A demonstration of how compilers give many different results for the same piece of code.

"g++" is GCC 4.1.2 on Linux. "icc" is the Intel C++ Compiler for Linux, 10.0.023. "cl" is Visual C++ 2008. All code compiled and run on 32-bit Core 2 CPU.

The code is: 

    T a = 1e20;
    T b = arg;
    T c = a*b;
    T d = (a*b)/a;
    T e = sqrt(a*a);
    T f = (a*b)-(a*b+1);
    T g = (a*a)-(a*a+1);
    // then printf everything, using "%f" or "%Lf" as appropriate
where 'arg' is a variable (not a compile-time constant) equal to 1e20.

Output from various compiler configurations, with various T types: (Differences from the first row are highlighted in yellow)

Compiler float double long double
g++ -O0 [0]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     inf
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): 100000002004087734272.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[1]
a=1e20:  100000000000000000000.000000
b=arg:   100000002004087734272.000000
a*b:     10000000200408773773888108121297313071104.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[2]
a=1e20:  100000000000000000000.000000
b=arg:   100000002004087734272.000000
a*b:     10000000200408773427384467440737095516160.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
g++ -O1 [3]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     inf
(a*b)/a: inf
sqrt(a*a): 100000002004087734272.000000
(a*b)-(a*b+1): nan
(a*a)-(a*a+1): nan
[same as 1] [same as 2]
g++ -O2 [4]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     10000000400817550870767646659478483369984.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): 100000002004087734272.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[same as 1] [same as 2]
g++ -O3 [same as 4] [same as 1] [same as 2]
g++ -O0 -ffast-math [same as 0] [same as 1] [same as 2]
g++ -O1 -ffast-math [5]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     10000000400817550870767646659478483369984.000000
(a*b)/a: 100000000834727641088.000000
sqrt(a*a): 100000002004087734272.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[same as 1] [6]
a=1e20:  100000000000000000000.000000
b=arg:   100000002004087734272.000000
a*b:     10000000200408773427384467440737095516160.000000
(a*b)/a: 100000002004087734280.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
g++ -O0 -ffloat-store [7]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     inf
(a*b)/a: inf
sqrt(a*a): inf
(a*b)-(a*b+1): nan
(a*a)-(a*a+1): nan
[same as 1] [same as 2]
g++ -O1 -ffloat-store [same as 7] [same as 1] [same as 2]
g++ -O2 -ffloat-store [same as 7] [same as 1] [same as 2]
g++ -O0 -frounding-math [same as 0] [same as 1] [same as 2]
g++ -O1 -frounding-math [same as 3] [same as 1] [8]
a=1e20:  100000000000000000000.000000
b=arg:   100000002004087734272.000000
a*b:     10000000200408773427384467440737095516160.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): 100000000000000001520.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
g++ -O2 -frounding-math [same as 4] [same as 1] [same as 8]
g++ -O0 -frounding-math -ffloat-store [same as 7] [same as 1] [same as 2]
g++ -O1 -frounding-math -ffloat-store [same as 7] [same as 1] [same as 2]
g++ -O2 -frounding-math -ffloat-store [same as 7] [same as 1] [same as 2]
g++ -O1 -msse -mfpmath=sse [same as 7] [same as 1] [same as 2]
g++-3.3.6 -O2 [9]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     inf
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): inf
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): nan
[same as 1] [same as 8]
g++-3.4.6 -O2 [10]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     10000000400817550870767646659478483369984.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): inf
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): nan
[same as 1] [same as 2]
icc -O0 [same as 0] [same as 1] [same as 2]
icc -O1 [11]
a=1e20:  100000002004087734272.000000
b=arg:   100000002004087734272.000000
a*b:     inf
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): inf
(a*b)-(a*b+1): nan
(a*a)-(a*a+1): nan
[same as 1] [same as 2]
icc -O2 [12]
a=1e20:  100000002004087734272.000000
b=arg:   100000000000000000000.000000
a*b:     10000000200408773773888108121297313071104.000000
(a*b)/a: 100000000000000000000.000000
sqrt(a*a): inf
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): nan
[13]
a=1e20:  100000000000000000000.000000
b=arg:   100000000000000000000.000000
a*b:     10000000000000000303786028427003666890752.000000
(a*b)/a: 100000000000000000000.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[14]
a=1e20:  100000000000000000000.000000
b=arg:   100000000000000000000.000000
a*b:     9999999999999999999783686092270256259072.000000
(a*b)/a: 100000000000000000000.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): -1.000000
icc -O3 [same as 12] [same as 13] [same as 14]
icc -O2 -fp-model double [same as 0] [same as 1] [same as 2]
icc -O2 -fp-model extended [same as 0] [same as 1] [same as 2]
icc -O2 -fp-model fast [same as 12] [same as 13] [same as 14]
icc -O2 -fp-model fast=1 [same as 12] [same as 13] [same as 14]
icc -O2 -fp-model fast=2 [same as 12] [same as 13] [same as 14]
icc -O2 -fp-model precise [same as 0] [same as 1] [same as 2]
icc -O2 -fp-model source [same as 7] [same as 1] [same as 2]
icc -O2 -fp-model strict [same as 0] [same as 1] [same as 2]
icc -O2 -mp [same as 0] [same as 1] [same as 2]
icc -O2 -mp1 [same as 12] [same as 13] [same as 14]
icc -O2 -prec-div [same as 12] [same as 13] [same as 14]
icc -O2 -prec-sqrt [same as 12] [same as 13] [same as 14]
icc -O2 -pc32 [15]
a=1e20:  100000002004087734272.000000
b=arg:   100000000000000000000.000000
a*b:     10000000288757980574615738795334859489280.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): inf
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): nan
[16]
a=1e20:  100000000000000000000.000000
b=arg:   100000000000000000000.000000
a*b:     10000000288757980574615738795334859489280.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[17]
a=1e20:  100000000000000000000.000000
b=arg:   100000000000000000000.000000
a*b:     10000000288757980574615738795334859489280.000000
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): 100000002004087734272.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): -1.000000
icc -O2 -pc64 [same as 12] [same as 13] [18]
a=1e20:  100000000000000000000.000000
b=arg:   100000000000000000000.000000
a*b:     10000000000000000303786028427003666890752.000000
(a*b)/a: 100000000000000000000.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): -1.000000
icc -O2 -pc80 [same as 12] [same as 13] [same as 14]
icc -O2 -fbuiltin [same as 12] [same as 13] [same as 14]
icc -O2 -fno-builtin [19]
a=1e20:  100000002004087734272.000000
b=arg:   100000000000000000000.000000
a*b:     inf
(a*b)/a: 100000002004087734272.000000
sqrt(a*a): inf
(a*b)-(a*b+1): nan
(a*a)-(a*a+1): nan
[same as 13] [same as 14]
cl /Od [20]
a=1e20:  100000002004087730000.000000
b=arg:   100000002004087730000.000000
a*b:     1.#INF00
(a*b)/a: 100000002004087730000.000000
sqrt(a*a): 1.#INF00
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[21]
a=1e20:  100000000000000000000.000000
b=arg:   100000002004087730000.000000
a*b:     10000000200408774000000000000000000000000.000000
(a*b)/a: 100000002004087730000.000000
sqrt(a*a): 100000000000000000000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[same as 21]
cl /O1 [same as 20] [same as 21] [same as 21]
cl /O2 [same as 20] [same as 21] [same as 21]
cl /Ox [same as 20] [same as 21] [same as 21]
cl /O2 /fp:fast [22]
a=1e20:  100000002004087730000.000000
b=arg:   100000002004087730000.000000
a*b:     10000000400817551000000000000000000000000.000000
(a*b)/a: 100000000834727640000.000000
sqrt(a*a): 100000002004087730000.000000
(a*b)-(a*b+1): 0.000000
(a*a)-(a*a+1): 0.000000
[same as 21] [same as 21]
cl /O2 /fp:precise [same as 20] [same as 21] [same as 21]
cl /O2 /fp:strict [same as 20] [same as 21] [same as 21]
cl /O2 /arch:SSE [same as 20] [same as 21] [same as 21]
cl /O2 /arch:SSE2 [same as 20] [same as 21] [same as 21]