diff mbox series

Decimal floats can never be an frange::singleton_p.

Message ID 20220905134701.3330685-1-aldyh@redhat.com
State New
Headers
Series Decimal floats can never be an frange::singleton_p. |

Commit Message

Aldy Hernandez Sept. 5, 2022, 1:47 p.m. UTC 
  As Jakub mentioned in the PR, because many numbers have multiple
possible representations, we can't reliably return true here.

I'll commit this if tests pass.

I wonder if its worth even handling decimal floats in frange, since
there's a lot of things we can't represent.  I suppose even though we
could never propagate an actual value with VRP, we could fold
conditionals (symbolic and stuff outside ranges, etc) ??.  

Thoughts?

	PR middle-end/106831

gcc/ChangeLog:

	* value-range.cc (frange::singleton_p): Return false for
	DECIMAL_FLOAT_TYPE_P.
---
 gcc/value-range.cc | 8 ++++++++
 1 file changed, 8 insertions(+)

Comments

Jakub Jelinek Sept. 5, 2022, 1:51 p.m. UTC | #1 
On Mon, Sep 05, 2022 at 03:47:01PM +0200, Aldy Hernandez wrote:
> As Jakub mentioned in the PR, because many numbers have multiple
> possible representations, we can't reliably return true here.
> 
> I'll commit this if tests pass.
> 
> I wonder if its worth even handling decimal floats in frange, since
> there's a lot of things we can't represent.  I suppose even though we
> could never propagate an actual value with VRP, we could fold
> conditionals (symbolic and stuff outside ranges, etc) ??.  
> 
> Thoughts?
> 
> 	PR middle-end/106831
> 
> gcc/ChangeLog:
> 
> 	* value-range.cc (frange::singleton_p): Return false for
> 	DECIMAL_FLOAT_TYPE_P.
> ---
>  gcc/value-range.cc | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/gcc/value-range.cc b/gcc/value-range.cc
> index c3f668a811a..12a3750d078 100644
> --- a/gcc/value-range.cc
> +++ b/gcc/value-range.cc
> @@ -647,6 +647,14 @@ frange::singleton_p (tree *result) const
>        if (HONOR_NANS (m_type) && !get_nan ().no_p ())
>  	return false;
>  
> +      // Because the significand is not normalized (there is no
> +      // implicit leading "1").  Most values with less than 7
> +      // significant digits have multiple possible representations.
> +      // Zero has 192 possible representations (or twice that for
> +      // signed zeros).

The exact details (7, 192) are dependent on the format, for
decimal64 it is (16, 768) and for decimal128 (34, 12288).

	Jakub
Aldy Hernandez Sept. 5, 2022, 3:55 p.m. UTC | #2 
Final version of the patch.  Disable frange for DECIMAL_FLOAT_MODE_P altogether.

Tested on x86-64 Linux with regstrap as well as mpfr tests.

On Mon, Sep 5, 2022 at 3:47 PM Aldy Hernandez <aldyh@redhat.com> wrote:
>
> As Jakub mentioned in the PR, because many numbers have multiple
> possible representations, we can't reliably return true here.
>
> I'll commit this if tests pass.
>
> I wonder if its worth even handling decimal floats in frange, since
> there's a lot of things we can't represent.  I suppose even though we
> could never propagate an actual value with VRP, we could fold
> conditionals (symbolic and stuff outside ranges, etc) ??.
>
> Thoughts?
>
>         PR middle-end/106831
>
> gcc/ChangeLog:
>
>         * value-range.cc (frange::singleton_p): Return false for
>         DECIMAL_FLOAT_TYPE_P.
> ---
>  gcc/value-range.cc | 8 ++++++++
>  1 file changed, 8 insertions(+)
>
> diff --git a/gcc/value-range.cc b/gcc/value-range.cc
> index c3f668a811a..12a3750d078 100644
> --- a/gcc/value-range.cc
> +++ b/gcc/value-range.cc
> @@ -647,6 +647,14 @@ frange::singleton_p (tree *result) const
>        if (HONOR_NANS (m_type) && !get_nan ().no_p ())
>         return false;
>
> +      // Because the significand is not normalized (there is no
> +      // implicit leading "1").  Most values with less than 7
> +      // significant digits have multiple possible representations.
> +      // Zero has 192 possible representations (or twice that for
> +      // signed zeros).
> +      if (DECIMAL_FLOAT_TYPE_P (m_type))
> +       return false;
> +
>        // Return the appropriate zero if known.
>        if (HONOR_SIGNED_ZEROS (m_type) && zero_p ())
>         {
> --
> 2.37.1
>
diff mbox series

Patch

diff --git a/gcc/value-range.cc b/gcc/value-range.cc
index c3f668a811a..12a3750d078 100644
--- a/gcc/value-range.cc
+++ b/gcc/value-range.cc
@@ -647,6 +647,14 @@  frange::singleton_p (tree *result) const
       if (HONOR_NANS (m_type) && !get_nan ().no_p ())
 	return false;
 
+      // Because the significand is not normalized (there is no
+      // implicit leading "1").  Most values with less than 7
+      // significant digits have multiple possible representations.
+      // Zero has 192 possible representations (or twice that for
+      // signed zeros).
+      if (DECIMAL_FLOAT_TYPE_P (m_type))
+	return false;
+
       // Return the appropriate zero if known.
       if (HONOR_SIGNED_ZEROS (m_type) && zero_p ())
 	{