From: Tom Lane <tgl@sss.pgh.pa.us>
To: pgsql-hackers@lists.postgresql.org
cc: Dean Rasheed <dean.a.rasheed@gmail.com>,
        Andres Freund <andres@anarazel.de>
Subject: NaN divided by zero should yield NaN
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Date: Thu, 16 Jul 2020 15:29:45 -0400
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Dean Rasheed questioned this longstanding behavior:

regression=# SELECT 'nan'::float8 / '0'::float8;
ERROR:  division by zero

After a bit of research I think he's right: per IEEE 754 this should
yield NaN, not an error.  Accordingly I propose the attached patch.
This is probably not something to back-patch, though.

One thing that's not very clear to me is which of these spellings
is preferable:

	if (unlikely(val2 == 0.0) && !isnan(val1))
	if (unlikely(val2 == 0.0 && !isnan(val1)))

I think we can reject this variant:

	if (unlikely(val2 == 0.0) && unlikely(!isnan(val1)))

since actually the second condition *is* pretty likely.
But I don't know which of the first two would give better
code.  Andres, any thoughts?

			regards, tom lane


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diff --git a/src/include/utils/float.h b/src/include/utils/float.h
index 8d4bbc51a6..e2aae8ef17 100644
--- a/src/include/utils/float.h
+++ b/src/include/utils/float.h
@@ -222,7 +222,7 @@ float4_div(const float4 val1, const float4 val2)
 {
 	float4		result;
 =

-	if (unlikely(val2 =3D=3D 0.0f))
+	if (unlikely(val2 =3D=3D 0.0f) && !isnan(val1))
 		float_zero_divide_error();
 	result =3D val1 / val2;
 	if (unlikely(isinf(result)) && !isinf(val1) && !isinf(val2))
@@ -238,7 +238,7 @@ float8_div(const float8 val1, const float8 val2)
 {
 	float8		result;
 =

-	if (unlikely(val2 =3D=3D 0.0))
+	if (unlikely(val2 =3D=3D 0.0) && !isnan(val1))
 		float_zero_divide_error();
 	result =3D val1 / val2;
 	if (unlikely(isinf(result)) && !isinf(val1) && !isinf(val2))
diff --git a/src/test/regress/expected/float4-misrounded-input.out b/src/t=
est/regress/expected/float4-misrounded-input.out
index 6c89af6394..38b847a1ad 100644
--- a/src/test/regress/expected/float4-misrounded-input.out
+++ b/src/test/regress/expected/float4-misrounded-input.out
@@ -143,6 +143,12 @@ SELECT 'nan'::float4 / 'nan'::float4;
       NaN
 (1 row)
 =

+SELECT 'nan'::float4 / '0'::float4;
+ ?column? =

+----------
+      NaN
+(1 row)
+
 SELECT 'nan'::numeric::float4;
  float4 =

 --------
diff --git a/src/test/regress/expected/float4.out b/src/test/regress/expec=
ted/float4.out
index d6c22c1752..310402b6ee 100644
--- a/src/test/regress/expected/float4.out
+++ b/src/test/regress/expected/float4.out
@@ -143,6 +143,12 @@ SELECT 'nan'::float4 / 'nan'::float4;
       NaN
 (1 row)
 =

+SELECT 'nan'::float4 / '0'::float4;
+ ?column? =

+----------
+      NaN
+(1 row)
+
 SELECT 'nan'::numeric::float4;
  float4 =

 --------
diff --git a/src/test/regress/expected/float8.out b/src/test/regress/expec=
ted/float8.out
index c635dd7dcb..2304b579d2 100644
--- a/src/test/regress/expected/float8.out
+++ b/src/test/regress/expected/float8.out
@@ -126,6 +126,12 @@ SELECT 'nan'::float8 / 'nan'::float8;
       NaN
 (1 row)
 =

+SELECT 'nan'::float8 / '0'::float8;
+ ?column? =

+----------
+      NaN
+(1 row)
+
 SELECT 'nan'::numeric::float8;
  float8 =

 --------
diff --git a/src/test/regress/sql/float4.sql b/src/test/regress/sql/float4=
.sql
index 393d98fb14..1fcf823c21 100644
--- a/src/test/regress/sql/float4.sql
+++ b/src/test/regress/sql/float4.sql
@@ -50,6 +50,7 @@ SELECT ' INFINITY    x'::float4;
 SELECT 'Infinity'::float4 + 100.0;
 SELECT 'Infinity'::float4 / 'Infinity'::float4;
 SELECT 'nan'::float4 / 'nan'::float4;
+SELECT 'nan'::float4 / '0'::float4;
 SELECT 'nan'::numeric::float4;
 =

 SELECT '' AS five, * FROM FLOAT4_TBL;
diff --git a/src/test/regress/sql/float8.sql b/src/test/regress/sql/float8=
.sql
index 288969aed6..f103871cdb 100644
--- a/src/test/regress/sql/float8.sql
+++ b/src/test/regress/sql/float8.sql
@@ -43,6 +43,7 @@ SELECT ' INFINITY    x'::float8;
 SELECT 'Infinity'::float8 + 100.0;
 SELECT 'Infinity'::float8 / 'Infinity'::float8;
 SELECT 'nan'::float8 / 'nan'::float8;
+SELECT 'nan'::float8 / '0'::float8;
 SELECT 'nan'::numeric::float8;
 =

 SELECT '' AS five, * FROM FLOAT8_TBL;

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