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Additionally, compilers may treat floating-point variables differently under different levels of optimization \[[Gough 2005|AA. C References#Gough 2005]\].

The reason for this behavior is that Linux uses the internal extended precision mode of the x87 floating-point unit (FPU) on IA-32 machines for increased accuracy during computation.  When the result is stored into memory by the assignment to {{c}}, the FPU automatically rounds the result to fit into a {{double}}. The value read back from memory now compares unequally to the internal representation, which has extended precision.  Windows does not use the extended precision mode, so all computation is done with double precision, and there are no differences in precision between values stored in memory and those internal to the FPU.  For gcc, compiling at optimization level 1 or higher eliminates the unnecessary store into memory, so all computation happens within the FPU with extended precision \[[Gough 2005|AA. C References#Gough 2005]\].

\[[Gough 2005|AA. C References#Gough 2005]\] [Section 8.6, "Floating-point issues"|http://www.network-theory.co.uk/docs/gccintro/gccintro_70.html]
\[[IEEE 754 2006|AA. C References#IEEE 754 2006]\]
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 5.2.4.2.2, "Characteristics of floating types {{<float.h>}}"
\[[ISO/IEC PDTR 24772|AA. C References#ISO/IEC PDTR 24772]\] "PLF Floating Point Arithmetic"