有没有办法在gfortran中强制将下溢刷为零?
我无法相信这是有人第一次问这个问题,但我在任何地方都找不到相关信息。如果这是重复的,请原谅。
我无法相信这是有人第一次问这个问题,但我在任何地方都找不到相关信息。如果这是重复的,请原谅。
ieee_support_underflow_control(X)
测试控制下溢模式是否受支持,该函数测试X的实际类型下溢控制是否受支持,并在受支持时返回逻辑真值。 如果受支持,则可以call ieee_set_underflow_mode(.false.)
以设置突变下溢模式。
下面是一个测试程序,您可以使用它来测试默认实数类型的下溢控制支持:
program test
use, intrinsic :: ieee_arithmetic
use, intrinsic :: iso_fortran_env, only: compiler_version, compiler_options
implicit none
logical :: underflow_support, gradual, underflow
real :: fptest
integer :: i
print '(4a)', 'This file was compiled by ', &
compiler_version(), ' using the options ', &
compiler_options()
fptest = 0.0
underflow_support = ieee_support_underflow_control(fptest)
if (underflow_support) then
print *,'Underflow control supported for the default real kind'
else
stop 'no underflow control support'
end if
call ieee_set_underflow_mode(.false.)
call ieee_get_underflow_mode(gradual)
if (.not.gradual) then
print *,'Able to set abrupt underflow mode'
else
stop 'error setting underflow mode'
end if
fptest = 2e-36
do i=1,50 ! 50 iterations max
fptest = fptest * 0.5
print '(e15.10)',fptest
call ieee_get_flag(ieee_underflow,underflow)
if (underflow) print *,'Underflow exception signaling'
if (fptest == 0.0) exit
end do
end program test
This file was compiled by GCC version 5.2.0 using the options -mtune=generic -march=x86-64 -fno-unsafe-math-optimizations -frounding-math -fsignaling-nans
Underflow control supported for the default real kind
Able to set abrubpt underflow mode
.1000000036E-35
.5000000180E-36
.2500000090E-36
.1250000045E-36
.6250000225E-37
.3125000112E-37
.1562500056E-37
.0000000000E+00
Underflow exception signaling
-fno-unsafe-math-optimizations -frounding-math -fsignaling-nans
建议在使用IEEE模块时始终使用,以确保遵守标准。-funsafe-math-optimizations
选项:
允许可能违反IEEE或ISO标准的数学优化
或者用其他话来说:
此模式启用了充分利用任意重组和转换的优化,但不保证准确性。 它也不会尝试保留零的符号。
例如,small.f
:
program test
real r
r=1e-40
print *,'r on next line'
print *,r
end program
如果没有任何标志,非零的非规格化(小)数将被显示,而不会出错:
$ gfortran -g small.f
$ ./a.out
r on next line
9.99994610E-41
当尝试打印该值时,陷入非规格化数会导致崩溃,需要对其进行处理:
$ gfortran -g -ffpe-trap=denorm small.f
$ ./a.out
r on next line
Program received signal SIGFPE: Floating-point exception - erroneous arithmetic operation.
Backtrace for this error:
#0 0x2aaaab05c26f in ???
#1 0x2aaaaac61aed in get_float_string
at ../../../libgfortran/io/write_float.def:1064
#2 0x2aaaaac6423d in list_formatted_write_scalar
at ../../../libgfortran/io/write.c:1889
#3 0x4008f1 in test
at /path/to/small.f:5
#4 0x400941 in main
at /path/to/small.f:6
Floating point exception
同时添加了一个标志,将其刷新为零:
$ gfortran -g -ffpe-trap=denorm -funsafe-math-optimizations small.f
$ ./a.out
r on next line
0.00000000