[Bug libc/31240] New: LONG DOUBLE: denormals: assigning a constant factor 100 slow,

["newbie-02 at gmx dot de" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31240

            Bug ID: 31240
           Summary: LONG DOUBLE: denormals: assigning a constant factor
                    100 slow,
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: libc
          Assignee: unassigned at sourceware dot org
          Reporter: newbie-02 at gmx dot de
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Created attachment 15297
  --> https://sourceware.org/bugzilla/attachment.cgi?id=15297&action=edit
snippet demonstrating slow evaluation of denormal constants

See subject, assigning 3.3E-4932 to a variable takes 100 times longer than
3.4E-4932. 

Boiled it into the attached file which here produces the following output: 

````  
Assigning a long double denormal constant to a long double variable seems very
slow: 
The '+ ( argc - 1 )' part is an attempt to block compiler cheating by compile
time assigning. 
First column is the time used for 1000000 iterations. Factor ~100 looks odd to
me. 
0.231966; 3.6451995318824746025284E-4951; x2l = ( LDBL_MIN_DEN + ( argc - 1 ) ) 
0.002198; 3.5000000000000000001381E-4932; x2l = ( 3.5e-4932l + ( argc - 1 ) ) 
0.002153; 3.4000000000000000000716E-4932; x2l = ( 3.4e-4932l + ( argc - 1 ) ) 
0.002115; 3.3621031431120935062627E-4932; x2l = ( 3.3621031431120935063E-4932l
+ ( argc - 1 ) ) 
Note the break here, above are 'normal, below 'denormal' values. 
0.209994; 3.3621031431120935058982E-4932; x2l = ( 3.362103143112093506E-4932l +
( argc - 1 ) ) 
0.207378; 3.3000000000000000000052E-4932; x2l = ( 3.3e-4932l + ( argc - 1 ) ) 
0.205288; 3.1999999999999999999388E-4932; x2l = ( 3.2e-4932l + ( argc - 1 ) ) 

Not observed without the '+ ( argc - 1 )' part, assume compiler cheating. 
0.002090; 3.6451995318824746025284E-4951; x2l = ( LDBL_MIN_DEN ) 
0.001863; 3.5000000000000000001381E-4932; x2l = ( 3.5e-4932l ) 
0.001814; 3.4000000000000000000716E-4932; x2l = ( 3.4e-4932l ) 
0.001865; 3.3621031431120935062627E-4932; x2l = ( 3.3621031431120935063E-4932l
) 
0.001811; 3.3621031431120935058982E-4932; x2l = ( 3.362103143112093506E-4932l ) 
0.001868; 3.3000000000000000000052E-4932; x2l = ( 3.3e-4932l ) 
0.001926; 3.1999999999999999999388E-4932; x2l = ( 3.2e-4932l ) 

Not observed when assigning long double values around double normal / denormal
break. 
0.063401; 4.9406564584124654417657E-324; x2l = ( DBL_MIN_DEN + ( argc - 1 ) ) 
0.002173; 2.3999999999999999999368E-308; x2l = ( 2.4e-308l + ( argc - 1 ) ) 
0.002116; 2.3000000000000000000299E-308; x2l = ( 2.3e-308l + ( argc - 1 ) ) 
0.002124; 2.2250738585072013999772E-308; x2l = ( 2.2250738585072014E-308l + (
argc - 1 ) ) 
0.002272; 2.2250738585072009999964E-308; x2l = ( 2.225073858507201E-308l + (
argc - 1 ) ) 
0.002590; 2.2000000000000000000024E-308; x2l = ( 2.2e-308l + ( argc - 1 ) ) 
0.002082; 2.0999999999999999999749E-308; x2l = ( 2.1e-308l + ( argc - 1 ) ) 

But! affecting evaluation of denormal double constants! Here penalty factor
'only' ~30. 
0.062303; 4.9406564584124654417657E-324; x2l = ( DBL_MIN_DEN + ( argc - 1 ) ) 
0.002470; 2.4000000000000000788233E-308; x2l = ( 2.4e-308 + ( argc - 1 ) ) 
0.002405; 2.2999999999999998902644E-308; x2l = ( 2.3e-308 + ( argc - 1 ) ) 
0.002234; 2.2250738585072013830902E-308; x2l = ( 2.2250738585072014E-308 + (
argc - 1 ) ) 
0.063479; 2.2250738585072008890246E-308; x2l = ( 2.225073858507201E-308 + (
argc - 1 ) ) 
0.062954; 2.2000000000000001957711E-308; x2l = ( 2.2e-308 + ( argc - 1 ) ) 
0.062987; 2.1000000000000000072122E-308; x2l = ( 2.1e-308 + ( argc - 1 ) ) 

No such problem when assigning to double variable. 
0.001588; 4.940656458412465442E-324; x2d = ( DBL_MIN_DEN + ( argc - 1 ) ) 
0.001528; 2.400000000000000079E-308; x2d = ( 2.4e-308 + ( argc - 1 ) ) 
0.001557; 2.299999999999999890E-308; x2d = ( 2.3e-308 + ( argc - 1 ) ) 
0.001582; 2.225073858507201383E-308; x2d = ( 2.2250738585072014E-308 + ( argc -
1 ) ) 
0.001528; 2.225073858507200889E-308; x2d = ( 2.225073858507201E-308 + ( argc -
1 ) ) 
0.001544; 2.200000000000000196E-308; x2d = ( 2.2e-308 + ( argc - 1 ) ) 
0.001483; 2.100000000000000007E-308; x2d = ( 2.1e-308 + ( argc - 1 ) ) 

Can't tell if hardware, compiler, library whatever. 
Assume evaluation of constant when read. 
````

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