op1, op2)); > > > } > > > > > > +// Flush denormal endpoints to the appropriate 0.0. > > > + > > > +void > > > +frange::flush_denormals_to_zero () > > > +{ > > > + if (undefined_p () || known_isnan ()) > > > + return; > > > + > > > + // Flush [x, -DENORMAL] to [x, -0.0]. > > > + if (real_isdenormal (&m_max) && real_isneg (&m_max)) > > > + { > > > + m_max = dconst0; > > > + if (HONOR_SIGNED_ZEROS (m_type)) > > > + m_max.sign = 1; > > > + } > > > + // Flush [+DENORMAL, x] to [+0.0, x]. > > > + if (real_isdenormal (&m_min) && !real_isneg (&m_min)) > > > + m_min = dconst0; > > > +} > > > + > > > // Setter for franges. > > > > > > void > > > @@ -317,6 +337,10 @@ frange::set (tree min, tree max, value_range_kind kind) > > > gcc_checking_assert (tree_compare (LE_EXPR, min, max)); > > > > > > normalize_kind (); > > > + > > > + if (flag_unsafe_math_optimizations) > > > + flush_denormals_to_zero (); > > > + > > > if (flag_checking) > > > verify_range (); > > > } > > > diff --git a/gcc/value-range.h b/gcc/value-range.h > > > index 3a401f3e4e2..795b1f00fdc 100644 > > > --- a/gcc/value-range.h > > > +++ b/gcc/value-range.h > > > @@ -327,6 +327,7 @@ private: > > > bool union_nans (const frange &); > > > bool intersect_nans (const frange &); > > > bool combine_zeros (const frange &, bool union_p); > > > + void flush_denormals_to_zero (); > > > > > > tree m_type; > > > REAL_VALUE_TYPE m_min; > > > -- > > > 2.37.1 > > > > > ^ permalink raw reply [flat|nested] 8+ messages in thread

*Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.2022-09-19 13:04 ` Aldy Hernandez 2022-09-19 13:06 ` Aldy Hernandez@ 2022-09-19 13:44 ` Richard Biener2022-09-20 5:22 ` Aldy Hernandez 1 sibling, 1 reply; 8+ messages in thread From: Richard Biener @ 2022-09-19 13:44 UTC (permalink / raw) To: Aldy Hernandez;+Cc:Jakub Jelinek, Richard Henderson, GCC patches On Mon, Sep 19, 2022 at 3:04 PM Aldy Hernandez <aldyh@redhat.com> wrote: > > On Mon, Sep 19, 2022 at 9:37 AM Richard Biener > <richard.guenther@gmail.com> wrote: > > > > On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches > > <gcc-patches@gcc.gnu.org> wrote: > > > > > > Jakub has mentioned that for -funsafe-math-optimizations we may flush > > > denormals to zero, in which case we need to be careful to extend the > > > ranges to the appropriate zero. This patch does exactly that. For a > > > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] > > > we flush to [+0.0, x]. > > > > > > It is unclear whether we should do this for Alpha, since I believe > > > flushing to zero is the default, and the port requires -mieee for IEEE > > > sanity. If so, perhaps we should add a target hook so backends are > > > free to request flushing to zero. > > > > > > Thoughts? > > > > I'm not sure what the intention of this is - it effectively results in > > more conservative ranges for -funsafe-math-optimizations. That is, > > if -funsafe-math-optimizations says that denormals don't exist and > > are all zero then doesn't that mean we can instead use the smalles > > non-denormal number less than (or greater than) zero here? > > > > That said, the flushing you do is also "valid" for > > -fno-unsafe-math-optimizations > > in case we don't want to deal with denormals in range boundaries. > > > > It might also be a correctness requirement in case we don't know how > > targets handle denormals (IIRC even OS defaults might matter here) so > > we conservatively have to treat them as being flushed to zero. > > Actually, rth suggested we always flush to zero because we don't know > what the target would do. Again, I'm happy to do whatever you agree > on. I have no opinion. My main goal here is correctness. Yes, I think we should do this. > > > > So ... if we want to be on the "safe" side then please always do this. > > > > At the same point you could do > > > > if (!HONOR_SIGNED_ZEROS ()) > > if (real_iszero (&m_max)) > > { > > if (real_iszero (&m_min)) > > m.min.sign = 1; > > m_max.sign = 1; > > } > > But wouldn't that set [-0.0, -0.0] when encountering [+0, +0] ?? Yeah, that's my laziness not adding a special case for m_min == m_max. > > > else if (real_iszero (&m_min)) > > m_min.sign = 0; > > Jakub actually suggested something completely different...just set +0 > always for !HONOR_SIGNED_ZEROS. Hmm, but the [-1, -0.] with known sign becomes [-1, +0.] with unknown sign? Richard. > Aldy > > > > > so that we canonicalize a zero bound so that the sign is known for a range. > > > > Richard. > > > > > gcc/ChangeLog: > > > > > > * value-range.cc (frange::flush_denormals_to_zero): New. > > > (frange::set): Call flush_denormals_to_zero. > > > * value-range.h (class frange): Add flush_denormals_to_zero. > > > --- > > > gcc/value-range.cc | 24 ++++++++++++++++++++++++ > > > gcc/value-range.h | 1 + > > > 2 files changed, 25 insertions(+) > > > > > > diff --git a/gcc/value-range.cc b/gcc/value-range.cc > > > index 67d5d7fa90f..f285734f0e0 100644 > > > --- a/gcc/value-range.cc > > > +++ b/gcc/value-range.cc > > > @@ -267,6 +267,26 @@ tree_compare (tree_code code, tree op1, tree op2) > > > return !integer_zerop (fold_build2 (code, integer_type_node, op1, op2)); > > > } > > > > > > +// Flush denormal endpoints to the appropriate 0.0. > > > + > > > +void > > > +frange::flush_denormals_to_zero () > > > +{ > > > + if (undefined_p () || known_isnan ()) > > > + return; > > > + > > > + // Flush [x, -DENORMAL] to [x, -0.0]. > > > + if (real_isdenormal (&m_max) && real_isneg (&m_max)) > > > + { > > > + m_max = dconst0; > > > + if (HONOR_SIGNED_ZEROS (m_type)) > > > + m_max.sign = 1; > > > + } > > > + // Flush [+DENORMAL, x] to [+0.0, x]. > > > + if (real_isdenormal (&m_min) && !real_isneg (&m_min)) > > > + m_min = dconst0; > > > +} > > > + > > > // Setter for franges. > > > > > > void > > > @@ -317,6 +337,10 @@ frange::set (tree min, tree max, value_range_kind kind) > > > gcc_checking_assert (tree_compare (LE_EXPR, min, max)); > > > > > > normalize_kind (); > > > + > > > + if (flag_unsafe_math_optimizations) > > > + flush_denormals_to_zero (); > > > + > > > if (flag_checking) > > > verify_range (); > > > } > > > diff --git a/gcc/value-range.h b/gcc/value-range.h > > > index 3a401f3e4e2..795b1f00fdc 100644 > > > --- a/gcc/value-range.h > > > +++ b/gcc/value-range.h > > > @@ -327,6 +327,7 @@ private: > > > bool union_nans (const frange &); > > > bool intersect_nans (const frange &); > > > bool combine_zeros (const frange &, bool union_p); > > > + void flush_denormals_to_zero (); > > > > > > tree m_type; > > > REAL_VALUE_TYPE m_min; > > > -- > > > 2.37.1 > > > > > > ^ permalink raw reply [flat|nested] 8+ messages in thread

*Re: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.2022-09-19 13:44 ` Richard Biener@ 2022-09-20 5:22 ` Aldy Hernandez2022-09-20 8:26 ` Jakub Jelinek 0 siblings, 1 reply; 8+ messages in thread From: Aldy Hernandez @ 2022-09-20 5:22 UTC (permalink / raw) To: Richard Biener;+Cc:Jakub Jelinek, Richard Henderson, GCC patches [-- Attachment #1: Type: text/plain, Size: 3382 bytes --] On Mon, Sep 19, 2022 at 3:45 PM Richard Biener <richard.guenther@gmail.com> wrote: > > On Mon, Sep 19, 2022 at 3:04 PM Aldy Hernandez <aldyh@redhat.com> wrote: > > > > On Mon, Sep 19, 2022 at 9:37 AM Richard Biener > > <richard.guenther@gmail.com> wrote: > > > > > > On Sat, Sep 17, 2022 at 10:25 AM Aldy Hernandez via Gcc-patches > > > <gcc-patches@gcc.gnu.org> wrote: > > > > > > > > Jakub has mentioned that for -funsafe-math-optimizations we may flush > > > > denormals to zero, in which case we need to be careful to extend the > > > > ranges to the appropriate zero. This patch does exactly that. For a > > > > range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] > > > > we flush to [+0.0, x]. > > > > > > > > It is unclear whether we should do this for Alpha, since I believe > > > > flushing to zero is the default, and the port requires -mieee for IEEE > > > > sanity. If so, perhaps we should add a target hook so backends are > > > > free to request flushing to zero. > > > > > > > > Thoughts? > > > > > > I'm not sure what the intention of this is - it effectively results in > > > more conservative ranges for -funsafe-math-optimizations. That is, > > > if -funsafe-math-optimizations says that denormals don't exist and > > > are all zero then doesn't that mean we can instead use the smalles > > > non-denormal number less than (or greater than) zero here? > > > > > > That said, the flushing you do is also "valid" for > > > -fno-unsafe-math-optimizations > > > in case we don't want to deal with denormals in range boundaries. > > > > > > It might also be a correctness requirement in case we don't know how > > > targets handle denormals (IIRC even OS defaults might matter here) so > > > we conservatively have to treat them as being flushed to zero. > > > > Actually, rth suggested we always flush to zero because we don't know > > what the target would do. Again, I'm happy to do whatever you agree > > on. I have no opinion. My main goal here is correctness. > > Yes, I think we should do this. Ok, I'm pushing the attached patch because it's conservatively correct everywhere. We can tweak it when y'all reach consensus. > > > > > > > So ... if we want to be on the "safe" side then please always do this. > > > > > > At the same point you could do > > > > > > if (!HONOR_SIGNED_ZEROS ()) > > > if (real_iszero (&m_max)) > > > { > > > if (real_iszero (&m_min)) > > > m.min.sign = 1; > > > m_max.sign = 1; > > > } > > > > But wouldn't that set [-0.0, -0.0] when encountering [+0, +0] ?? > > Yeah, that's my laziness not adding a special case for m_min == m_max. > > > > > > else if (real_iszero (&m_min)) > > > m_min.sign = 0; > > > > Jakub actually suggested something completely different...just set +0 > > always for !HONOR_SIGNED_ZEROS. > > Hmm, but the [-1, -0.] with known sign becomes [-1, +0.] with unknown sign? Huh. I guess that's true. Does that happen often enough in practice to matter? I suppose we're going into uncharted territory with folks asking for no signs on zeros, but them appearing in the source code? My gut feeling is that we should take whatever signs are in the source code (similar to what we're doing for NANs), but don't introduce any additional ones. Again, no strong opinions. Feel free to add whatever adjustment you think is necessary. Aldy [-- Attachment #2: 0001-frange-flush-denormals-to-zero.patch --] [-- Type: text/x-patch, Size: 2231 bytes --] From 2b61ed838c7f3f4bf54d4530c0f053b420623beb Mon Sep 17 00:00:00 2001 From: Aldy Hernandez <aldyh@redhat.com> Date: Sat, 17 Sep 2022 10:17:13 +0200 Subject: [PATCH] frange: flush denormals to zero For some architectures (or for -funsafe-math-optimizations) we may flush denormals to zero, in which case we need to be careful to extend the ranges to the appropriate zero. This patch does exactly that. For a range of [x, -DENORMAL] we flush to [x, -0.0] and for [+DENORMAL, x] we flush to [+0.0, x]. gcc/ChangeLog: * value-range.cc (frange::flush_denormals_to_zero): New. (frange::set): Call flush_denormals_to_zero. * value-range.h (class frange): Add flush_denormals_to_zero. --- gcc/value-range.cc | 23 +++++++++++++++++++++++ gcc/value-range.h | 1 + 2 files changed, 24 insertions(+) diff --git a/gcc/value-range.cc b/gcc/value-range.cc index 67d5d7fa90f..a8e3bb39bab 100644 --- a/gcc/value-range.cc +++ b/gcc/value-range.cc @@ -267,6 +267,26 @@ tree_compare (tree_code code, tree op1, tree op2) return !integer_zerop (fold_build2 (code, integer_type_node, op1, op2)); } +// Flush denormal endpoints to the appropriate 0.0. + +void +frange::flush_denormals_to_zero () +{ + if (undefined_p () || known_isnan ()) + return; + + // Flush [x, -DENORMAL] to [x, -0.0]. + if (real_isdenormal (&m_max) && real_isneg (&m_max)) + { + m_max = dconst0; + if (HONOR_SIGNED_ZEROS (m_type)) + m_max.sign = 1; + } + // Flush [+DENORMAL, x] to [+0.0, x]. + if (real_isdenormal (&m_min) && !real_isneg (&m_min)) + m_min = dconst0; +} + // Setter for franges. void @@ -317,6 +337,9 @@ frange::set (tree min, tree max, value_range_kind kind) gcc_checking_assert (tree_compare (LE_EXPR, min, max)); normalize_kind (); + + flush_denormals_to_zero (); + if (flag_checking) verify_range (); } diff --git a/gcc/value-range.h b/gcc/value-range.h index 3a401f3e4e2..795b1f00fdc 100644 --- a/gcc/value-range.h +++ b/gcc/value-range.h @@ -327,6 +327,7 @@ private: bool union_nans (const frange &); bool intersect_nans (const frange &); bool combine_zeros (const frange &, bool union_p); + void flush_denormals_to_zero (); tree m_type; REAL_VALUE_TYPE m_min; -- 2.37.1 ^ permalink raw reply [flat|nested] 8+ messages in thread

*2022-09-20 5:22 ` Aldy HernandezRe: [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations.@ 2022-09-20 8:26 ` Jakub Jelinek0 siblings, 0 replies; 8+ messages in thread From: Jakub Jelinek @ 2022-09-20 8:26 UTC (permalink / raw) To: Aldy Hernandez;+Cc:Richard Biener, Richard Henderson, GCC patches On Tue, Sep 20, 2022 at 07:22:03AM +0200, Aldy Hernandez wrote: > > > Jakub actually suggested something completely different...just set +0 > > > always for !HONOR_SIGNED_ZEROS. > > > > Hmm, but the [-1, -0.] with known sign becomes [-1, +0.] with unknown sign? > > Huh. I guess that's true. Does that happen often enough in practice Sure, if you -fno-signed-zeros/-ffast-math and some variable can be zero, copysign/signbit is undefined. The option basically asserts you don't care about it... Jakub ^ permalink raw reply [flat|nested] 8+ messages in thread

end of thread, other threads:[~2022-09-20 8:26 UTC | newest]Thread overview:8+ messages (download: mbox.gz / follow: Atom feed) -- links below jump to the message on this page -- 2022-09-17 8:24 [PATCH] frange: flush denormals to zero for -funsafe-math-optimizations Aldy Hernandez 2022-09-19 7:37 ` Richard Biener 2022-09-19 8:25 ` Jakub Jelinek 2022-09-19 13:04 ` Aldy Hernandez 2022-09-19 13:06 ` Aldy Hernandez 2022-09-19 13:44 ` Richard Biener 2022-09-20 5:22 ` Aldy Hernandez 2022-09-20 8:26 ` Jakub Jelinek

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